Fire use by hominins: an example of rapid cultural evolution?

July 26, 2021 • 9:15 am

Yesterday we discussed the possibility of cultural evolution (dissemination of a behavior or skill through imitation and learning) in cockatoos, which attracted a lot of attention, probably because of its parallel with human cultural evolution. (The cockatoos seem to have learned to open garbage bins by watching each other.) And in our species there are a gazillion examples, especially since transportation allowed innovations to be spread quickly and widely. You can think of lots of cases: blue jeans, cuisines from other places, music, and, earlier than that, printing, the wheel (some cultures never got it) and even religion.

The new paper in Proc. Nat. Acad. Sci. below, however, suggests what may have been the very first behavior that spread though species of Homo (not only H. sapiens, but perhaps Neanderthals, which some consider a different species) through movement of individuals: the use of fire.  Click on the screenshot to read the article (free) below, or get the pdf here. The reference is at the bottom.

Fire, of course, has many uses: besides cooking meat and tubers, it can be used to harden wood to make spear points, change the quality of stone to make it easier to flake, and to keep yourself warm. Other uses are given in the Wikipedia article “Control of fire by early humans.”

The MacDonald et al. paper collects evidence of fire use from species of Homo, concluding that it got started about 400,000 to 350,000 years ago and then spread rapidly throughout the species. The rapidity of spread then led them to propose what kind of social structure was present in humans at that time.  This contradicts speculations H. erectus controlled the use of fire about 1.5 million years ago; the authors find that evidence unconvincing.

The problem is to distinguish anthropogenic (“human caused”) fire from natural wildfires. But there are ways of doing this, as the article summarizes. Hearths and charred animal bones are one way. Here’s another bit of evidence: a fire-hardened wooden spear from, coincidentally, about 380,000-400,000 years old, part of a group of artifacts found in Germany:

I can’t evaluate the quality of the evidence, but the authors summarize a lot of data to conclude that regular fire use began about 400,000 years ago, and spread quickly throughout the Old World, with evidence coming from Portugal, Spain, France, Israel, and Morocco. Two quotes:

. . . a review by Roebroeks and Villa identified a clear pattern for Europe: there the record strongly suggests that anthropogenic fire use was very rare to nonexistent during the first half of the Middle Pleistocene, as exemplified by the absence—bar a few dispersed charcoal particles—of fire proxies in deeply stratified archaeological karstic sequences, such as the Atapuerca site complex in Spain or the Caune de l’Arago at Tautavel (France), as well as from such prolific open-air sites as Boxgrove in the United Kingdom. In contrast, the record from 400 ka onward is characterized by an increasing number of sites with multiple fire proxies (e.g., charcoal, heated lithics, charred bone, heat-altered sediments) within a primary archaeological context.

. . . The spatiotemporal pattern of the appearance in the archaeological record of an innovation provides evidence relevant for identifying how the innovation came to be widely distributed: that is, through independent innovation, demic processes, cultural diffusion, or genetic processes. The fact that regular fire use appeared relatively quickly across the Old World and in different hominin subpopulations strongly suggests that the behavior diffused or spread from a point of origin rather than that it was repeatedly and independently invented.

Since fire appeared in both warm and cold places around the same time, the authors suggest that its inception was not correlated with “environmental pressures” (e.g., cold). And because the spread was so rapid, the authors claim, correctly, that the spread throughout the Old World was very unlikely to have been caused by the diffusion of genes producing the tendency to create fire, which would spread only very slowly. Likewise, the near-simultaneity makes it seems unlikely that the use of fire was invented independently by several groups.

If fire use did spread through imitation and learning, then, what does that say about the social structure of early humans? If we were divided up into groups of xenophobic hunter-gatherers who didn’t interact, that would not facilitate the spread of fire. Why would a group give the skill to a competitor group? There are two alternatives.

The first, “demic diffusion,” is that a “deme” (a cohesive populations of hominins) spread rapidly, taking with it the fire use they invented. This seems unlikely given that the spread was more rapid than one could imagine a single population could migrate.

The alternative comprises groups that tolerated each other, and were at least somewhat friendly. As the authors suggest, there was a more “fluid social structure with multiple levels of clustering in social networks”. In other words, perhaps hominims were more interactive than we thought.

Well, we have no direct evidence for that, and it would be hard to come by. And I’ll let other physical anthropologists judge the “simultaneous spread” hypothesis. But I wanted to bring this up because the scenario is at least plausible, and it may be the first evidence for cultural evolution in our genus.

There’s one other trait they add in to the mix as another behavior that spread by cultural evolution: the “Levallois technology” for knapping stone (striking flakes off a stone like flint to make weapons and other implements). This, say the authors, can be learned only through “close and prolonged observation combined with active instruction.” Here’s the Levallois method, which involves producing a flint core in such a way that sharp flakes, useful for tools, can be easily struck off:

The authors posit that this technology also originated in one place, but about 100,000 years later than fire (and surely in a different place), and then spread rapidly among groups in a similar way: non-hostile group interactions in a multi-level social network.

I’ll close with the authors’ final paragraph, summarizing their views:

We hypothesize that around 400 ka, cultural processes supported change in technology across wide areas. This indicates, at a minimum, a degree of social tolerance for individuals from different groups, and suggests the less minimal but still plausible hypothesis that more intensive cooperative interactions within larger-scale networks were already in place, occasionally crossing the boundaries between what we usually infer to have been different biological populations within the wider hominin metapopulation. [JAC: I think they’re referring to movement between “modern H. sapiens and Neanderthals. After all, these groups did mate with each other] We conclude that the spatial and temporal pattern of the appearance of regular Middle Pleistocene fire use documented in the archaeological record signals more than the advent of an important tool in the hominin toolbox: the presence of cultural behavior more like that of humans today than of our great ape relatives. We suggest that long before the cultural florescence associated with the late MSA/Middle Pleistocene and to a greater extent LSA/Upper Paleolithic periods, hominins were beginning to develop the capacities for complexity, variability, and widespread diffusion of technology and behavior that we tend to associate only with H. sapiens.

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MacDonald, K., F. Scherjon, E. van Veen, K. Vaesen, and W. Roebroeks. 2021. Middle Pleistocene fire use: The first signal of widespread cultural diffusion in human evolution. Proceedings of the National Academy of Sciences 118:e2101108118.

Faith versus fact: the problem of Native American creationism and paleoanthropology in North America

June 14, 2021 • 9:30 am

This article in Quillette caught my eye because it was about science—paleoanthropology—and its conflict with faith. The authors are a pair of anthropologists who have written a book about the topic, which is the perennial conflict between scientists on the one hand and Native Americans claiming ancient human remains that, they say, are their ancestors.

Click on the screenshot to read:

The title refers to a meeting of the SAA in April when Weiss gave a talk about the obstructionism of Native American creation myths as they affect paleoanthropology in North America. Although the talk is certainly germane, and provides food for thought (see below), it was in effect “erased” by the SAA, who refused to post it despite their pledge to do so. It was the only talk that wasn’t posted. The reason is clear: going up against the claims of Native Americans, even if those claims can’t be supported, is a no-win situation. As Weiss writes:

The new SAA president, Deborah Nichols, subsequently contacted me to let me know that the video of our live talk would not be posted by the SAA for others to view, due to reports of hurt feelings. (We had previously relied on the SAA’s emailed assurance to presenters that “sessions will be available for viewing on demand within 24–36 hours after their original broadcast, until July 17, 2021.”) Furthermore, we learned, the SAA would not even provide us with the video. (And so we re-recorded the talk, which you can find here.) Another SAA statement was then put out to inform readers that “the SAA board finds the presentation does not align with SAA’s values,” and mentioned that “the board categorically rejects the Weiss-Springer position.”

Here’s the 13-minute talk that Weiss re-recorded after the SAA refused to post it:

There was substantial other pushback on both professional and social media. Here’s an example of a reaction by an “indigenous archeologist” to Weiss’s abstract of the talk: archaeologist.”

Hurt feelings again!

Well, we all know about these conflicts, and it’s conceivable that for some of them the Native Americans have a right to the bones and artifacts found by archaeologists, who of course lose the chance to study them.  But in most cases that “right” is dubious, for the genetic connection between those claiming the bones and the person whose bones are claimed is tenuous at best. Often it rests solely on creation myths: many Native Americans claim that despite scientific evidence that the Americas were populated by “modern” H. sapiens who crossed the Bering Strait from Asia about 15,000 years ago, their ancestors have lived in America forever. Further, to establish that the bones belong to a specific tribe is almost impossible, because bones from people of multiple “tribes” have been found in one locality, and there was considerable migration within North and South America.

Genetic analysis could conceivably settle the question, but without the bones you don’t have the DNA, and even so it’s hard to narrow down ancient DNA to a specific existing group of Native Americans, who are fairly closely related to each other. It’s even worse because the U.S. government passed laws saying that establishing ancestral (i.e., genetic) affinity isn’t necessary: “cultural evidence”, like oral traditions and creation myths, is sufficient. That immediately puts science at loggerheads with superstitions, superstitions that can be demonstrably incorrect.

As an example, Weiss and Springer discuss the famous Kennewick Man, 8,400- year-old remains of a man found in 1996 in Kennewick, Washington. It’s one of the most complete ancient North American skeletons ever found (see photos at bottom), and dates pretty close to the time when Asians began populating the Americas. But the remains were claimed by several tribes of Native Americans, although, according to the authors “the oldest known American tribe, the Hopi, reliably dates its history to only about 2,000 years ago.”

This started a decade-long court case between Native Americans, scientists, and the U.S. Army Corps of Engineers.  The case wasn’t resolved until 2004, and in favor of the scientists, though the remains have since been returned to a “coalition of Pacific Basin tribes” for reburial.

In the meantime, scientific studies of the skeleton, including use of DNA, showed that it was actually more closely related to “modern American Indian populations of Central and South America who are not ‘Native American'” by the U.S. government’s regulations. Using ancestry rather than creation myths, those other populations would have a stronger claim to the bones than would North American tribes.

In the meantime, scientists were able to find out a great deal of information from the skeleton. As the article states,

The greatly delayed scientific study was finally carried out, and the result was a magnificent peer-reviewed 2014 volume, edited by Jantz and Douglas Owsley of the Smithsonian Institution, titled: Kennewick Man: The Scientific Investigation of an Ancient American Skeleton. The studies revealed Kennewick Man’s age, sex, bone morphology, and bone chemistry, as well as modifications to the skeleton incurred during his life. This information, in turn, allowed inferences as to his food intake, food production, and other physical activities, and diseases and injuries he’d endured.

His affinities with other prehistoric and modern populations and individuals were also revealed by these studies. Kennewick Man’s dietary reconstruction from nitrogen isotopes (elements found in a variety of food sources that settle in bones and teeth, and which can be used to reconstruct eating and weaning patterns) revealed a diet composed mainly of marine foods. This differed from the previous view of Paleoindians as big game hunters. Kennewick Man also had bony growths in his ear canals called external auditory exostoses, which some have argued may have impacted his hearing and were related to chronic ear infections.

Kennewick Man had multiple injuries—including a projectile point (a spear head) in his pelvic bone. Chatters argued that this injury never healed properly and likely caused lifelong pain. Anthropologist Della Cook, on the other hand, suggested that the lack of reactive bone (which is evidence of bone healing from injury or infection) in the CT-scans suggests that Kennewick Man’s injury healed quickly. Interestingly, these two perspectives were both published in this 2014 book—an example of the open-minded manner in which science should be conducted and evaluated.

All of this information would have been lost had the repatriationists been successful. No other Paleoindian is as well studied as Kennewick Man, and many were reburied with just a simple osteological report. Such reports may include only the remains’ antiquity, sex, estimated age, and other basic information; and often are written up by undertrained students and marginal scholars who are not subject to peer review, and who do not report their findings in a way that contemporaries can validate. . .

Weiss and Springer describe other cases in which cultural tradition blocked scientific study, as well as scientific study that did succeed in finding out stuff about early Native Americans.

So what should the rule be? Of course, as a scientist who values scientific fact over creation myths or oral tradition, I’m biased in favor of empirical study. But if remains can be traced to a specific tribe or group of tribes, showing the bones to be more closely related to that group than to other tribes, one might consider tribal claims to be valid.  Even so, perhaps there should be an allowed period of scientific study, say two years, before the remains are returned to present-day tribes for reburial or various rites. After all, it’s not as if these bones belong to a present family of Native Americans, like the remains of someone’s son recovered and returned after a battle.

But I don’t think that claims based only on “oral tradition” or “creation myths,” should be honored at all. In such a case, the remains should then be available to scientists. After all, if we honor such superstitious claims, we are also tacitly honoring the creation myths of anybody, including Christians, Scientologists, and Muslims, each of which has its own creation story. That is government entanglement with religion.

And it’s a double entanglement: one with the myths of Native American groups, and the other with the religion of Wokeism, which makes the SAA into an organization that renders decisions based not on empirical considerations, but on ideology and identity politics.

Here’s the skeleton and skull of Kennewick Man:

CHIP CLARK/SMITHSONIAN INSTITUTION

World’s oldest representational art: an Indonesian warty pig from 45,000 years ago

January 23, 2021 • 1:45 pm

Here from Science Advances via National Geographic, is the painting of a wild pig from the Indonesian island of Sulawesi. AT 45,000 years old, it’s world’s oldest cave art, and in fact the oldest known representational art of any sort.

Photo: MAXIME AUBERT

Here’s the paper reporting it (click on screenshot), and a free pdf is here:

The very oldest art comes not from Europe or Africa, but from Indonesia; but surely there was much earlier representational art. The subject is presumably a Celebes warty pig (Sus celebensis), a species still with us, and the artist presumably an anatomically modern human (H. sapiens sapiens).

Here’s the subject. Not a bad representation, eh?

From Wild Pig, Peccary & Hippo Specialist Groups.

And a few words from the authors (“AMH” means “anatomically modern humans”)

On the basis of the presently available evidence, we are unable to definitively conclude that the dated figurative rock art depiction from Leang Tedongnge is the handiwork of cognitively “modern” members of our species. However, this seems to be the most likely explanation given the sophistication of this early representational artwork and the fact that figurative depiction has so far only been attributed to AMH everywhere else in the world.

If so, the dated pig image from Leang Tedongnge would appear to provide some of the earliest evidence, if not the earliest, for the presence of our species in Wallacea. The minimum age of this artwork is compatible with the earliest established indications of AMH from excavated deposits in the Lesser Sunda islands, which formerly provided the oldest archaeological evidence for H. sapiens in Wallacea (~44.6 ka cal BP). Hence, dating results for the Leang Tedongnge painting underline the view that representational art, including figurative animal art and depictions of narrative scenes, was a key part of the cultural repertoire of the first AMH populations to cross from Sunda into Wallacea—the gateway to the continent of Australia.

Are human facial expressions universal in which emotions they express?

August 24, 2020 • 1:00 pm

The latest issue of Science Advances has a provocative and clever piece of research aimed at answering a long-standing question—one considered by Charles Darwin himself: are the facial expressions associated with human emotions universal across all cultures? And, if so, is that the result of evolution? The paper suggests that, at least for a limited set of emotions, the answer is yes to both questions. You can read the paper by clicking the screenshot below, or reading the pdf here. The full reference is at the bottom.

Cowen and Keltner note that there have been a fair number of attempts to answer this question, all involving going to remote areas where there is little contact with Westerners, and seeing if people in those areas match photos of Western expressive signals (joy, anger, sympathy) with similar words in their native language. The results have been mixed. That, one would think, already suggests that perhaps facial expressions aren’t universal. (Cultural similarities could be due to either cross-cultural transmission of expressivity, or to a common evolutionary heritage, but dissimilarities among different populations already suggest that culture can’t cause similarities.)

However, the authors suggest these tests are not dispositive, and suggest a better way to see if expressions are universal. What they did was to look at ancient Mesoamerican artworks that depict facial expressions associated with different situations, isolated the facial expressions, got modern observers to guess the emotions depicted, and then asked whether Westerners would judge those expressions to be ones appropriate to the situation depicted by the artwork.  If there was a correlation between the judgements of which emotions were shown on faces, and those that other people guessed would appear in the situations depicted in the sculptures, this would suggest (but not prove) that expressions are not only universal, but perhaps universal because evolution made them universal.

The authors examined tens of thousands of Mesoamerican artifacts (going back to 1500 B.C.) archived by museums, and found 63 pieces of art that met their three criteria for this study:

  1. subjects were portrayed “within one of eight identifiable contexts” (torture, being held captive, carrying a heavy object, embracing another person, holding a baby, in fighting position, playing a ball sport, and playing music).  Examples of each of these are given in the figure below.
  2. The facial expressions could be clearly visible.
  3. The artworks (sculptures and vessels) were deemed authentic by authorities.

Here are examples of the eight contexts for emotion; see the caption for details:

(From paper): Fig. 1 Ancient American sculptures with discernible faces and contexts. (A) Captive from Tonina archeological site (Mexico, 690–700 CE). Photo credit: Mauricio Marat, Instituto Nacional de Antropología e Historia. https://www.inah.gob.mx/images/boletines/2016_215/demo/#img/foto5.png (1 July 2019). (B) Tortured, scalped prisoner from Campeche (Mexico, 700–900 CE). Baltimore Museum of Art, Kerr Portfolio 2868, photo by J. Kerr. (C) Maya man carrying large stone (Mexico, 600–1200 CE). Kerr Portfolio 8237, photo by J. Kerr. (D) Joined couple (Mexico, 200–500 CE). Los Angeles County Museum of Art (LACMA) AC1996.146.21, gift of C. M. Fearing. (E) Maya woman holding child (600–800 CE). Princeton University Art Museum 2003-26, gift of G. G. Griffin. (F) Kneeling Maya warrior with facial tattoos and shield (Mexico, 600–800 CE), detail. Earthenware and pigment, 15.9 cm by 10.8 cm. Fine Arts Museums of San Francisco 2009.38.2, gift of G. Merriam and J. A. Merriam. (G) Maya ballplayer (Mexico, 700–900 CE). University of Maine HM646, William P. Palmer Collection. (H) Colima drummer (Mexico, 200 BCE–500 CE). LACMA, Proctor Stafford Collection, purchased with funds provided by Mr. and Mrs. Allan C. Balch.

They then isolated the faces from the contexts so that the modern subjects could just look at the faces but not see what the subject was experiencing in the artwork. The 325 subjects were asked them to describe the facial expressions using 30 categories of emotion (e.g., “awe”, “anger”, “shame”, “fear”, and so on and 13 “affective features” like “arousal” and “dominance” (see categories in figure below).

Independently, they asked 114 different subjects, also modern Westerners, to judge what emotions they would expect someone to show in each of the eight situations above. These people were not shown the artworks but simply given the eight situations verbally and asked what emotions you’d expect people to evince.  They then correlated the first subjects’ judgments on what emotions were portrayed by the faces with the expectation of what emotions should be shown from the eight situations, described in words, chosen for analysis.

Here are the correlations between the “read” of subjects from looking just at the faces with the emotions expected judged from subjects looking at the context described in words.

(from paper): Fig. 2 Accordance between emotions perceived in sculptures’ isolated face depictions and Western expectations for the emotions that unfold in eight portrayed contexts. To calculate the accordance between sculptures’ expressions and Westerners’ expectations, we correlated the participants’ average judgments of the emotions and affective features associated with each isolated face and each context across the eight contexts and divided by the maximum attainable correlation given sampling error (see Materials and Methods). Correlations are generally positive, indicating that facial muscle configurations portrayed in ancient American sculptures align, in terms of the emotions they communicate to Westerners, with Western participants’ expectations for the emotions that unfold in different contexts. Error bars represent SEs. Here, we excluded 10 emotions and 1 affective feature used seldom enough that <1/3 of the covariance in judgments was explainable, as a result of which SEs were very large.

Correlations above zero are positive; that is, the expectations of what you’d feel aligned with the subjects’ “read” of the artworks’ faces. As you see, of the 32 emotion categories assessed, 27 had correlations above zero, which alone says something (if there were no correlation, roughly half of the values would be above zero and half below). More important, only six of the 27 positive correlations had standard errors that overlapped zero, or “no correlation”; the rest were statistically significant.

This shows that, in general, the emotions expected by people hearing about the eight contexts for the artwork were those actually discerned by people who looked at the faces of the artwork. And that means that the emotions we expect to see in a modern situation were those that people read from artwork of ancient civilizations. This is turn suggests that at least some facial expressions are “universal”, and people can read them fairly accurately, even in sculptures, as reflecting what people are feeling in a situation. Because these cultures were far removed from ours, this also suggests that the facial expressions weren’t culturally inherited across the ages—that is, we don’t learn the expressions to show determination or distress or the other emotions from others, but express them innately.

One could, I suppose, argue that the Incas and Mayans simply passed on their facial expressions culturally to us as their descendants, but that’s a big stretch since the route for that kind of cultural transmission is dubious: we’re not the descendants of these Mesoamericans. Alternatively, the expressions could have been culturally inherited from our mutual ancestor who lived at least 60,000 years ago when H. sapiens left Africa to populate the world.  If you’re a blank-slater, that’s the argument you’d make.

But it seems more plausible to me that smiling and scowling and showing distress and pain are innate features of our behavior, though I can’t adduce hard evidence for it except that a). babies who haven’t been socialized to show facial expressions of joy or distress still show easily interpretable expressions, presumably before they’re socialized; b). other primates have readable facial expressions without our kind of culture; and c.) one can make a plausible evolutionary hypothesis that humans might have evolved to show expressions that other humans could read: “honest signals.” None of this adds up to a strong evolutionary-psychology argument, but one can agree, from the results above, that there is some universality across millennia in how humans show emotions on their faces.

Finally, the authors used a method called “principal preserved components analysis” to gather in one basket the main “dimensions” (combinations of emotions) that best explain the correlations shown above. From this they determined that three combinations of factors combine to explain at least five types of facial expressions on sculptures. Those expressions, the ones most easily seen as “universal, are these:

  1. “pain”, often in the context of torture
  2. “determination”/”strain”, in sculptures showing heavy lifting
  3. “anger”, usually in sculptures depicting combat
  4. “elation”, seen in “contexts of familial or social touch”, and
  5. “sadness”, as in being held captive after defeat.

The paper has other analyses, but these are the main conclusion. The authors also give several caveats, and you can have a look at those.

The tentative takeaway lesson is that, even after millennia, we can pretty accurately read the facial expressions of people from different cultures.  That is, there are some universals, at least between ourselves and Mesoamericans, in the expression of emotions. Whether that be the result of cultural inheritance or a universal code for expression that resides in our DNA (or a combination of these factors) can’t yet be discerned, but I’m betting on a largely biological-determinism explanation. And so are the authors, at least judging from their final paragraph:

The present results thus provide support for the universality of at least five kinds of facial expression: those associated with pain, anger, determination/strain, elation, and sadness. These findings support the notion that we are biologically prepared to express certain emotional states with particular behaviors, shedding light on the nature of our responses to experiences thought to bring meaning to our lives.

Here are a few more faces and the contexts in which they appear (see paper for explanation), but I think you can guess pretty well.

h/t: cesar
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Cowen, A. S. and D. Keltner. 2020. Universal facial expressions uncovered in art of the ancient Americas: A computational approach. Science Advances 6:eabb1005.

A very old tool

October 16, 2018 • 2:00 pm

Here’s my hand holding a 130,000 year old flint spearhead created by a Neanderthal living near what is now Krapina, Croatia. It’s a beautiful point, and amazing to think that this was chipped by a hominin so long ago.

I learned a ton today at the Croatia Natural History Museum, as we had a special visit to the Neanderthal collection and got a close-up view of the stunning bones and artifacts. This required special permission, and I am most grateful to the curator, Dr. Davorka Radovčić, for taking the time to show us the specimens and give us detailed explanations.

I will do a whole post on our visit, with lots of cool photos, but here’s a teaser:

Okay, one more. This is a very special skull (all bone, no reconstruction), but I’ll tell you about it later:

 

Did modern Homo sapiens evolve in different parts of Africa?

July 15, 2018 • 9:30 am

The general view of human evolution among both scientists and laypeople is that “modern” Homo sapiens emerged from one single area in East Africa: perhaps from just a single population. That population supposedly evolved from an earlier ancestor of unknown identity—perhaps Homo erectus—underwent the transformation into the group of characters that identify our species, and then spread throughout the world. (This is independent of the evolution of what I consider other extinct subspecies of H. sapiens, like Neanderthals and Denisovans, which may have originated from single populations.)

A new paper by Eleanor Scerri and many colleagues, however, questions this conventional wisdom. The paper, an opinion piece rather than a scientific paper, appears in Trends in Ecology and Evolution (click on screenshot to see it, free with UnPaywall; the reference is at bottom, and a free pdf is here), limns what is known as the “multiregional hypothesis” for modern H. sapiens. This is the view that there were many semi-isolated populations of our ancestor scattered throughout Africa, and that each evolved quasi-independently because there were geographic and climatic barriers that reduced gene flow between the groups.

Each population then evolved different traits—of behavior, morphology, and mentation—that eventually combined through more extensive migration into one species that then spread throughout Africa and then the world. It is as if a group of tailors were to craft a garment, each responding to local tastes and materials, and then those garments were combined into one garment that became the “standard and universal” one.

“Anatomically modern humans” date from about 315,000 years ago, with the first finds being in Moroccan caves at Jebel Irhoud. These finds contravened the idea that the earliest humans evolved in East Africa, and suggest that already at that time modern H. sapiens was widely distributed throughout Africa. However, the Jebel Irhoud skulls, while having most of the features of modern human skulls, were relatively elongated. Here’s the earliest find (left) compared with a more recent (95,000 years old) modern H. sapiens skull. The elongation of the earlier one is evident.

Multiregional evolution is supported by the authors’ contention that human populations were structured morphologically as late as 15,000 years ago, with some retaining more ancestral skeletons and others being more advanced—but all considered by anthropologists to be part of the single species H. sapiens sapiens (“modern” Homo sapiens).

Likewise, different populations of H. sapiens sapiens had different stone tools at about the same time, with some being more advanced than others, and this regional differentiation also held for other cultural artifacts like bone tools and shell beads. This too suggests a subdivided species, perhaps with different populations having different degrees of mental/neurological evolution.

What caused this differentiation? The authors aren’t sure, but say a likely cause was different environments that not only isolated populations from each other, but imposed different selection pressures on them. Climate is an obvious difference, with the figure below showing big differences in precipitation among parts of Africa that housed different populations of H. sapiens sapiens (areas within colored squares). Note the variation among localities even at the same time. These would cause obvious barriers as dry places would impede migration between wet places (the Sahara region, with less precipitation, is an obvious barrier).

(from paper): Middle and Late Pleistocene African Environmental Variability in Space and Time. (Left) Map of Africa with key archaeological and fossil sites discussed in the text. Colored boxes indicate averaged regions for simulated precipitation changes from the transient glacial/interglacial LOVECLIM climate model experiment [81]. (Right) Precipitation changes (%) relative to the long-term 784 thousand year mean in the key regions highlighted in left panel, as simulated by transient 784 thousand year-long LOVECLIM climate model simulation [81]. From top to bottom the regions are eastern equatorial Africa, southern Africa, northwestern Africa, and the central Sahara region.
Precipitation would affect the vegetable and animal food available, and hence the means of foraging and food preparation, and that would affect selection for tool usage and for forming different kinds of tools. Those pressures, in turn, could affect social evolution and the structure of our brains. Further, random differences in the frequencies of genes arising by genetic drift could act as different beginning substrates for natural selection, as the course of natural selection depends on its genetic and morphological starting points.

So there is some evidence for the authors’ multiregional hypothesis, which, as Ed Yong characterizes in an article in The Atlantic like this:

The best metaphor for this [the authors’ “multiregionalism” hypothesis] isn’t a tree. It’s a braided river—a group of streams that are all part of the same system, but that weave into and out of each other.

These streams eventually merge into the same big channel, but it takes time—hundreds of thousands of years. For most of our history, any one group of Homo sapiens had just some of the full constellation of features that we use to define ourselves. “People back then looked more different to each other than any populations do today,” says Scerri, “and it’s very hard to answer what an early Homo sapiens looked like. But there was then a continent-wide trend to the modern human form.” Indeed, the first people who had the complete set probably appeared between 40,000 and 100,000 years ago.

But there’s evidence against the hypothesis as well, the most telling being genetic analyses suggesting that different populations of humans in Africa didn’t diverge from each other 300,000 years or so ago, but between 100,000 and 150,000 years ago, suggesting a genetic and populational uniformity before that.

So we have a conflict between what the genetic data tell us and what morphology and tool use tell us.  This may not be a problem since the genetic data is not yet rock-solid. Nevertheless, we should take the paper as tentative but suggestive, which is why it’s an “opinion” piece rather than a regular scientific article.

The implications aren’t that important for non-paleobiologists. Although the multiregional theory suggests that our species emerged in a patchwork fashion, like Yong’s braided rivers, we know that as that species spread throughout the world in perhaps several egresses from Africa, those spreads did not emanate from many different places in Africa. And they don’t overturn the indubitable fact that humans are, in essence, complex African apes.

_________

Scerri, E. M. L. et al. 2018. Did Our Species Evolve in Subdivided Populations across Africa, and Why Does It Matter? Trends Ecol Evol., online publication,  DOI: https://doi.org/10.1016/j.tree.2018.05.005

Finally: a sensible discussion of “race”

March 23, 2018 • 11:15 am

And by “sensible,” of course, I mean a discussion that aligns with my own views. I’ve often written that while there are no finite and strongly genetically demarcated human “races”, there are meaningful and statistically diagnostic differences between populations, ethnic groups, or whatever you want to call them. This is in opposition to the common Left-wing view that races are purely “social constructs” having no biological reality.

Well, there aren’t a finite number of groups whose members are 100% genetically differentiated from other groups. But when you take all genes together, there are sufficient average frequency differences that one can discern statistical clusters that, in turn, allow you to use lots of genes to pretty much diagnose where somebody’s from and who their ancestors were. These “statistical clusters” are real, not social constructs, for they fall out regardless of the politics or biases of the investigator.

Recognizing their existence by no means justifies bigotry or stereotyping, but we shouldn’t dismiss the existence of those clusters simply because, in the past, people with an incorrect idea of “race” have used differences to justify segregation and prejudice. Yet all too often, as with genetic differences among ethnic groups, behavioral differences between the sexes, and evolutionary psychology, those on the Left simply dismiss entire fields because of a fear that scientific research will justify discrimination. And in theory it could, as it did in the past, but it’s better to know the facts and at the same time absorb the idea that the moral and legal equality of all humans, and the equality of opportunity they deserve, does not depend on evolutionary or genetic details. For if it did, then scientific findings could be used to justify prejudice—something that all humanists reject. Asserting that entire fields, like genetic analysis of human ethnic groups, are simply parsing “social constructs” is a form of anti-intellectualism that will stifle scientific progress. If some Leftists had their way, for instance, there would be no evolutionary psychology, no attempt to understand the evolutionary roots of modern human behavior. Do we really want to impose a moratorium on such work?

The recognition of genetic clusters as meaningful entities is the point that David Reich makes in the article given below. Reich, as you may know, is an accomplished professor of genetics at Harvard who’s done a lot of work on DNA-based human and primate phylogenies, human disease genes, interbreeding among ancient lineages of hominins (e.g., Denisovans, Neanderthals, etc.), and mapping human ancestry by looking at statistical grouping. (There’s a big NYT article about his work here.)

I highly recommend you read his essay in the New York Times‘s Sunday Review (click on screenshot):

I’ll give just two quotes from Reich: one about the scientific data and the other about its moral implications—or lack thereof. But read the article!

The data:

[After the 1972 paper of my advisor Dick Lewontin], a consensus was established that among human populations there are no differences large enough to support the concept of “biological race.” Instead, it was argued, race is a “social construct,” a way of categorizing people that changes over time and across countries.

It is true that race is a social construct. It is also true, as Dr. Lewontin wrote, that human populations “are remarkably similar to each other” from a genetic point of view.

But over the years this consensus has morphed, seemingly without questioning, into an orthodoxy. The orthodoxy maintains that the average genetic differences among people grouped according to today’s racial terms are so trivial when it comes to any meaningful biological traits that those differences can be ignored.

The orthodoxy goes further, holding that we should be anxious about any research into genetic differences among populations. The concern is that such research, no matter how well-intentioned, is located on a slippery slope that leads to the kinds of pseudoscientific arguments about biological difference that were used in the past to try to justify the slave trade, the eugenics movement and the Nazis’ murder of six million Jews.

I have deep sympathy for the concern that genetic discoveries could be misused to justify racism. But as a geneticist I also know that it is simply no longer possible to ignore average genetic differences among “races.”

Groundbreaking advances in DNA sequencing technology have been made over the last two decades. These advances enable us to measure with exquisite accuracy what fraction of an individual’s genetic ancestry traces back to, say, West Africa 500 years ago — before the mixing in the Americas of the West African and European gene pools that were almost completely isolated for the last 70,000 years. With the help of these tools, we are learning that while race may be a social construct, differences in genetic ancestry that happen to correlate to many of today’s racial constructs are real.

Recent genetic studies have demonstrated differences across populations not just in the genetic determinants of simple traits such as skin color, but also in more complex traits like bodily dimensions and susceptibility to diseases. For example, we now know that genetic factors help explain why northern Europeans are taller on average than southern Europeans, why multiple sclerosis is more common in European-Americans than in African-Americans, and why the reverse is true for end-stage kidney disease.

I am worried that well-meaning people who deny the possibility of substantial biological differences among human populations are digging themselves into an indefensible position, one that will not survive the onslaught of science. I am also worried that whatever discoveries are made — and we truly have no idea yet what they will be — will be cited as “scientific proof” that racist prejudices and agendas have been correct all along, and that those well-meaning people will not understand the science well enough to push back against these claims.

And how we should handle the future discoveries of genetics:

For me, a natural response to the challenge is to learn from the example of the biological differences that exist between males and females. The differences between the sexes are far more profound than those that exist among human populations, reflecting more than 100 million years of evolution and adaptation. Males and females differ by huge tracts of genetic material — a Y chromosome that males have and that females don’t, and a second X chromosome that females have and males don’t. [JAC: I find this statement somewhat misleading, because he’s talking about “biological” differences, not differences in genetic content, and the Y chromosome doesn’t have many genes.]

Most everyone accepts that the biological differences between males and females are profound [JAC: Again, it’s not clear what he means by “profound,” but I’d agree that they are there and that they do explain differences between the sexes in both morphology and behavior.] In addition to anatomical differences, men and women exhibit average differences in size and physical strength. (There are also average differences in temperament and behavior, though there are important unresolved questions about the extent to which these differences are influenced by social expectations and upbringing.)

How do we accommodate the biological differences between men and women? I think the answer is obvious: We should both recognize that genetic differences between males and females exist and we should accord each sex the same freedoms and opportunities regardless of those differences.

It is clear from the inequities that persist between women and men in our society that fulfilling these aspirations in practice is a challenge. Yet conceptually it is straightforward. And if this is the case with men and women, then it is surely the case with whatever differences we may find among human populations, the great majority of which will be far less profound.

An abiding challenge for our civilization is to treat each human being as an individual and to empower all people, regardless of what hand they are dealt from the deck of life. Compared with the enormous differences that exist among individuals, differences among populations are on average many times smaller, so it should be only a modest challenge to accommodate a reality in which the average genetic contributions to human traits differ.

It is important to face whatever science will reveal without prejudging the outcome and with the confidence that we can be mature enough to handle any findings. Arguing that no substantial differences among human populations are possible will only invite the racist misuse of genetics that we wish to avoid.

Between the unwarranted pseudoscientific statements of Nicholas Wade and James Watson on one hand (both criticized in Reich’s article) and the genetic blank-slateism of various ideologically-biased scientists and cultural anthropologists (who don’t act like scientists) on the other, lies the reasonable position—the one limned by Reich.

Greg’s Take on Reich’s Article

by Greg Mayer

I also like Reich’s article, but if he hopes to be able to talk about genetic differentiation, he’s going to have to stop accepting the “race is a social construction” fallacy, because that means everyone who thinks race is a social construction, or been convinced it is because they keep getting told it is, will ignore everything else he says. As he points out, there is measurable genetic variation; that that variation can be important (clinically, cognitively, etc.); and that that variation allows the identification of the geographic origin of individuals– and the latter is what race means. (As always, I use the zoological definition of a geographic race or subspecies. Subspecies may be described when it is the case that if you show me a specimen I can tell you where it is from, and, conversely, if you tell me where it is from I can tell you what it looks like.) The mass of genetic data on humans now allows us to divide indigenous populations  (i.e. pre-Columbian) into so many races that fit the zoological definition of a race that one of the chief arguments against recognizing races is that there are too many recognizable races– 23 and Me is selling microracial identification on television! Very fine scale genetic data make recognition of geographic groupings so easy that the problem with subspecies isn’t that you can’t tell them apart, but rather you can tell everything apart, even local populations.  Nomenclaturally, subspecies are optional, and there could be reasons, both practical and social, not to name them.

Reich cites Dick Lewontin‘s 1972 apportionment of diversity finding (which, of course, is true), but then doesn’t mention (or perhaps even realize) that that finding  says nothing about whether there are recognizable races. What Reich does do, although more indirectly than I would, is to argue that human moral equality must not rest upon an empirical finding of no genetic differences, because then the finding of genetic differences will undermine the argument for moral equality. I 100% endorse him on the principle that human moral equality should NOT depend on an empirical argument about genetic differentiation. The problem with basing human moral and civil equality on empirical claims about human biological similarity is that such claims may prove to be mistaken. Tony Edwards, in his commentary on Dick’s 1972 paper, says it quite nicely:

“But it is a dangerous mistake to premise the moral equality of human beings on biological similarity because dissimilarity, once revealed, then becomes an argument for moral inequality.”

[Also, Reich seems terribly naive if he thinks “Most everyone accepts that the biological differences between males and females are profound.” I predict he will be assailed from the left on this point. And, Jerry and I wrote our commentaries independently of one another.]

h/t: Rodney, Greg

Readers’ fieldwork pictures

January 15, 2018 • 8:45 am

I’ve put up posts before from Dorsa Amir, a graduate student in biological anthropology at Yale. Here’s her latest contribution: a set of photos from her fieldwork (more photos here). Dorsa’s notes are indented:

As an anthropologist, one of my goals is to explore human behavioral variation across diverse contexts. In conjunction with the larger Shuar Health & Life History project, I have had the amazing opportunity to work with the Shuar, a population indigenous to the Ecuadorian Amazon. The Shuar are forager-horticulturalists who still hunt, fish, and cultivate crops for most of their subsistence, and largely function as small-scale societies. Working with communities of people still living in many ways like their ancestors has been an eye-opening experience when pondering human evolution and culture change. One of the most salient differences between their way of life and ours, in my experience, has been the resilience and independence of Shuar children, who furnish many of their own calories, care for their siblings, and live a much more independent life than their Western counterparts. (I’ve written a bit more about this in a piece for Nautilus). Below are some photos from several summer fieldwork trips.

Note the tamarin:

In the spirit of wildlife photos, here’s Shuar woman with her pet kinkajou [Potos flavus]:

Dorsa had an orange and white cat named Emerson, but wants us to know she’s expanded the felid family to include Hamilton (“Hammy”), named after the late evolutionary biologist W. D. Hamilton. Here’s the happy couple:

 

Nature paper suggests humans inhabited North America 130,000 years ago

April 27, 2017 • 5:43 pm

by Greg Mayer

As Jerry noted yesterday, in a new paper in Nature, Steven R. Holen and colleagues report finding the remains of a butchered 130,000 year old mastodon in San Diego. (If you haven’t already done so, do go take a look at Jerry’s post, which includes a video press release, and illustrations from the paper.)

The key words in the first sentence are ‘butchered’* and ‘San Diego’. The first word indicates that people had taken the bones of the 130,000 year old mastodon apart– which in itself would be a “neat, but what’s the fuss” result. It’s ‘San Diego’ that’s the cause of the fuss. The peopling of the Americas has been a contentious topic for some time, but virtually all the debate has concerned a relatively slim time interval– 12-30 kya (see here for a previous discussion at WEIT, and this news piece in Science about two recent papers with contrasting conclusions). The San Diego find is thus 100,000 + years earlier!

So what evidence do they have for this early arrival? First, they have the mastodon, whose bones were fractured in ways which they find inconsistent with damage by carnivores or the environment, but which appear consistent only with being struck with implements. They did a lot of breaking of elephant bones in order to try to simulate the damage to the mastodon, and concluded that tools alone could do the trick. The mastodon’s remains were radiometrically dated at 130.7 ± 9.4 kya. In addition to the mastodon, they also found stone tools, which they interpret as hammerstones and anvils.

These results would have many important implications for human evolutionary history; but first we must ask, are the results correct?

I must admit I’m dubious. The anvil and hammerstones are not the sorts of objects which are unquestionably manufactured– they are not like finely fluted spear points, whose human origin cannot be doubted. The breakage patterns in the bones do indicate that the breaks occurred perimortem, but I’m not sure the breaks could not be due to non-human causes. The dating is directly on the mastodon, which is good– they’ve not dated some possibly extraneous item which could have been redeposited from earlier strata. But, nonetheless, dating is subject to various artifacts.

As Carl Sagan used to say, “Extraordinary claims require extraordinary evidence.” What makes the current claim extraordinary is that there’s no other evidence of human presence in the Americas for ca. 100+ K years after this find. And it’s not like the late Quaternary of America is an unstudied or poorly known stretch of time! I don’t regard fracture patterns and crude tools to be sufficiently extraordinary evidence to overcome, in a single go, the weight of that 100,000 year absence. It is much more reasonable to think that the new data can be reconciled with all the past data in a way that does not require us to discount the past data. And, thinking, “they must have made a mistake somewhere with the new data”, is a perfectly plausible way of reconciling the two. This conservatism in the face of anomalies is a key part of the method of science– it properly proportions belief to the evidence.

On the other hand, the new data do not threaten to overturn any fundamental principles, merely a seemingly well-attested fact of evolutionary history, and such facts have been overturned before. So, we must ask, but what if they’re right?

The most interesting implication, to me, is that if there were people here 130 kya, they went completely extinct.  It means that human habitation of an entire hemisphere is an iffy thing. The real first Americans got wiped out by something– disease, predators, climate, competitors, whatever. Who would these now extinct people have been? Well, if they got to America not too long before the radiometric date, they would probably be Neanderthaloid (by which I mean the varied archaic Eurasian subspecies of Homo sapiens with which anatomically modern humans interbred after their spread from Africa). If they came much earlier, they might have been Homo erectus (which would make Harry Turtledove’s A Different Flesh, in which the first European settlers of America encounter not Indians, but “sims“, prophetic!).

There would also be a possibility that these first Neanderthaloid Americans survived, and that the anatomically modern human colonizers of ca. 20 kya, interbred with them in the course of replacing them, just as their forebears did in Asia. However, because American Indians are not, as far as I know, enriched for Neanderthaloid alleles relative to other Eurasians (who are 1-4% Neanderthaloid; a bit higher in Melanesia), this seems unlikely. (There are claims out there that Indians are enriched for Neanderthaloid genes, but I don’t know how that got started; East Asians, from which, at least generally, American Indians descend, are Neanderthaloid enriched relative to Western Europeans, which seems to indicate more than one episode of interbreeding on the course of their migration from Africa.)

* I use “butchered” here in the sense of “processed for eating”, as the bones were presumed broken apart to get at the marrow. The paper uses “butcher” in the narrower sense of “cut with a knife or similar implement”. The paper does not say the mastodon was cut with a knife or other sharp tool.


Holen, S. R., T. A. Deméré, D. C. Fisher, R. Fullagar, J. B. Paces, G. T. Jefferson, J. M. Beeton, R. A. Cerutti, A. N. Rountrey, L. Vescera, and K. A. Holen. 2017. A 130,000-year-old archaeological site in southern California, USA. Nature 544:479-483.

The evolution of sexual dimorphism in humans: Part 2

December 21, 2016 • 11:00 am

In a post one week ago, “The ideological opposition to biological truth,” I argued that sexual dimorphism for body size (difference between men and women) in humans is most likely explained by sexual selection, and that it also reflects behavioral differences between males and females: males compete for females, and greater size and strength give males an advantage. That competition results from females—in many species, not just ours—being a “scarce” resource for males, since the number of males capable of breeding far exceeds the number of females who cannot breed because they’re tending offspring or in gestation. This disparity can be categorized in two ways:

  • The behavioral operational sex ratio: the ratio of sexually active males to fertilizable females at a given time. This is about 11.7 in humans!
  • The physiological operational sex ratio, the same ratio but for all individuals capable of reproducing (rather than those actually engaged in mate-hunting). This is about 8.6 in humans.

The ratios are greater in some primates (gorillas have values of about 84!), but if they’re greater than 1, there’s room for sexual selection, since there are more males seeking females than there are females available as mates. This itself is one bit of evidence for the operation of sexual selection in humans.

Now how the sexual selection actually operated in our ancestors is not perfectly clear. Some of it, as the data suggest, involves male-male competition: fights between males to control females, as we witness in gorillas, deer, and elephant seals. Females are more or less constrained to mate with the winning males. Or females may prefer to mate with the biggest and strongest males, for those males may protect their offspring—and hence the female’s genes—better than do smaller, weaker males. (This gives an evolutionary advantage to those females who can discern and choose the best males.)

Both of these factors can, of course, work at the same time, and there are other more arcane forms of sexual selection I won’t mention, including other signs in males of “good genes”. But any sexual-selection scenario goes along with a difference in sexual behavior, explaining why, even today, males are more promiscuous and willing to mate than are the choosier females.

A further possibility is that there could be an ecological distinction between males and females, with males hunting, and thus needing size and strength, while females do gathering (presumably females don’t have time to hunt because they’re rearing children). That doesn’t involve sexual selection, but it also fails to explain all the data, like the correlation between sexual dimorphism and polygyny within humans, and the fact that in our primate relatives there’s not only the same correlation among species, but no palpable division of labor among males and females. It also doesn’t explain the existence of traits like beards, lower voices, or same-sex aggression among human males but not females. Nevertheless, there’s no reason why several forces couldn’t work together to cause men to have evolved larger body size and increased musculature (as well as other features) in our ancestors. But surely sexual selection is one, for the evidence below fits no other hypothesis.

As I noted, these relatively uncontroversial ideas about sexual selection (not mine, actually; they’re the conventional wisdom among evolutionists beginning with Darwin), was challenged by Holly Dunsworth, an associate professor of anthropology at the University of Rhode Island, on her website. Dunsworth, who called my theory (supported by lots of data cited in my original post) a “story”, offers her own speculations, which really are a story because they lack empirical support and don’t explain a lot of observations. Here’s what she said:

It’s not that Jerry Coyne’s facts aren’t necessarily facts, or whatever. It’s that this point of view is too simple and is obviously biased toward some stories, ignoring others. And this particular one he shares in this post has been the same old story for a long long time.

What about the other side of the body size sexual dimorphism story?

What about the women?

Selection could well be the reason they stop growing before men and why they end up having smaller bodies than men, on average.

Perhaps men can make babies while growing, but perhaps women can’t. Energetically, metabolically. So reproduction wins over growth. We reach sexual maturity and stop growing. Is that just a coincidence?

Why doesn’t this (and other tales) fit alongside the big-aggressive-males-take-all explanation for sexual dimorphism? #evolution

But as I noted in the piece she criticized, selection on females—through either evolution of female preference or on differential ecological roles between the sexes—could affect sexual dimorphism. But Dunsworth conveniently ignored that bit. Her criticisms were echoed by an article by Jesse Singal in New York Magazine, which claimed, as did Dunsworth, that I was offering mere “stories”—unevidenced speculation. Singal said this:

In Dunsworth’s view, all she is asking for is some nuance and, well, skepticism. “People love to boil complex processes down to their preferred (intentional or not) story,” she wrote, “with some in leading roles and others completely absent, and we don’t have to take that anymore.” Her tweeted example about growth nicely captures this: It could be that Coyne’s aggressiveness story leaves out important details about why men are bigger than women, or fails to explain certain aspects about that differences. Overall, it certainly seems like people are quicker to latch onto evo-psych stories that reinforce certain views of men and women.

That last sentence is a veiled accusation that my piece was sexist. I reject that completely.

As I noted in part I of this response, neither Singal nor Dunsworth appreciated that I have a long published history of criticizing “just-so” stories in evolutionary psychology. I don’t like unevidenced speculation when it’s promoted as truth. But the sexual selection theory for human sexual dimorphism is supported by a lot of evidence. It is manifestly not a mere “just-so” story.  In my original piece I adduced this evidence (revised slightly):

  • In human societies studied by Richard Alexander, those societies that are more polygynous (in which males compete more intensively for females) show greater sexual size dimorphism than societies that are more monogamous. This was a prediction made before the data were acquired—a prediction derived from sexual selection theory. And it was fulfilled. UPDATE: I see now that Alexander’s finding wasn’t reproduced in another experiment, so consider this conclusion questionable.
  • Among species of primates, there’s a good correlation between the polygyny of a species and sexual dimorphism: those species in which males have a higher variance in offspring number, and in which males thus compete more intensely for females, also show a greater ratio of male/female body size, even when corrected for phylogeny. (Too, in primate species in which males fight each other over females, the relative size of the canine teeth, used in battle, is larger than in species showing less direct male-male competition.)
  • In humans, as in many other species in which males compete for females, the sex ratio at birth favors males. They then die off at a higher rate due to higher risk-taking and exploratory behavior, and also senesce faster, which is why among older humans there are so many more females than males. (Check out any Gray Line tourbus.) This is predicted by sexual selction theory.
  • In line with the above, in humans and other primates, males show from the outset great exploratory and risk-taking behaviors, and as adults show many other behaviors that differ from those of females, such as greater dispersal. Is this due to the Primate Patriarchy? Probably not, given that these differences in behavior are shown in many species besides ours and make evolutionary sense.

There’s more evidence, too, which I’ll mention shortly.

But what’s the evidence for Dunsworth’s theory? As far as I can see, there isn’t any. Her theory claims that 1) females can’t reproduce while growing, while males can. 2) There’s a tradeoff between growth and reproduction, so if you stop growing as a female, you can start reproducing earlier. Conclusion: females stop growing before males because reproduction is all-important, and therefore they’re smaller than males as adults.

But the data don’t even support her theory. Puberty begins in females at about ages 10 and 11, and in males between 11 and 12.  (The age of both appears to be decreasing in recent years.) Yet males keep growing this whole period and well beyond, as do females. There’s no indication that females stop growing when they become reproductively competent. Here are growth curves (stature and weight) for both males and females. Stature begins tapering off at about ages 14-15 in both sexes (a slower taper in males), but both sexes continue to grow until age 20.

Females:

growth-2-20-girls

Males:growth-2-20-boys

Now we don’t know about body sizes and ages of puberty in our ancestors, which is the really important information, and I doubt we’ll have that given that it’s virtually impossible to ascertain the age of puberty in fossils. But clearly there’s no support in any data for Dunsworth’s hypothesis that “perhaps men can make babies while growing, but perhaps women can’t. Energetically, metabolically.” Both men and women can make babies while they’re still growing. But men continue to grow not only faster but also bit longer than do women (see above), something which explains sexual dimorphism. But since men are reproductively competent when they hit puberty, why do they keep getting bigger? Dunsworth doesn’t tell us, but sexual selection theory does. Men achieve greater stature and muscle mass by both growing faster than females, and tapering off a bit later.

So Dunsworth’s hypothesis is not only unsupported by data, but fails to explain the growth data that do exist.

More important, her theory doesn’t explain the four points given above—points that are well explained by sexual selection theory.  She and New York Magazine fail to realize that the sexual-selection explanation for human sexual dimorphism is not a “story”, but makes supported predictions and clarifies previously obscure observations. How irritating to see these people distort what we know about evolutionary theory and human biology!

As I mentioned in earlier posts, I think Dunsworth is blinkered by her ideology, because she thinks that sexual selection theory ignores females. Well, straight male-male competition without female choice does involve evolution mainly in males, but there are forms of sexual selection that involve female choice, too, and that has surely happened in species like birds and fish. In those groups, and others, males show ornaments and colors not useful in male-male competition, but are the object of female choice. And some of that process may have happened in our own lineage. The competing theories are not zero-sum, so that only one can be right. All these processes can work together. But surely one is sexual selection.

Regardless, sexual selection as an explanation implies that there are also sexual differences in behavior: differences we see in modern experiments and are probably not purely cultural because a. they’re predicted by the differences in body size and b. we see the same difference in mate choosiness in many other species—and not just primates. It’s an ineluctable consequence of the difference in reproductive investment between males and females.

I’ll now list some other observations about human mating and morphology that are explained by sexual selection theory but not explained at all by Dunsworth’s theory. Some of these come from the references given at the bottom of the post.

  • In other sexually dimorphic primates, including chimpanzees and gorillas, direct contests between males can be observed, and probably existed in our ancestors since paleoanthropological data show that many more males were killed by violence than females, possibly reflecting inter-group battles, which in modern hunter-gatherer societies are often over females. Many societies also show “bride theft”, capture of females by bands of males—common in Amazonian hunter-gatherer societies.
  • Male humans have more robust skulls than do females, including mandibles and brow ridges. This may reflect evolution to withstand blows to the head. (Males also have a higher tolerance for pain.)
  • Men are not only taller and heavier than women, but are stronger, particularly in the upper body. While size differences are about 8%, and body mass about 15-20%, women’s bodies have a higher percentage of fat, so that when you look at fat-free body mass, men are 40% heavier, have 60% more lean muscle mass, 80% greater arm muscle mass, 75% more upper-body muscle mass, and 50% more lower body mass. This difference in relative amount of muscle mass cannot be explained by Dunsworth’s theory, which is purely about growth, but is explained by male-male competition under sexual selection—and perhaps by female preference as well. This is reflected in differential athletic performance, and is why men and women usually compete separately in athletics. Even for men and women of equal sizes, men are far stronger; as Hill et al. note, “the average man is stronger than 99.9% of women (some of this, of course, may be because men work out; I haven’t checked the references.)
  • In every society studied, men are physically more aggressive than women, both in play as kids and as adults. The vast majority of murderers are men, and this aggressive activity peaks during men’s peak reproductive years, when they would be competing for mates most strongly. These data do not include killings in war.
  • Traits like beards and lower voices in men (men’s vocal folds are 60% longer than women’s, giving them lower voices) have been shown to act as indicators of dominance; both are evolved morphological traits. (The evidence supporting all these claims can be found in the papers cited below.) Women also prefer larger men and deeper voices, so there may have been an element of female choice in sexual selection, though of course the observations we make are on modern rather than ancient hominins.
  • Sexual dimorphism is also seen in our ancestors like Australopithecus and H. erectus, implying that it’s been acting on our lineage a long time. But there’s also some evidence, cited by Plavcan, that the degree of sexual dimorphism has waxed and waned as females got either bigger or smaller over time, implying that there may have been some separate natural selection in females that could increase or decrease sexual dimorphism (but never effaced it).
  • Finally, Buss’s article and others not cited outline the psychological and behavioral differences between males and females that make sense under sexual selection. These not only include the greater promiscuity of males than females, but also the greater sexual jealousy of males toward women than vice versa (our male ancestors weren’t always sure who the father of their mate’s children was, while women were far more certain). There is also a big difference between males and females in their attitudes towards casual sexual experiences (guess in which direction), and in how exacting their standards are for a short-term mate (guess again). Men have lower psychological thresholds for risk-taking. And so on. As Buss wrote, “Large sex differences appear reliably for precisely the aspects of sexuality and mating predicted by evolutionary theories of sexual strategies.”

I’ve adduced about a dozen pieces of evidence supporting the sexual selection explanation for human morphological and behavioral dimorphism—none of which can be explained by Dunsworth’s hypothesis. (And that hypothesis was dead in the water anyway, contradicted by the known data.) Since all hypotheses must, at bottom, be supported by the weight of accumulated scientific evidence, it is clear that sexual selection, and male-male contest competition in particular, is a compelling explanation for human sexual dimorphism. In contrast, Dunsworth’s hypothesis isn’t in the least compelling. That doesn’t mean we shouldn’t keep evaluating the evidence or suggesting new hypotheses, but simply that these should be supported by data rather than ideological preference.

I urge readers to look at the papers below, and use the data (and that from other papers) to evaluate theories about human behavioral and sexual dimorphism. I don’t propose to engage in a dialogue with Dr. Dunsworth about this, but I would like to know how her theory can explain the dozen-odd observations given above.

Dunsworth must have emitted something like twenty tweets about her piece, impugning me; and she even issued this over-the-top pronouncement:

Well, there’s fighting material above, but I’ve had my say. Still, I can’t believe that simply my writing a post on human sexual dimorphism and its implications would drive anybody away from studying human evolution. After all, the give-and-take of hypotheses, critical thinking, and data are the very meat of science, and if you disagree with somebody, you don’t simply walk away from a field. I sure as hell am not leaving evolutionary biology because Dunsworth and New York Magazine took out after me!

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UPDATE: Things are getting worse: Peter Boghossian is arguing with Dunsworth on Twitter (I’m not involved, as I avoid Twitter Wars), but now we’re getting lumped with some rather unsavory types (except for the “evolutionists”):

h/t: Steve, David

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Buss, D. M. 1995. Psychological sex differences. Amer. Psychologist 50:164-168.

Hill, A. K., D. H. Bailey, and D. A. Puts. 2017. Gorillas in our midst? Human sexual dimorphism and contest competition in men. pp. 235-249 in: On Human Nature: Biology, Psychology, Ethics, Politics, and Religion. in M. Tibayrencand F. J. Ayala (eds.) .On Human Nature, M.  Tibayrenc and F. J. Ayala, eds. Academic Press.

Puts, D. A. 2010. Beauty and the beast: mechanisms of sexual selection in humans. Evolution and Human Behavior 31:157-175.

Plavcan, J. M. 2012. Sexual size dimorphism, canine dimorphism, and male-male competition in primates. Where do humans fit in? Human Nature 23:45-67.