Category: human biology

How do we tell left from right?

April 6, 2022 • 12:15 pm

I’m sure this topic has been covered by scientists before, but I haven’t researched it, so I’m raising it as a naive question.

First, it’s easy for you to tell up from down because down is where your feet are and up is what you see when you look away from your feet and toward the sky. Or you could drop something; the direction it falls is “down”.

It’s also easy for you to tell your front from your back. Your front is what you see when you look down, and the other side of your body is your back.

But how do you tell right from left at any given moment?

Now of course there are a number of cues that we could use to tell right from left. The side our heartbeat is most detectable by touch is on the left (unless you have situs inversus!), I wear my watch on my left wrist and my ring on my right hand, and so on. If you drive a car in the US, the steering wheel is on the left side.

But we don’t actually use these cues. When someone tells you “turn right” when you’re asking directions, you just know which way to go.  But HOW?

Presumably we learn right from left when we’re kids: a parent presumably points out your right hand and says “that’s the right hand” and vice versa. But again, what cues do we use now? Surely not the hands! (I’m sure the answer is out there somewhere, but if a reader provides it, many of us will have learned something.)

That’s my question, but it’s related to a genetics question that I pondered for years before any answer was ever given. It’s about asymmetry in animals.  There are basically two types of ways a bilaterally  symmetrical animal can be asymmetrical in some ways. I’ve posted on this three times before (here ,here, and here), so have a look at those posts. Here’s just a brief summary.

1.) Fluctuating asymmetry. Individuals are asymmetrical for some features, but the direction of asymmetry varies from individual to individual. Handedness in humans is this way, though it has a genetic component, too, making right-handed people more common. Lobsters have asymmetrical claws: one is a “cutter” and the other a “crusher”, and it’s random whether the crusher claw is on the right or left. (We know, by the way, how this comes about. Young lobsters start their lives with identical claws, but the claw that is used most often provides more neurological activity, and that activity irrevocably creates the asymmetry, which lasts for life.The most-used one becomes the grinder.) Some species of flounders are randomly flat on the left or right sides, though all start off being vertically postured fish who develop into flat fish, with the eye on the bottom migrating to the top. Many human facial features are examples of fluctuating asymmetry: the right sides of our faces are not the same as the left, but the kind of differences differ in direction from person to person. Fluctuating asymmetry is also called “anti-symmetry” since the sides are different, but not in a consistent direction.

2.) Directional asymmetry. This is what always puzzled me. There are some basically bilaterally symmetric animals, like us, in which there are some asymmetries that are directional. That is, the right side always differs from the left in a consistent way. The narwhal tusk (a hyper-developed canine tooth) is always on the left side, some owls use directionally asymmetrical ears as a way to locate prey, I’ve mentioned the human heart before, and there are many examples. (In some flounder species, individuals are always right-flat, while individuals of other species are left-flat.)

The question I always had about this rests on the observation that because every individual is directionally asymmetrical the same way, that asymmetry must somehow rest on genes for those traits that are active in development. But how does a gene know it’s on the right or left side so it can turn on or off? Given a bilaterally symmetrical individual, it’s easy to genetically specify “front” and “back”, and “up and down”, but once those are specified, then the internal features of the organism should be identical on the right and left side. So how does a gene for say, hyper-development of the canine tooth “know” that it’s on the left side to become activated? There has to be some consistent physiological or metabolic difference between the right and left sides of an animal to provide the relevant developmental cues.  But how could that occur?

We’re beginning to find out now, though we’re far from a complete understanding of the phenomenon. There are two suggestions I know of, based on either the asymmetry in the way embryonic cilia beat (causing an asymmetry in the flow of embryonic fluid) or in the “handedness” of our constituent amino acids. I describe these in the second post I wrote in the series.

Of course, once a single directional asymmetry has evolved in an animal or plant, then the evolution of further directional asymmetries can evolve using developmental cues provided by the first one.

But this is irrelevant to the question above, so I repeat it:

How do you know the difference between left and right?

Two questions about human history

December 26, 2021 • 9:30 am

I’m sure that historians have pondered the first question at length, but I haven’t read their lucubrations. According to Wikipedia, the first definitive use of the wheel on transportation was in Mesopotamia around 3500 B.C. We don’t know how many times it was invented independently, but probably more than once (see below):

So, my first question is this: Why was the wheel not devised in the New World? The Americas had plenty of civilizations, including many Native American groups, and the Aztecs, Incas, and Maya as well as many other groups, but none of them had the wheel, with one exception (see below). Why? Further, the Diquis culture had stone spheres beginning about 300 A.D., so they certainly knew that something round could roll. But this wasn’t adapted for carts or other rolling entities. Yet the Incas are said to have used wooden rollers to roll large stones for their walls and cities. Why no wheels, then?

According to The Straight Dope (I just looked this up), there was one exception:

The wheel evidently was familiar to the ancient Mexicans, the only known instance of its having been invented independently of the Sumerian version. Unfortunately, it apparently never occurred to anyone at the time that wheels had any practical application, and their use was confined to little clay gadgets that are thought to be either toys or cult objects.

That link also gives you an explanation that Cecil Adams considers definitive, but I don’t know. See for yourself.  I am guessing that Jared Diamond pondered this question in Guns, Germs, and Steel but I read it so long ago I can’t recall. Go to the link, read “the” answer to my question above, and see if you agree with Cecil.

My second question is this:  How did our ancestors keep their fingernails and toenails at reasonable length?

I thought of this question while clipping my nails the other day, and thought, “Scissors and nail clippers, and even steel knives were not invented in fairly late in human history. But yet our ancestors did without them for millions of years.  How did they keep their nails short?

Now you might say, “They didn’t need to: their nails wore down from hunting, gathering, and walking barefoot.” But I am not sure this is the case. How would walking barefoot wear down your toenails? And we know that, at least in modern society, if you don’t trim your fingernails and toenails, they get ungodly long (see below).  Did the ancients use flint? And what did they do before they had flint implements? Or did they bite their fingernails?

Now we could surely answer this question by observing what hunter-gatherers do, if anything, to keep their nails short. But I am not going to look it up; I’d rather have readers speculate or, if they know the answer, tell me.

Below: a video showing what happens if you don’t trim your nails: here’s a man who didn’t trim the nails on one hand for 66 years. (He explains why.) He has, on that hand, the longest known fingernails in history.

Of course he had to cut his nails on his right hand so he could do stuff (and I’m betting he’s a rightie). Nobody would marry him, and you can imagine the trouble he had just living from day to day. It’s all in the video

At the end they cut his nails:

 

Sex with a stranger? Evolutionary psychology and sex differences in behavior

June 6, 2021 • 9:15 am

In the early days of evolutionary psychology—that is, when it was just beginning to be applied to humans—I was rather critical of the endeavor, though not so much about “sociobiology”, the application of evolutionary principles to animal behavior. A lot of the early evo psych stuff on humans was weak or overly speculative.

Since then, I’ve mellowed somewhat in light of replicated research findings about human behavior that show phenomena predicted by or very consistent with the theory of evolution. Not only are the phenomena predicted and replicated, but they are in line with what other animals show. Further, researchers have also falsified some alternative explanations (“culture” or “patriarchy” is the most common one).

I’ll add here that the disturbingly common claim that evolutionary psychology is “bogus” or “worthless” as an entire field is ridiculous, both in principle and in practice. In principle, why should human behavior, or behavioral differences between the sexes, be the one area that is exempt from evolutionary influence, especially given that we evolved in small hunger-gatherer groups for at least five million years, on top of which is overlaid a thin veneer (about 20,000 years) of modern culture? That position—that all differences between men and women, say, are due to cultural influence—is an ideological and not an empirical view. If physical differences, both between sexes and among groups, are the result of evolution, why not mental ones? After all, our brain is made of cells just like our bodies!

In practice, there are several types of human behavior that, using my mental Bayes assessment, I consider likely to reflect at least some of the workings of evolution, past and present, although culture may play a role as well. There will be an upcoming paper on these fairly solid evo-psych behaviors (I’m not an author), but I’ll highlight it when it’s published.

In the meantime, we have one behavior, described in this 2017 article from Areo Magazine, that describes a “universal human behavior” involving sex differences, and a behavior that’s likely to reflect our evolutionary heritage. Although the article is four years old, it’s worth reading. The author, David P. Schmitt, has these bona fides:

David P. Schmitt, PhD, is Founding Director of the International Sexuality Description Project, a cross-cultural research collaboration involving 100s of psychologists from around the world who seek to understand how culture, personality, and gender combine to influence sexual attitudes and behaviors.

See also his Wikipedia page, which describes him as “a personality psychologist who founded the International Sexuality Description Project (ISDP). The ISDP is the largest-ever cross-cultural research study on sex and personality.”

The article, which I recommend you read, is chock-full of data. Click on the screenshot for a free read:

 

The behaviors Schmitt discusses in this longish but fascinating and readable piece are summarized in the first two paragraphs (there are lots of references should you want to check his claims):

Choosing to have sex with a total stranger is not something everyone would do. It probably takes a certain type of person. Quite a bit of evidence suggests, at least when it comes to eagerly having sex with strangers, it might also take being a man. Let’s look at the evidence.

Over the last few decades almost all research studies have found that men are much more eager for casual sex than women are (Oliver & Hyde, 1993; Petersen & Hyde, 2010). This is especially true when it comes to desires for short-term mating with many different sexual partners (Schmitt et al., 2003), and is even more true for wanting to have sex with complete and total strangers (Tappé et al., 2013).

Of course this is “common wisdom” in American culture: it is the heterosexual guy who does the pursuing, and does so without many criteria beyond the lust object having two X chromosomes, and he’s still often rejected, while women are far choosier about who they mate with.

There are many studies, described and cited by Schmitt (usually using lab experiments or good-looking students on campus approaching members of the opposite sex) that show the same thing. An attractive man propositioning a woman for sex is accepted about 0% of the time, while, in the opposite situation far more than half the males accept a sexual proposition from an attractive female stranger. Here are two studies, but there are more:

In a classic social psychological experiment from the 1980s, Clark and Hatfield (1989) put the idea of there being sex differences in consenting to sex with strangers to a real life test. They had experimental confederates approach college students across various campuses and ask “I’ve been noticing you around campus, I find you to be very attractive, would you go to bed with me tonight?” Around 75 percent of men agreed to have sex with a complete stranger, whereas no women (0 percent) agreed to sex with a complete stranger. In terms of effect size, this is one of the largest sex differences ever discovered in psychological science (Hyde, 2005).

Twenty years later, Hald and Høgh-Olesen (2010) largely replicated these findings in Denmark, with 59 percent of single men and 0 percent of single women agreeing to a stranger’s proposition, “Would you go to bed with me?” Interestingly, they also asked participants who were already in relationships, finding 18 percent of men and 4 percent of women currently in a relationship responded positively to the request.

This of course jibes with the behavior of many animals (in my flies, for example, males will court almost any female, even wooing pieces of dust or small blobs of wax), while females repeatedly reject males. It’s true of primates in general, and of many animal species. And it makes evolutionary sense. If a male mates with five females instead of one, he’s likely to have five times more offspring. In the reverse situation, though, a female who mates with five males in a short period will have roughly the same number of offspring as if she mated just once. That’s because she makes a huge investment in eggs and (in some species like ducks) maternal care, and so she should be selected to be choosy about her mates, looking for a male who is fit, healthy, may have good genes, and, if there’s parental care, will be an attentive father. Since the male has far less to lose, and far more to gain, by repeatedly mating with different females, this explains the strategy of “wanton male versus choosy female” sexual preference. These are likely to be evolved sexual behaviors.

This of course is a generalization. There are certainly picky men and women who are less choosy about their partners. But it’s a generalization that holds up not only in the “choice” studies I just mentioned, but in other aspects as well. Psychological studies show that (here I quote Schmitt, bolding is his)

. . . men have more positive attitudes towards casual sex than women, have more unrestricted sociosexuality than women, and generally relax their preferences in short-term mating contexts (whereas women increase selectivity, especially for sexual attractiveness.

. . . Cognitively and emotionally, men are more likely than women to have sexual fantasies involving short-term sex and multiple opposite-sex partners, men perceive more sexual interest from strangers than women, and men are less likely than women to regret short-term sex or “hook-ups.”

Considering sexual fantasies, men are much more likely than women to report having imagined sex with more than 1,000 partners in their lifetime (Ellis & Symons, 1990).

Behaviorally, men are more likely than women to be willing to pay for short-term sex with (male or female) prostitutes, men are more likely than women to enjoy sexual magazines and videos containing themes of short-term sex and sex with multiple partners, men are more likely than women to actually engage in extradyadic sex, men are more likely than women to be sexually unfaithful multiple times with different sexual partners, men are more likely than women to seek one-night stands, and men are quicker than women to consent to having sex after a very brief period of time (for citations, see Buss & Schmitt, 2011).

Here’s a table reproduced in the Areo paper taken from Buss and Schmitt (2011), where you can find the original references. Click to enlarge.

These patterns hold in nearly all studies in different parts of the world. That in itself suggests that culture may play an insignificant role in the difference I’m discussing.

Now if you’re thinking hard, you can think of at least four non-evolutionary explanations for these behaviors (I’ve combined disease and pregnancy in #3 below). Both, however, have been shown to be unlikely to be the major explanation for the sex difference in choosiness.

1.) Patriarchy: These could be cultural differences enforced by the patriarchy and socialization. Why a patriarchy exists itself may be evolutionary (e.g., males are stronger and thus can control females more easily than the other way around), but male dominance itself is not the explanation we’re testing here. Schmitt explains why (beyond observed cultural universalism), this is unlikely to explain the entire behavioral difference (all emphases are the author’s):

For instance, Schmitt (2015) found sex differences in the sociosexuality scale item “I can imagine myself being comfortable and enjoying ‘casual’ sex with different partners” were largest in nations with most egalitarian sex role socialization and greatest sociopolitical gender equity (i.e., least patriarchy, such as in Scandinavia). This is exactly the opposite of what we would expect if patriarchy and sex role socialization are the prime culprits behind sex differences in consenting to sex with strangers.

How can this be? Why are these sex differences larger in gender egalitarian Scandinavian nations? According to Sexual Strategies Theory (Buss & Schmitt 1993), among those who pursue a short-term sexual strategy, men are expected to seek larger numbers of partners than women (Schmitt et al., 2003). When women engage in short-term mating, they are expected to be more selective than men, particularly over genetic quality (Thornhill & Gangestad, 2008). As a result, when more egalitarian sex role socialization and greater sociopolitical gender equity “set free” or release men’s and women’s mating psychologies (which gendered freedom tends to do), the specific item “I enjoy casual sex with different partners” taps the release of men’s short-term mating psychology much more than it does women’s. Hence, sex differences on “I enjoy casual sex with different partners” are largest in the most gender egalitarian nations.

Overall, when looking across cultures, reducing patriarchy doesn’t make these and most other psychological sex differences go away, it makes them larger (Schmitt, 2015). So much for blaming patriarchy and sex role socialization.

2.) Fear of injury. In general, men are stronger than women (this is almost surely the result of evolution affecting competition for mates). Perhaps women are leary of accepting propositions from unknown men because they might get hurt, as do many prostitutes. But several studies show that safety alone cannot be the whole explanation:

Clark (1990) was among the first to address the issue of physical safety. He had college-aged confederates call up a personal friend on the phone and say “I have a good friend, whom I have known since childhood, coming to Tallahassee. Joan/John is a warm, sincere, trustworthy, and attractive person. Everybody likes Joan/John. About four months ago Joan/John’s five year relationship with her/his high school sweetheart dissolved. She/he is was quite depressed for several months, but during the least month Joan/John has been going out and having fun again. I promised Joan/John that she/he would have a good time here, because I have a friend who would readily like her/him. You two are just made for each other. Besides she/he has a reputation as being a fantastic lover. Would you be willing to go to bed with her/him?” Again, many more men (50%) than women (5%) were willing to have sex with a personally “vouched for” stranger. When asked, not one of the 95% of women who declined sex reported physical safety concerns were a reason why.

3.) Fear of pregnancy and/or disease. Since venereal diseases can be passed in both directions, I’m not sure that disease is a good explanation, though perhaps women are more likely to get serious disease than are men. As far as pregnancy is concerned, there’s at least one study showing it can’t be the sole factor:

Surbey and Conohan (2000) wondered whether worries of safety, pregnancy, stigma, or disease were what was holding women back from saying yes to sex with a stranger. In a “safe sex” experimental condition, they asked people “If the opportunity presented itself to have sexual intercourse with an anonymous member of the opposite sex who was as physically attractive as yourself but no more so (and who you overheard a friend describe as being a well-liked and trusted individual who would never hurt a fly), do you think that if there was no chance of forming a more durable relationship, and no risk of pregnancy, discovery, or disease, that you would do so?” On a scale of 1 (certainly not) to 4 (certainly would), very large sex differences still persisted with women (about 2.1) being much less likely to agree with a “safe sex” experience with a stranger compared to men (about 2.9).

So, sex differences in agreeing to sex with strangers are not just a matter of safety issues, pregnancy concerns, slut-shaming stigma, or disease avoidance. Controlling for all of that, researchers still find large sex differences in willingness to have sex with a stranger.

There’s a lot more in this paper, including Schmitt’s critique of the two papers cited widely as disproving the “pickiness” hypothesis. Both papers, however, suffer from extreme methodological flaws, and in both cases the results support the “pickiness” hypothesis when the flaws are corrected.

You can read the hypothesis and judge for yourselves, but I think this is one of the best examples we have of evolutionary psychology explaining a difference between men and women in behavior*. As I said, it’s shown up throughout the world in different cultures, it’s paralleled in many species of animals, alternative explanations fail to explain the data, other, unrelated data support at least a partial evolutionary basis of the choice difference, and the few papers that claim to disprove it wind up actually supporting it.

Aside from “universal” behavior like sleeping, eating, or wanting to reproduce, which are surely instilled in us by evolution (and nobody questions those), we shouldn’t ignore differences between groups, especially the sexes, as having an evolutionary origin. It’s likely that morphological differences between geographic populations, like the amount of melanin in the skin, are adaptive responses to natural selection, so why is behavior the one trait that is always off limits to evolutionary explanation?  It’s ideology, Jake.

h/t: Steve Stewart-Williams

 

*As a reader points out below, and even more obvious evolutionary difference is that the vast majority of men are sexually attracted to women, and vice versa. That would be hard to explain as a result of the patriarchy or of socialization.

Why do backs itch so much?

January 31, 2021 • 12:00 pm

A while back, someone gave me a Chinese backscratcher: a piece of bamboo with a hand carved at the end with slightly separated fingers.  I use it every couple of days when my back works up a good itch, and believe me, it provides substantial relief!  Here’s what mine looks like:

But when I use it I’ve pondered two questions.

A.) Is the relief you get when scratching your back pleasure, or simply the removal of discomfort? (Or are they equivalent?) This same question applies when you finally make it to the restroom after having to hold your bowels or bladder for a long time. I wonder if philosophers have debated this question.

B.) Why do backs itch so much? I have two theories here, which are mine. The first is that they itch no more than do fronts (i.e., your chest), but we’re unconsciously scratching our fronts all the time, while we can’t reach our backs without a special implement. But I don’t notice myself scratching my chest.

The other is that dirt and oils accumulate on your back more than they do on your front, simply because you can’t reach your back so easily in the bath or shower. I don’t have a sponge on a stick or anything like that, and so am forced to wash my back by reaching around with a piece of soap. I’m never sure that does a great job because it’s hard to reach all the places.  If any accumulated back schmutz makes you itch, this could be an explanation.

Maybe there are other theories as well, but these are the only two I’ve thought of.

Why your eyes can rotate: an evolutionary explanation

September 26, 2020 • 2:30 pm

Reader Bryan sent me a link to this video by Steve Mould discussing how our ability to rotate our eyes may be an exaptation (and a vestigial remnant) from ancestors who had eyes on the sides of their head. It’s a fascinating idea, and may well be true. Or, to quote the YouTube notes:

Torsional eye movement, known as cycloversion and cyclovergence have a fascinating evolutionary history via our evolutionary ancestors that had eyes on the sides of their heads. Your eyes twist in this way whenever you tilt your head!

After 10:07 there are ads, so you may wish to skip the last two minutes of the video.

Did humans occupy the New World over 30,000 years ago? New paper suggests it.

July 23, 2020 • 9:45 am

This new paper in Nature (click on screenshot, pdf here, reference at bottom) has the potential to be the big human-paleobiology story of the last several years.  It reports finding human occupancy of a high-altitude cave in Mexico during the last glacial maximum (LGM): about 26,000 years ago.  And that, say the authors, implies that humans have been in the New World since more than 30,000 years ago—more than doubling the time we thought they’d been here. Previously, the best guess was that humans crossed the Bering Strait from Siberia about 15,000 years ago, and then spread through the Americas.

Click on the screenshot below to get the paper (free through the legal app Unpaywall, or you can make a judicious inquiry).

Before we accept these results as overturning the received wisdom about humans in the New World, though, there has been some criticism of the paper, as you can see in a precis in Science by Andrew Curry.

The cave where the finds were made sits atoop a remote mountain in the Mexican state of Zacatecas, about 2,740 m high, and has been studied since 2012.  Although dry and barren now, it was thought to be verdant during the LGM, with water, plants, and plenty of edible animals nearby.  Researchers worked there for a month at a time, camping in the cave and hauling water and food by donkey from the nearest town.

What made the researchers suppose that the cave was occupied by humans were several things, most prominently 2000 specimens of what looked like sculpted tools. Here’s a figure showing some of these putatively manufactured objects:

(from paper): a, Core. b–e, Flakes; inlay in b emphasizes an isolated platform. f–j, Blades. k–o, Points. Scale bar, 3 cm. Most items are from component SC-B; d and m belong to SC-C. One Pseudotsuga sp. (Douglas fir) charcoal fragment closely associated with the bifacial preform shown in m in stratum 1223 was dated to 27,929 ± 82 uncalibrated radiocarbon years BP (PRI-5414). More lithic finds are shown in Extended Data Figs. 5, 6.

 

Now I would have thought that by now paleoanthropologists would be able to distinguish non-human rock artifacts from real, chipped tools, but apparently that’s not the case. As one critic says in the Science writeup:

Critics point out that the tools are simple and don’t resemble other toolkits from the Americas, raising the possibility they’re the product of natural breakage. “They look like they could be artifacts, but why aren’t they found anywhere else in the landscape?” wonders David Meltzer, an archaeologist at Southern Methodist University. The tools’ consistency is also remarkable, he says. “If these tools are real, why are they only found—so far at least—in this one spot over a 10,000-year period? Humans adapt and adopt new technology.”

The tool-making conclusion, at least, must remain tentative. There was also burned wood that was radiocarbon dated, implying human campfires, but the critics again say that this could derive be from “wind-blown” wildfires. The researchers also used OSL dating of quartz from the sediments, which tells you when the mineral was last exposed to light, ergo when it was laid down.

Finally, the researchers trawled the cave for DNA, which they could sequence to see what kind of animals and plants were there. The fauna included bats, mice and other rodents, marmots, goats, and sheep, as well as birds, though this could have come from more recent occupancy. Plant DNA included forest species like spruce, pines, grasses, and palms. The disappearance of cold-adapted species and forest trees that gave way later to Joshua trees and grasses suggests that the sediments in the cave did go through the late Glacial Maximum, which was followed by a period of dryness.

Notably, no human or humanoid DNA was found in the cave, which would have gotten people much more excited about this find.

THE UPSHOT

How strong is the evidence for human presence in the Americas beginning 30,000 years ago? The 30,000 years is a guess by the authors, derived from guessing how long it would take humans to get to a 26,000-year-old cave residence in Mexico after crossing from Siberia. In terms of the age of the cave itself, that seems reasonable, but the evidence for human occupation is largely the “tools”, and their provenance is doubtful. And if humans inhabited the cave continuously for millennia, as the authors suppose, then why wasn’t human DNA found there? My judgment, and I’m a tyro here, is that the evidence is intriguing but not terribly strong. A lot hinges on whether the “tool-like” stone artifacts really were chipped by hominin hands.

On the other hand, the Science article says that there is a cave in the Yukon that’s yielded dates as old as the Mexican cave (about 24,000 years), but although it contains thousands of animal bones, there are “few stone tools or cut marks.” But other researchers are beginning to think that people came to America earlier than we thought, and could have spread quickly by traveling along the West coast by boat, avoiding the largely frozen interior. Here’s a tweet (h/t: Matthew) showing sites where there could have been earlier habitations:

How good are the dating methods?  From what I read, they seem fairly reasonable, and they used at least two methods that give about the same dates. The question is not how old the cave is, but whether humans lived there and made the tools and charcoal.

What happened to the people? Part of the reason we think humans have been in the New World for only 15,000 years is not just evidence from habitation, but from DNA of Native Americans (note: there are some older estimates). If that’s the case, why doesn’t the DNA give a consistent age of 30,000 years from when Native Americans branches off from East Asians? One possibility is that the early arrivers went extinct without leaving descendants, so we wouldn’t find a genetic signature of their existence. Given that some paleoanthropologists see evidence of an early arrival from other sites, like that in the Yukon, the possibility of extinction seems unlikely.

All in all, this is an exciting finding, and may well be right, but we’ll have to let the experts fight it out.

Excavating in the cave, a photo from the Science precis:

(from precis): Researchers dug nearly 3 meters deep in Chiquihuite Cave and found almost 2000 stone tools. DEVLIN A. GANDY

h/t: Matthew Cobb

____________

Ardelean, C.F., Becerra-Valdivia, L., Pedersen, M.W. et al. 2020. Evidence of human occupation in Mexico around the Last Glacial Maximum. Nature (2020). https://doi.org/10.1038/s41586-020-2509-0

Note and correction on the “Lewontin fallacy”

April 23, 2020 • 1:00 pm

Yesterday I wrote about Angela Saini’s misguided claim that human populations and races (I prefer “ethnic groups” rather than “races”) are basically genetically identical. So identical, in fact, that, as Saini argued, it’s entirely possible (or even likely) that the genomes of a South Asian and a white Canadian could be more similar than the genomes of two South Asians. That is wrong, but plays into Saini’s ideological bias that there are no appreciable or meaningful difference between biological races.

As I indicated, we now have sufficient data to show that the chances that her assertion is true is close to zero. Looking at the whole genome, you’re not going to find many South Indians whose DNA is more similar to that of a white Canadian than to that of another South Asian.

In trying to understand why Saini would make such a statement, I speculated that she had bought into the “Lewontin fallacy“: the claim by my ex Ph.D. advisor that the vast bulk of genetic variation segregating in our species occurs among individuals within populations, rather than among populations within a classically-defined “race” or among races.

From his mathematical analysis, Lewontin concluded that the term “race” has no biological reality. The error of Lewontin’s claim was pointed out by geneticist A.W.F. Edwards, who noted that Lewontin was treating each gene as independent. But they’re not, because the constraints of history, geographic separation, and evolution ensures that differences among populations and races at different genes are correlated. Taking these correlations into account, Edwards concluded this (characterized in Wikipedia):

In Edwards’ words, “most of the information that distinguishes populations is hidden in the correlation structure of the data.” These relationships can be extracted using commonly used ordination and cluster analysis techniques. Edwards argued that, even if the probability of misclassifying an individual based on the frequency of alleles at a single locus is as high as 30 percent (as Lewontin reported in 1972), the misclassification probability becomes close to zero if enough loci are studied.

And the use of cluster analysis is in fact the way that population-genetic studies are able to describe evolutionary history and ancestry from DNA data. I cited cluster analysis of the genetic structure of the British Isles as an example of how well one can deduce someone’s ancestry and geographic origin from looking at half a million base pairs—a small fraction of the total DNA in the human genome (about 0.02%).

I stand by my claim that Saini was wrong, but I did err on one count, one that doesn’t affect my conclusions but that I wanted to point out to be scientifically accurate.

And that is this: Lewontin’s original claim about the apportionment of genetic variation among individuals, populations, and races was incorrect. This was pointed out to me by reader, biologist, and polymath Lou Jost, who works at a field station in Ecuador. I vaguely remembered that Lou had done some work on this, but had forgotten, as he just reminded me, that work completely invalidates Lewontin’s method.

Lou has written several papers on this error, one of which you can access for free. (I have the other papers if you want the pdfs.) Click on the screenshot:

The math behind Lou’s arguments is above the pay grade of many of us (including me), but I, at least, am convinced that Lewontin was wrong for reasons beyond Edwards’s claim: he was wrong because, as Lou showed, he “used a measure of ‘differentiation’ that doesn’t really measure differentiation.” Lou presented an alternative diversity-based model that will allow you to compare differentiation within and among groups (this holds for species diversity in ecology as well as genetic diversity in and among populations), but he didn’t apply it to Lewontin’s data, because those data are outmoded now (they were based on electrophoretically derived allele frequencies).

The take-home lesson is that Lewontin’s conclusion is wrong not only because it applies to single loci assumed to be uncorrelated, but also because he used the wrong metric to compare within- versus between-group diversity. As Lou noted, the take-home lesson of Lewontin’s paper—that most of the genetic diversity in our species is present in any population of our species, with only smaller amounts added by looking at different populations or races—may still be right. But until Lou’s metrics are applied to the new and better data we have, we just won’t know.

Again, this correction affects an idea that I thought Saini might have been erroneously pondering when she made her misleading statement. It does not affect the fact that her statement is misleading, and that we really can distinguish populations and ethnic groups very well using genetic data—the more data the better. And it doesn’t affect my claim that Saini is either deliberately misleading people or is ignorant about the data on population differentiation in our species, and that her ignorance, willful or not, plays into her ideological narrative about “races.”

I stand corrected on the Lewontin issue, and thank to Lou Jost for setting me straight about the “Lewontin fallacy.”