I’ve written quite a bit about the Homo floresiensis controversy: these are the small (1-meter tall) individuals whose remains were found in a cave on the Indonesian island of Flores, originally dated about 18,000 years old. They are remarkable because of their size, their small brains (about the size of a chimp’s) and their remarkably recent age — a time when the much taller and brainier Homo sapiens had already infested the world. But there was doubt about whether these diminuitive fossils represented a real species: some people thought that the one relatively complete skull represented a diseased or microcephalic individual.
Two papers in the latest issue of Nature (links below) address this controversy, and both come down on the side of H. floresiensis being a real species, not an aberration. (There’s also a very nice two-page summary by Daniel Lieberman if you don’t want to plow through the articles.) More bones have been found in the Liang Bua cave on Flores (see below), most notably a pretty complete foot. The foot, like the rest of the skeleton, shows an intriguing mixture of primitive and derived traits. There are now enough bones to pretty much rule out the “aberrant individual” theory.
So what was H. floresiensis? As the diagram below shows, it could have been a descendant of an earlier hominin species, H. habilis, or perhaps a very early form of H. erectus; in both cases it would have had to reach the island several million years ago. Yet the brains of these two ancestors were larger than that of the hobbit, so how did it get such a small titer of gray matter? (Hobbit brains were about 400 cc in volume, the size of a modern chimp or of the smallest australopithecines.) Here the notable dwarfism of many species on islands (including elephants on Flores) comes in: like other mammal species, H. floresiensis could simply be an evolutionarily dwarfed form of an earlier hominin. Weston and Lister’s paper gives relevant data taken, oddly enough, from dwarf hippos on Madagascar , but I’ll leave you to follow their reasoning in their article.
This is perhaps the most bizarre and interesting twist in hominin evolution yet, and it’s completely unsettled. A whole new group of questions open up. How did this weird species coexist with the much larger and brainier H. sapiens for so long? (H. floresiensis was not dumb, by the way: they managed to colonize the island over water and made fairly sophisticated tools). If hobbits are a remnant of the H. habilis lineage, how come we don’t find habilis fossils elsewhere outside of Africa?
Ling Bua cave on Flores: where the hobbits were found. Photograph by C. Turney, University of Wollongong
Homo floresiensis might be most closely related to early H. erectus, but also shows potential affinities with H. habilis. In either case, recognizing H. floresiensis as a species will require us to re-examine how we define species of the genus Homo and how they were related to each other. Reasonably well-known relationships are indicated by solid arrows; less secure relationships are indicated by dotted arrows. Broken vertical bars indicate uncertainties about when species evolved or went extinct. (Figure and caption from Lieberman’s summary.)
The hobbit continues to be a mystery: perhaps the deepest mystery about human evolution. Today’s New York Times has a longish and interesting report on the status of the “hobbit,” Homo floresiensis, that I’ve posted about several times. This is a diminutive (3-foot-tall) human skeleton found on the island of Flores, in Indonesia, that has a brain case not much larger than those of modern chimps. It is, however, modern in time, going back only about 18,000 years ago (see chapter 8 of WEIT). Based on its wrist bones and other skeletal characteristics, scientists are now beginning to think it was not an aberrant or diseased individual, but a representative of a distinct species, perhaps an earlier species of hominin that became isolated on Flores hundreds of thousands or even millions of years ago:
Scientists who reviewed hobbit research at a symposium here last week said that a consensus had emerged among experts in support of the initial interpretation that H. floresiensis is a distinct hominid species much more primitive than H. sapiens. On display for the first time at the meeting was a cast of the skull and bones of a H. floresiensis, probably an adult female. . . . .
. . . Some prominent paleoanthropologists are reserving judgment, among them Richard Leakey, the noted hominid fossil hunter who is chairman of the Turkana Basin Institute at Stony Brook University. Like other undecided scientists, he cited the need to find more skeletons at other sites, especially a few more skulls. Mr. Leakey conceded, however, that the recent research “greatly strengthened the possibility” that the Flores specimens represented a new species.
It’s possible that hobbits represented a much earlier migration out of Africa than previously thought, earlier than the migration that gave rise to the widespread Homo erectus. It could even represent a migration of the very early australopithecines! As John Noble Wilford, the writer, says, lots of puzzles remain:
Indeed, the more scientists study the specimens and their implications, the more they are drawn to heretical speculation.
¶Were these primitive survivors of even earlier hominid migrations out of Africa, before Homo erectus migrated about 1.8 million years ago? Could some of the earliest African toolmakers, around 2.5 million years ago, have made their way across Asia?
¶Did some of these migrants evolve into new species in Asia, which moved back to Africa? Two-way traffic is not unheard of in other mammals.
¶Or could the hobbits be an example of reverse evolution? That would seem even more bizarre; there are no known cases in primate evolution of a wholesale reversion to some ancestor in its lineage.
Stay tuned; I’ll provide further information on this strange branch of our family tree as more research is published. Be sure to listen to the 20-minute podcast on the Times website. Meanwhile, courtesy of the NYT, here is the hobbit’s tiny foot (notice that the ruler is 5 cm long: about 2 in., which makes the foot about 6 inches long).
Also in today’s New York Times is a piece by John Noble Wilford about the display of a skeleton of “the hobbit,” (Homo floresiensis) at Stony Brook this week. It includes the head and much of the body (see below). If you’ve read my book or followed this website (see here), you know that whether H. floresiensis is a real species rather than an aberrant individual is a subject of real controversy, though the latest evidence suggests it was indeed a real species. This individual, an adult, was only three feet tall. Imagine how small that is — just put a yardstick up against your leg, with its end on the floor, to see.
Here is what’s on display (it’s actually a cast, not the real skeleton). Look at that tiny braincase! If you’re anywhere near Stony Brook, go have a look.
Some human populations in South America, Africa, and Southeast Asia are of extraordinarily small stature: they are called pygmies. All of them inhabit rainforests that are warm and humid. Although they bear a common name, genetic work shows that each group has evolved independently, so it is better to speak of “the pygmy phenotype” (“phenotype” refers to any aspect of an organism that can be observed or measured). A population is said to show the pygmy phenotype when the height of adult males averages about 160 cm (5′ 3″) or smaller. (The smallest pygmy population comprises the Efe hunter-gatherers of the Congo, where adult males and females are 143 and 136 cm tall respectively [4′ 8″ and 4′ 6″]).
A new paper in Trends in Ecology and Evolution summarizes what is known about the distribution, genetics, and evolutionary basis of the pygmy stature. The first thing we learn is that we’re not absolutely sure if the height difference is due to genetic differences between pygmies and populations of “normal” stature. Could it instead be due to differences in nourishment alone? The evidence is against this because pygmy populations show no clinical signs of malnutrition. Still, other nongenetic environmental factors could be responsible. The best way to show that the height difference is based on genes is, of course, to rear pygmy and non-pygmy infants in a common environment and show that the height difference remains; surprisingly, this has not been done (though surely there are some pygmy children brought up in different environments— the authors don’t discuss this).
Why are pygmies so short? If the this phenotype is indeed genetically based, the obvious hypothesis is that natural selection in warm tropical environments causes humans to evolve smaller size. But why? The authors give four hypotheses:
1. There is a scarcity of food in the rainforest, and this selects for smaller individuals who are able to maintain their bodies with fewer calories. The evidence for this hypothesis is mixed: pygmy populations don’t especially suffer a dearth of calories, although this may be due to their recent trading for food with other populations outside the rainforest.
2. Living in a hot, humid environment selects for smaller bodies because smaller individuals have a higher ratio of body surface are to body volume. This allows them to lose, through sweating and heat transfer, relatively more heat than larger individuals. The weakness of this idea is that among pygmy populations there is no correlation between body size and ambient humidity.
3. It is easier to move through dense, tangled forest if you are small. Bending down repeatedly while walking apparently uses quite a bit of energy. There is only anecdotal evidence for this idea, but it may be true.
4. If there is high mortality, then it may pay you to mature and reproduce early because otherwise you could die and leave no genes. The authors note that infant mortality up to age 5 of African rainforest poulations are 27-40%, about twice that of nearby populations that live in other habitats. This idea predicts that among populations, there will be a positive correlation between life expectancy and average adult height. This is indeed observed, providing some support for the idea.
Of course, all or some of these factors could work together. At present, we have some fruitful ideas about why the pygmy phenotype evolved, but nothing definitive yet. It is surprising that of the many differences in appearance between human ethnic groups– differences that involve skin color, hair configuration, facial configuration, height, body configuration, and physiology –the only one whose evolution we understand fairly well is pigmentation (see WEIT for the explanation, based on sun exposure). The rest is mystery. In my book I broach the idea that sexual selection may account for some of these, though it’s hard to explain the short pygmy phenotype this way (why would the target of sexual selection be correlated with humid, forested habitats?).
Note: Another population that was abnormally short were the “hobbits”: Homo floresiensis, a population (described as a species) of humans that lived on the Indonesian island of Flores. Again described in WEIT, this species is based on a single skull from one individual and an arm bone from another. Their apparent height was about 106 cm (3′ 6″), they weighed about 50 pounds, and were the size of a modern 5-year-old child. They lived about 18,000 years ago, when H. sapiens of modern stature already lived throughout the world. Although some think that the single tiny individual was really diseased and not a “normal” individual, it does appear from the arm bone that they really were this small. But they had one feature not present in modern pygmy populations: very small brains, about half the size of modern human brains (the brains of pygmies scale roughly the same as short humans elsewhere). H. floresiensis did not represent the pygmy phenotype, and were more likely an ancient population of a different species of Homo that, isolated on its island, was bypassed by the evolution of other populations into the modern human phenotype.
Here is a figure from Perry and Dominy’s paper showing the distribution of pygmy populations throughout the world (red dots) and some pictures of pygmy individuals:
Figure 1. Association of the human pygmy phenotype with tropical rainforest habitats. (a) Approximate locations of small-bodied hunter-gatherer populations discussed in this article, with mean adult male stature estimates , , ,  and . The smallest modern human statures (mean adult male height < 155 cm) are always associated with tropical rainforests (red circles). Some hunter-gatherer populations occupying savanna-woodlands (black circles) are also relatively small, such as the Hiwi of the Venezuelan llanos, the Hadza of Tanzania and the !Kung San of Botswana and Namibia. Precipitation data are from the Tropical Rainfall Measuring Mission (Goddard Space Flight Center, National Aeronautics and Space Administration; http://trmm.gsfc.nasa.gov). (b) Yanomamö male, Venezuela (photograph by Raymond Hames, with permission). (c) Efe male, Democratic Republic of Congo (photograph by William Wheeler, with permission from the National Anthropological Archives, Smithsonian Institution). (d) Batek male, Malaysia, with white-handed gibbon (Hylobates lar) hunted by blowdart (photograph by Kirk Endicott, with permission).
Photo and caption from Perry, G. H., and N. J. Dominy. 2009. Evolution of the human pygmy phenotype. Trends in Ecology & Evolution 24:218-225.
Research hot off the lab bench: My friend Graham Coop at The University of California at Davis sends me this note that they may be zeroing in on the genes responsible for the pygmy phenotype:
We’ve just published a large scan for selection across various human populations: two of the of the top hundred SNPs [single nucleotide polymorphisms] whose allele frequency most differentiate Bantu populations from pygmy populations fall next to genes in the insulin growth factor signaling system (discussed on the bottom of page 6). Obviously these will require much more followup, but these seem like reasonable candidates for genomic regions habouring variation affecting height in pygmies.
As recounted in WEIT, one of the most remarkable hominin fossils is that of Homo floresiensis, discovered on the island of Flores in Indonesia in 2003. This creature was remarkable in that although it lived only 18,000 years ago, when modern H. sapiens had already evolved, it was only a meter tall, weighed 50 pounds, and had a brain of less than 500 cc.–similar in size to of our distant cousin Australopithecus afarensis (“Lucy”). It seemed that some relict populations of Homo had survived on this Indonesian island, bypassed by modern humans.
Ever since H. floresiensis (dubbed “The Hobbit”) was found, it has been the center of heated controversy. Some have said that rather than being a long-surviving ancient hominin, for example, the one good specimen found is simply that of a modern human afflicted with a growth disease (such as goiterious cretinism) that produced a small skull. Others counter-claim that the wrist bones of the hobbit are clearly not that of a modern human, but of an earlier relative.
Now another criticism has surfaced–the claim that the hobbit’s teeth show dental work! In particular, an anthropologist at the University of Adelaide in Australia, Maciej Henneberg, claims that a lower molar of H. floresiensis shows a filling (and possibly a root canal) of the type performed in Indonesia in the 1930s. (See the articles about this claim here and here.) Could the hobbit be another Piltdown Man, a fraud foisted on a credulous scientific community?
Well, probably not. In a careful analysis of the dentition of H. floresiensis and a comparison with other ancient skulls, Peter Brown, one of the hobbit’s discoverers, debunks Henneberg’s claims. X rays and careful analysis (see the pictures on Brown’s page) show absolutely no evidence of dental work. Thus this claim, at least, has been debunked.
It is starting to look as if H. floresiensis really was a genuine species, but an anomalous one: a small population of tiny humans who hunted dwarf elephants with miniature spears. There will undoubtedly be more argument before this is settled.