Human Evolution: The scientists studying human origins are engaged in lively debate
Science is by its nature a feisty enterprise, with researchers evaluating each other's work -- not always kindly -- and constantly seeking better ways to test ideas against the hard reality of data. When a scientific field is active, it is alive with controversy, with the slam-around give-and-take of aggressive inquiring minds. This is nowhere more true than in the field of paleoanthropology, the study of human evolution. On 24 June, two paleontologists e mailed a scathing eight-page critique to dozens of their colleagues around the world complaining of how two other researchers had evaluated their work on Neanderthal evolution. This latest eruption of disagreement is best understood in the context of three related questions about human evolution, all of them controversial.
1. Is the human evolutionary tree a bush? When I first taught human evolution to Washington University freshmen over 25 years ago, the story seemed very simple: The first species of human, Homo habilis, evolved in Africa a few million years ago, only to be replaced by a second bigger-brained species, homo erectus. Homo erectus spread out of Africa to Asia and the rest of the world, only to be replaced in turn by our species, Homo sapiens.
In the intervening 25 years, the picture has proven to be considerably more complex than habilis to erectus to sapiens. Researchers now recognize (with considerable bickering about the details) five distinct species of human. The stem stock is H. ergaster, which appeared in Africa about 1.9 million years ago. This first human was the descendant of australopithecine ancestors which would have seemed to you, had you met them, to be upright-walking chimpanzees. Homo erectus evolved from H. ergaster in Africa about a million years ago, and later migrated out, reaching Asia but perhaps not Europe. A third species evolved some 600,000 years ago in Africa, H. heidelbergensis. This species also moved out of Africa, taking with it sophisticated tool-making technology. From it arose, perhaps half a million years ago, and again in Africa, H. neanderthalansis ("Neanderthal man"). Neanderthals arrived in Europe from Africa about 130,000 years ago. At about this time, and again in Africa, the fifth and last species arose, H. sapiens, us. Outside of Africa and the Middle East, there are no clearly-dated fossils of Homo sapiens older than 30,000 years, suggesting the last human migration out of Africa was very recent.
While the details of this story are still quite controversial among paleontologists, the emerging consensus is that there have been at least five species of humans, all of them evolving in Africa and four of them migrating out in successive waves to Europe and Africa. The human evolutionary tree is a bush, rooted in Africa.
2. Did our species evolve only in Africa? Some paleontologists do not accept the last stage in this story. They reject the Out-of-Africa theory that Homo sapiens evolved in Africa then migrated out, replacing earlier human species as it spread out over the face of the globe. Instead, they put forth a multi-regional proposal, sometimes called the Regional Continuity Theory. This theory proposes a single very early migration out of Africa to Europe and Asia, followed by two million years of evolution in place. African, European, and Asian lineages evolved separately, adapting to local conditions, yet all were linked evolutionarily by occasional migration. While distinct human races developed, the exchange of genes promoted by occasional migrants prevented major genetic differences from persisting, so no race was able to become a separate species. Eventually this evolving stream of populations became what we now recognize as Homo sapiens.
Who is right? One obvious way to sort out this problem without resorting to statistical shenanigans is to find enough genetic variation to make clear comparisons, and then ask if humans in Africa, Europe, and Asia have the same amount of it. Species accumulate genetic mutations over time, and if multi-regionalism is right, then humans in the three regions should have about the same amount. On the other hand, if our species originated only in Africa, and migrated out to Europe and Asia later, then you would expect that many more genetic mutations would have accumulated in Africa, for our species would have been there much longer.
This presents a significant problem in studying human evolution, as human species have evolved within the last two million years, too short a time to accumulate many gene differences (humans and chimps differ by less than 2% of their DNA).
The solution is to compare genes that evolve a lot faster than average ones. On the human chromosomes, certain genes evolve very fast indeed. A clear test of the multiregional proposal is possible using such highly variable portions of chromosomal DNA, as they provide a large number of "markers" to compare.
In 1996 a large team of scientists from six countries set out to settle the matter once and for all. They carried out a comprehensive study of DNA from human populations all over the world, analyzing two highly variable segments on chromosome 12. Sixteen hundred individuals were examined in 42 populations.
A total of 24 different versions of the two segments were found, each a different but related gene sequence. Fully 21 of them were present in African populations, while 3 were found in Europeans, and only 2 in Asians and in Americans.
Since DNA accumulates mutations over time, the oldest populations of humans should show the greatest number of genetic differences. This result thus argues forcefully that human chromosome 12 has existed in Africans far longer than among non-African humans. Said differently, the European and Asian humans haven't been around nearly as long as African ones. It is difficult to avoid the conclusion that the Out-of-Africa theory was right all along.
While multi-regionalists still argue the point ferociously, the tentative consensus among evolutionary biologists is that our species evolved in Africa recently, and spread from there to the rest of the world, eventually replacing earlier human species in Europe and Asia.
3. Are Neanderthals our cousins, or a separate species? DNA analysis of Neanderthal bones over 30,000 years old reveal that Neanderthals have quite different DNA than Homo sapiens, supporting the view that Neanderthals were a separate species. Their very distinct DNA also indicates that Neanderthals did not interbreed with the modern humans that began to supplant them about 50,000 years ago. Now comes the vituperous e mail mentioned at the outset of this article. Researchers studying a 25,000 year old Neanderthal jaw bone from Portugal reported this June that it contained a mix of modern human and Neanderthal traits, and concluded that humans and Neanderthals must have hybridized after all. Other researchers in a companion report criticized this conclusion as subjective and based on flimsy evidence, and the fray was joined. Who is right? While there is as yet no consensus, my money is on the DNA analysis, suggesting that Neanderthals were a separate dead-end species of human rather than our direct ancestors.
This fuzzy picture, with conflicting ideas sticking out akwardly here and there, is the sort that excites scientists. Science at work is not pretty -- but it sure is fun.