he ancestors of Africans and non-Africans split apart almost 200,000 years ago, much earlier than previously thought, a study of DNA in living populations has found.The estimate is so early that it precedes by some 70,000 years the first known fossils of modern humans.
The authors of the study say they cannot tell in which branch of the split population the genes that shape modern humans first developed, but they believe the genes spread quickly throughout the other branch because of the advantage they conferred.
The study, published in Proceedings of the National Academy of Sciences, also reports the first fixed DNA difference between African and non-African populations. The difference, which may be rendered less absolute when more people are tested, is in a single chemical unit of DNA that has no functional significance. Several geneticists said the finding did not alter the well-established fact that different racial groups differ very little at the genetic level.
The new finding, if sustained, is surprising in the light of two widely held assumptions about human origins. One is the view that modern humans evolved in sub-Saharan Africa and, in a later split, a small band then emigrated to populate the rest of the world.
The other is the expectation that only a handful of minor genes underlying superficial characteristics like hair and skin color would show sharp differences between major population groups. The gene under study is part of an enzyme, known as pyruvate dehydrogenase, which controls an important step in glucose metabolism.
The report, by Dr. Eugene E. Harris and Dr. Jody Hey of Rutgers University in New Jersey, suggests that the archaic human population split first, probably in Africa, and that the modernity typical of people today developed later.
Dr. Hey said that he and Dr. Harris selected the pyruvate gene purely for reasons of convenience: it lies on the X chromosome, and by studying the gene in men, who have only one copy of the X chromosome, they had to determine the sequence of DNA units only once in each individual.
They sequenced a 4,200-unit-long segment of DNA from the pyruvate gene in 16 Africans, 19 non-Africans and 2 male chimpanzees. The forebears of chimps and of humans are believed to have split into separate species some five million years ago, which yielded a time scale for measuring the rate of mutation in the DNA segment. Though the number of DNA samples was small, the researchers said their statistical analysis produced a meaningful result.
The Rutgers biologists noted 25 positions in the DNA segment at which their human subjects tended to have different chemical letters from the chimp sequence. The differences are caused by successive changes or mutations in units of DNA, allowing a family tree of the changes to be constructed.
Assuming the mutations occur at regular intervals, like a steadily ticking molecular clock, the Rutgers biologists concluded that the ancestral hominid sequence is 1.86 million years old. This is about the time of an archaic human species known as Homo habilis, and confirms the idea that much of the genetic variation in living populations is very old.
Much later, at about 189,000 years ago, the researchers said, the gene tree split into African and non-African branches, a point marked by a new mutation found only in the non-African subjects. At a place in the gene where Africans have the chemical sequence GCG, non-Africans have GCA, the letters standing for different units in DNA. In the genetic code, both GCG and GCA specify the same amino acid unit of a protein, so the difference has no practical effect.
However, the date it implies for an African/non-African split is substantially earlier than estimates so far derived from other parts of the human genome, which range from 100,000 to 156,000 years ago.
Dr. Hey said these estimates did not take into account, as his does, the subsequent flow of genes between the separate branches, an omission which lowers the apparent splitting date.
The Rutgers biologists cannot say from their study where the split took place, though presumably it was somewhere in Africa.
But from the amount of variability seen in their DNA sequences, they can estimate the effective size of the ancestral human population, which they put at a mere 18,000 people. Estimates from other genes range from 11,000 to 18,000, numbers thought not to have been greatly exceeded until a population expansion began some 50,000 years ago.
Dr. Hey said that 18,000 people, even hunter-gatherers, did not take up a lot of room, and that the African/non-African population split could still have left the two groups close to each other, even if geographically separated.
When the gene changes that led to modernity evolved, they would have reached and quickly spread through the other because of the powerful selective advantage they conferred.
Dr. Hey said his genetic data gave no clue on whether the genes for modernity first evolved in the African or non-African sub-population. Given that the transitional fossils and many of the earliest modern fossils appear in Africa, the African population seemed to him the likelier candidate, he said.
Defenders of the established view that modern humans arose from archaic hominids in sub-Saharan Africa may wait for confirming evidence before changing their minds. In a commentary on the new report, Dr. Rosalind M. Harding, a geneticist at the John Radcliffe Hospital in Oxford, writes that true dates could be twice or half the estimates derived at present from genetic data, such is the range of uncertainty in genetic dating methods.
Population genetics is a rigorous science but is statistical in nature and depends on several general assumptions that may not always perfectly mirror actual events. Those who study human origins from the fossil evidence believe their dates, usually based on radiocarbon, are more firmly grounded than the geneticists' molecular clocks. Richard Klein, an archeologist at Stanford University, said he gave even odds on whether the African/non-African split occurred before human modernity, as the new study says, or after.
"In the end the genetics will have to accommodate the fossil and archeological records, not the reverse," he said.
Dr. Ofer Bar-Yosef, an archeologist at Harvard University, said the archeological record supported the geneticists' calculation that the early human population was very small. But the archeological evidence is silent, he said, on whether there was a population split 200,000 years ago.
But Dr. Henry C. Harpending, a geneticist at the University of Utah, said he too had seen signs, from study of a different genetic region, of splits in the pre-modern human population. "I like the idea," he said.
The second surprise in the Rutgers report relates to the well-established finding that humans as a species show very little genetic difference between their various subpopulations. All people have exactly the same set of genes, as far as is known, but the genes come in slightly different flavors, embodied by minor variations in the DNA sequence.
Overall, there is much more variation among people within a subpopulation than between populations.
The gene fragment studied by Dr. Hey and Dr. Harris is unusual in that the versions of the gene found in Africans and non-Africans are quite different. Dr. Hey said he viewed the gene as an anomaly and that "our data cannot be interpreted as supporting the idea of very distinct races or populations."
"Human biologists have always known that would be some genes out there that would show this pattern, such as those for hair color, so finding this gene doesn't change the overall story that most genes do not show this kind of pattern," he said.
The pyruvate gene has no obvious correlation with any known difference among major population groups.
"It is surprising to find this all-or-none difference when previously we would have expected just frequency differences," said Dr. James F. Crow, a geneticist at the University of Wisconsin. He described the pyruvate gene finding as a "striking exception" to the rule that different versions of a gene tend to be present to varying extents in all populations.
Dr. Hey believes that Africans and non-Africans may have distinctly different versions of the gene because of a "genetic sweep" that erased much of the variability in the non-African version of the gene. A genetic sweep occurs when some gene or element of a gene confers so large a survival advantage that a wide region of DNA on either side of it becomes embedded in the whole population.
Since none of the mutations found by the Rutgers group make any practical difference, Dr. Hey believes the point of selective advantage may lie in another part of the gene or even in a neighboring gene.
The selective advantage, whatever it was, may have conferred an edge against some threat -- perhaps a disease or climatic change -- confronted only by non-Africans. "It was some kind of environmental difference but I don't have a clue as to what," Dr. Hey said.