By Sean Henahan, Access Excellence

NEW HAVEN, CT- A survey of genetic information contained in the Y chromosomes of modern day men suggests that Homo sapiens descended from a single group of male ancestors, and that this occurred more recently than previously believed.

Researchers at Yale University evaluated samples of DNA from 38 men from all over the world. Surprisingly. no sequence variations were found in a 729-base pair intron near a gene thought to be involved in sperm or testes development. In contrast, the corresponding sequence in gorillas, chimpanzees, and orangutans did show significant polymorphisms.

Based on the assumption that this lack of sequence variation is attributable to ancestry and not other causes, the researchers estimate the divergence occurred about 270,000 years ago. Previous estimates have put mankind's origins much further back in time.

This discovery adds to growing evidence that modern humans share the same basic genetic makeup, and that population differences represent relatively minor variations in the overall spectrum of human diversity, said Robert L. Dorit, an assistant professor of biology at Yale University. Dr. Dorit collaborated with researchers from the University of Chicago and Harvard University on the project.

"If we all descended from a recent common ancestor, and if the history of human populations is a history of movement and gene flow, then the differences between us, as socially striking as we may wish to make them, are largely irrelevant from a biologist's standpoint," Dorit said.

The new findings appear to corroborate the timeline of the controversial "African Eve" hypothesis, first proposed in 1987. That hypothesis holds that all humans are descended from female ancestors who lived in Africa about 200,000 years. Those studies focused on purported regular, clock-like mutations of genes located in the mitochondria, energy-producing structures outside the cell nucleus that are present in the egg and thus inherited only from mothers.

The Yale study focused on the even slower clock-like mutations in the Y chromosome, the male sex chromosome passed from father to son. The researchers selected the Y chromosome because it contains the only genetic material besides mitochondrial DNA that is inherited from just one parent. The genetic material on all other chromosomes is inherited from both parents and is recombined, thus making it more difficult to reconstruct an evolutionary history.

The new research also appears to rules out an opposing theory that modern humans simultaneously evolved in different regions of the Old World from an earlier human ancestor - Homo erectus - who migrated out of Africa perhaps 1 million years ago. The lack of genetic variation found in the Yale study makes it highly unlikely that independent Y chromosome lineages have been evolving for a million or more years along separate paths, Professor Dorit said.

"The lack of genetic variation in the Y chromosome region we examined also makes it impossible for us to reconstruct the geographic location of the last common ancestor," Dorit noted. "The African Eve hypothesis, on the other hand, is based on a mutation rate in mitochondria that is at least 10 times faster than the mutation rate in the Y chromosome. Therefore, the greater number of mutations found in the mitochondria of native Africans indicates a longer history and a probable African origin for modern humans. We hope to be able to confirm an African origin by looking at another segment of the Y chromosome that is mutating slightly faster than our original segment, which could reveal subtle regional genetic differences," he said.

Dr. Dorit and colleagues compared the same sequence of 800 base pairs of nucleotides in humans to that of chimpanzees, gorillas and orangutans so as to establish the mutation rate in the Y chromosome. Humans are believed to have shared a common ancestor with chimpanzees and gorillas about 5 million years ago, and with orangutans about 14 million years ago.

"The idea was to get a snapshot of this part of the Y chromosome in a worldwide sample of humans that would help us establish some kind of evolutionary tree connecting human populations. We were very surprised to find no genetic differences in humans, although we found mutations as expected when we studied the primates. This probably means we are a very young species," noted Dorit.

It is still possible to hypothesize that Homo sapiens is actually much older than 270,000 years, emphasized Dr. Dorit. For example, it is possible the "genetic slate" was wiped clean recently by a beneficial mutation that caused a rapid sweep of that individual's genes throughout the population by natural selection. Another possibility is a significant reduction in the population of Homo sapiens, which population geneticists call a "bottleneck,". Such an event would significantly reduce genetic diversity, he said.

"However, evidence now is coming in from other parts of the human genome besides the mitochondria pointing to a recent origin for modern humans. In addition, molecular clocks appear to be ticking at various speeds in different genes that together can help us measure human evolution with greater accuracy. We must remember, however, that we are reconstructing the history of molecules here. While that history is not independent of the history of the organisms in which they are found, molecules have agendas of their own," Dorit said.

For more information on this study please refer to: Science, 5/26/95, Dorit et al.