Genes tell of man's near extinction

Sun Journal

Doom: Our limited genetic diversity may be explained by a disaster less than a million years ago that reduced our ancestors to a few hundred.

May 16, 1999|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

One scientist has called it man's "brush with extinction."

Genetic studies have provided new evidence suggesting that early humans, or their immediate ancestors, were nearly wiped out by a calamity that struck in Africa within the past million years.

The event -- an epidemic, perhaps, or a natural disaster or series of them -- may have killed off all but a few hundred or a thousand of their numbers.

So few were left to repopulate the Earth, researchers say, that the human race today is left with less genetic diversity than even small communities of chimpanzees now living in Africa.

"From the point of view of a chimp, we must look like a bunch of clones," says Pascal Gagneux, a molecular geneticist at the University of California, San Diego.

Gagneux and the university's Christopher Wills led an international team in a comparative study of the genetic diversity of modern humans, one extinct Neanderthal man and today's chimps, gorillas and orangutans. The results of their work were published last month in Proceedings of the National Academy of Sciences.

Our genetic poverty today contrasts sharply with the rich diversity of our ancient past. There is fossil evidence of at least 15 species of extinct hominids -- humanlike primates -- that lived at various times within the past 5 million years.

Scientists don't know how many early humans (or our direct ancestor, called Homo erectus) were around when they had their brush with extinction. Some have estimated there were at least 10,000, similar to the number of African great apes today.

Likewise, no one knows for sure what might have caused the crash. "Very good candidates, obviously, are disease and environmental disasters," Gagneux says. "Very few people think a single event would have been the cause."

"Personally, I'm very intrigued by the change of lifestyle our ancestors adopted," he says. For example, apes build sleeping nests in new locations each night. But early Homo sapiens, and Homo erectus before them, returned nightly to the same camps, the same parasites and waste products.

New interactions with dogs, increased mobility or social exchanges may also have exposed early humans to new illnesses, Gagneux says. "So many things have changed with the life of early humans compared with apes that it is going to be really hard to know which contributed most to potential population crashes."

The "genetic bottleneck" theory is not new, and it remains controversial.

"Many population geneticists would agree that, at some point in our past, there was a reduction in effective population size. We are trying to understand how drastic that population reduction was," says Dr. Michael Bamshad, a University of Utah geneticist and professor of pediatrics who did not participate in the Gagneux study.

He says Gagneux's work is important because it demonstrates that whatever narrowed man's genetic diversity did not have the same impact on other African primates. "But that low level of diversity begs explanation."

For example, Gagneux's team looked at mitochondrial DNA (mtDNA), a genetic code specific to females.

The lack of genetic diversity they found could indicate that prehistoric women mated or migrated more widely than men, reducing the diversity of that female marker across the population. Repeating the work with male markers, and others, might provide a clearer picture of what was going on.

"The human genome reflects the history of our species," Bamshad says. "With the availability of many different molecular tools we can explore different facets of that evolutionary history."

The notion that all modern humans can trace their origins to a very small population in Africa has made news before.

In 1988, Allan Wilson, Rebecca Cann and others at the University of California, Berkeley, presented genetic evidence that all modern humans could trace their origins to an individual woman who lived in Africa perhaps 200,000 years ago.

This "African Eve" was obviously not alone in the world, scientists say. But only her genetic lineage has survived to the present.

While mitochondrial DNA is passed down nearly intact from mother to daughter, it does mutate slowly over time. Scientists can measure the differences and estimate how much time has passed since related individuals or species shared a common ancestor.

Studies of modern humans have found the greatest mtDNA diversity in Africa, strong evidence that our species originated there and had the longest time there to diversify.

Modern humans -- Homo sapiens -- emerged first in Africa and migrated out about 100,000 years ago. But among the modern human genes sampled, Gagneux says, "nothing outside of Africa looked very different. It looks like everybody is African under the skin."

Human racial distinctions are very recent, biologically meaningless regional differences, he said. "There is no natural counterpart to the concept of race in humans."

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