Scientists search modern genes for clues to human origin > Garden of Eden theorists say genes passed along only by mothers confirm one woman as our common ancestor

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NEW ORLEANS — Did we all descend from Eve, an African woman whose offspring quite literally conquered the world?

Though of course no one knows her name, some biologists and anthropologists believe that all humanity traces its roots to just one woman. This idea, known as the Eve hypothesis or the Garden of Eden theory, is intriguingly simple:

Eve lived in sub-Saharan Africa 100,000 to 200,000 years ago. As the ancestors of modern humans, her children and grandchildren spread throughout Africa and eventually across the world, triggering the extinction of other primitive peoples.

Or so some scientists believe.

Others ridicule the idea. The theory, based on a comparison of the genes of people around the world, is one of the most hotly contested controversies in the often-rancorous field of human origins.

At a meeting earlier this year of the American Association for the Advancement of Science, both sides argued their cases with facts, speculation and sarcasm.

Allan C. Wilson of the University of California at Berkeley, one of the architects of the Eve hypothesis, said the evidence is so strong that the “argument is done, clinched, finished. It’s over.”

Not quite, countered the other side.

“There are serious and obvious flaws in the Eve hypothesis,” said Geoffrey G. Pope of the University of Illinois.

Most everyone agrees that small-brained, humanlike creatures arose in Africa 1.5 million to 2 million years ago. They spread throughout Africa, Europe and Southeast Asia and flourished for hundreds of thousands of years.

But what became of them? The theory favored by Pope and many fossil experts holds that those creatures, known as homo erectus, are the true ancestors of us all.

If they are right, these early hominids parted company in Africa but went on to evolve in unison. Useful genetic changes that arose in small pockets of population slowly spread throughout the world as neighboring tribes mingled. After thousands of centuries, they became us — modern humans who differ only in superficial racial features.

However, Garden of Eden theorists believe those early settlers disappeared leaving no trace in humanity’s genes. Instead, they say they have evidence that a new kind of people arose in Africa much more recently. Their kin also spread throughout the prehistoric world, somehow killing off or squeezing out the European Neanderthals and other descendants of the first wave out of Africa.

The difference between the scenarios is so stark that there seems to be no middle ground, no room for compromise.

“There is no question on this issue,” said Alan G. Thorne of the Australian National University, an Eve skeptic. “One of us is absolutely wrong.”

The debate began about three years ago with the use of new scientific techniques to look for signs of the family tree inside the genes of modern humans. This intrusion of the glitzy tools of molecular biology have not always been accepted graciously by the traditional fossil hunters, who rely largely on meticulous measurements and comparisons of bone fragments to fathom human origins.

David W. Frayer, a Neanderthal expert from the University of Kansas, said the quick acceptance of Eve reflects a preference for sensational catastrophe over mundane gradual change, as well as a “willingness of some to accept without criticism, or even suspicion, the results of molecular biology.”

Molecular biology’s role has been to decipher the genetic information inside mitochondria. These are structures within human cells that help produce energy.

Mitochondria have their own genes, separate from the chromosomes that carry all the other blueprints for human beings. Ordinary genes are a mishmash contributed equally by mothers and fathers. Mitochondrial genes are passed along only by mothers.

Therefore, each new generation gets an almost exact copy of the mitochondrial genes of their forebears. The only difference is the occasional variation caused by genetic mutations.

These mutations are the key to reading mitochondrial genes as a clock. Biologists believe they know how quickly the mutations occur. By comparing the degree of mutation in the genes of people from different parts of the world, they believe they can trace the various lines back to the point at which they all converge: Eve.

The latest study of this sort, conducted by Linda Vigilant at Berkeley, uses hair roots plucked from 199 people from Europe, Asia, New Guinea and seven ethnic groups in Africa. Analyzing the root genes, she calculates all the people descended from a common ancestor who lived in Africa 207,000 years ago. This female’s descendants evolved into physically modern humans by 100,000 years ago.

As neat as the idea seems, anthropologists attack it on several grounds.

One is whether the mutations in mitochondrial genes occur like clockwork. The critics argue that the biologists are overly confident that they know the tempo at which the mitochondrial clock is ticking.

Milford H. Wolpoff of the University of Michigan contends the clock’s rhythm is hopelessly thrown off by the dead ends of all-boy families. Whenever a family produces no girls, the orderly accumulation of genetic mutations stops cold. No mitochondrial genes are passed to the next generation.

“If there is a clock,” said Thorne, “it needs new batteries.”

Other scientists point to the fossil record, the teeth and bits of bone that remain from the ancient beings.

If the Eve theory is correct, the new wave of humans migrating out of Africa 1,000 centuries ago did not mate and mingle their genes with the ancient hominids that already lived in Europe and Asia. Instead, they replaced them completely.

Some fossil experts believe they have found clear signs that traits of those ancient peoples have been passed down to modern humans. For instance, Pope argues that 1 million-year-old fossils from ancient Asia have the same cheekbones and wisdom teeth as modern Asians.

This similarity in facial features “indicates that the Eve hypothesis is completely untenable in Asia.”

Frayer also believes he can see echoes of Neanderthal skulls and teeth in later generations of Europeans.

Allison S. Brooks of George Washington University questions Eve on behavioral grounds. She said that if modern humans overran Europe, replacing the dimwitted natives, there should also have been a cultural revolution.

However, she finds no abrupt change showing dramatically more advanced stone tools or hunting techniques in Europe after the supposed arrival of Eve’s offspring.