Moving closer in the search for human roots, three scientists today offered new genetic evidence that all men are descended from a common ancestor who lived about 270,000 years ago, lending some support to the theory that modern humanity evolved in Africa a relatively short time ago.
For the first time, researchers at Yale, Harvard and the University of Chicago have traced the genetic roots of the human family through the male line, by analyzing the Y chromosome that makes an embryo become male. Until now, the effort to reconstruct the history of human heredity has concentrated on the genes that mothers alone pass along to their children.
In work made public today in the journal Science, the team of prominent biologists corroborates the idea that modern humanity originated around 200,000 to 300,000 years ago, rather than a million years earlier, as other experts have theorized.
"It suggests we really are an extremely recent species on this planet," said Yale University biologist Robert L. Dorit, who led the group. "We are definitely rookies . . . noisy, but new."
The new research also provides additional evidence that modern humans--despite their apparent racial and ethnic differences--all share the same basic genetic makeup. In one section of the chromosome that makes a person male, men around the world are virtually identical, the scientists discovered.
For their analysis, they sequenced a portion of the male sex chromosome that apparently changes quite slowly over the eons. To ensure that their sample would encompass the range of human geographic diversity, they used chromosomes from 38 men chosen from every continent. They discovered virtually no differences, despite the fact that genetic mutations usually occur at a predictable rate.
"Originally we were terribly surprised by the lack of variation," said Harvard University biologist Walter Gilbert, a Nobel laureate who worked on the study. "We realized very early that meant a recent common male ancestor."
Their research is based on the premise that scientists can reconstruct the history of human evolution by analyzing variations in the genes every person has in common. Researchers can use subtle mutations in human genes to track how prehistoric people evolved, ventured into new lands and adapted to new conditions.
"Recently, perhaps even more has been learned about human evolution in molecular genetics laboratories than at archeological excavations or in museum collections," said Svante Paabo, a University of Munich biologist who reviewed the research for Science.
While most scientists agree that humanity's earliest ancestors evolved in Africa, there are conflicting theories on when and where humankind took on its present form. "Part of the emotional charge in this debate is a racial one to many people," said Gilbert. "A very striking, very general message from these human DNA sequences is how closely all humans are related."
Some researchers have argued, based on fossil evidence, that modern humans evolved simultaneously around the world from a number of ancestral groups, all of which migrated out of Africa more than a million years ago. By that theory, many modern physical racial differences would be as old as humanity itself.
Others analyzed the regular clocklike evolution of genetic material contained in the cell's energy-producing complexes called mitochondria, which are inherited through maternal ancestors, and reached a dramatically different conclusion. They believe that modern humans emerged from Africa much more recently and edged those earlier pre-human groups into extinction.
Their work led to the 1980s theory of the "African Eve," which traces modern humanity to a common female ancestor who lived in Africa a few hundred thousand years ago.
In recent months, several new genetic studies have buttressed that view by refining estimates of when modern humans spread across the continents.
While the three groups that conducted the studies differ slightly in the estimated timing of human events, they together provide the strongest evidence yet that modern humans originated recently in Africa.
Emory University geneticist Douglas Wallace recently calculated that modern humans left Africa about 130,000 ago, settled Asia about 75,000 years ago and peopled Europe about 51,000 years ago.
Working with a slightly different molecular clock also calibrated by human genetic variation, Satoshi Horai at the Genetics Institute of Japan suggested that modern humans left Africa about 143,000 years ago, reaching East Asia about 94,000 years ago and Europe about 79,000 years ago.
Luigi Luca Cavalli-Sforza, a Stanford Medical School expert on human genetics and evolution, developed a third--and potentially more accurate--method to measure ancient human migrations. It relies on slight variations in DNA sequences called microsatellites, found scattered uniformly throughout people's chromosomes.
He suggests that modern humanity left its African homeland a mere 112,000 years ago.