A new mystery evolves on trail of early humans

Monday, June 26th 2000, 12:00 am
By: News On 6

World's common male ancestor seems to have postdated female counterpart

Adam and Eve would have had a tough time getting together. They may have lived in different eras.

In their effort to find out when the human species sprouted, diversified and spread around the globe, scientists have been hunting for an ancestral "Adam and Eve," the most recent common ancestors of all living humans.

New studies of the Y chromosome, the bundle of DNA that distinguishes men from women, suggest that current branches of the human family tree derive from a male ancestor who may have lived only 50,000 years ago, scientists reported last week in the Proceedings of the National Academy of Sciences.

Previous studies, based on a type of DNA passed on only by women, indicate that the most recent common female ancestor, or "Eve," lived at least 150,000 years ago.

That would mean that humans were around long before today's populations evolved from a common male ancestor. Earlier male lineages on the human family tree died out, leaving one relatively recent common father, or "Adam."

"Something happened to the record 50,000 to 60,000 years ago," said Peter Oefner, a biologist at the Stanford DNA Sequencing and Technology Center and one of the authors of the study. "We started at ground zero again."

Scientists can use DNA to figure out when populations diverged because stretches of the hereditary material have changed over time as human groups have drifted apart. The longer two populations have been distinct, the more differences show up between their DNAs.

Previous attempts to track human evolution relied mainly on evidence from DNA in cell parts called mitochondria. That DNA offers a way to trace female lineages because only the mother passes down mitochondrial DNA to her children.

Male studies use DNA from the cell nucleus, which contains 23 pairs of chromosomes, or packages of DNA. The 23rd pair, the sex chromosomes, distinguishes females (who have two X chromosomes) from males (who have an X and a Y).

The other chromosomes pair up before passing from parents to children. When that happens, pieces of paired chromosomes often get reshuffled like cards in a deck. But the lone Y chromosome, having no twin to mix with, remains fairly consistent within population groups from generation to generation. So scientists can use it to accurately classify males from different populations.

"Studying sex chromosomes has really given us a completely different and interesting handle on human diversity and human evolution," said F. Clark Howell, an anthropologist at the University of California, Berkeley.

The new evidence, based on analysis of the DNA of 72 males from 46 populations, is striking, Dr. Oefner said.

"All of us, the world as a whole, all the different ethnic groups, are all of very recent origin," he said. "There is very little that basically separates us. We are all coming out of the same boat not so long ago."

Y chromosome research suggests some intriguing scenarios for how humans migrated and diversified, said Jaume Bertranpetit, a biologist at the University Pompeu Fabra in Barcelona, Spain.

Research on the Y chromosome has already suggested that, throughout history, females have migrated from their home populations more than males, Dr. Bertranpetit said.

Other cultural behaviors, like polygyny, in which one male mates with many females, can influence the way male vs. female DNA is inherited, Dr. Oefner said.

Dr. Oefner is quick to warn that his results are not the final word on when humans diverged from the most recent common male ancestor. The average estimate coming out of the new data is 50,000 years, he said, but that male could have lived anywhere from 40,000 to 140,000 years ago. There just isn't enough data yet, he added. More DNA evidence from more people in more countries should help paint a clearer picture.