W A S H I N G T O N, Nov. 14, 2000 -- Science may have caught up with theBible, which says that Adam and Eve are the ancestors of allhumans alive today.
But in the scientists’ version, based on DNA analysis, “Adam,” the genetic ancestor of all men living today, and “Eve,”the genetic ancestor of all living women, seem to have livedtens of thousands of years apart.
How could this be?
Peter Underhill and colleagues at Stanford University inCalifornia have an explanation. “They had different molecularclocks,” Underhill said in a telephone interview. “Fewer menparticipated in reproduction than women did.”
Tracing Women to Earlier Time
His team, working with top geneticists across the UnitedStates, Europe, Israel and Africa, did a genetic analysis of DNAsamples from the Y chromosomes of more than 1,000 men from 22geographic areas and determined that their most recent commonancestor was a man who lived in Africa around 59,000 years ago.
Only men have Y chromosomes and researchers can look atgradual genetic mutations in them to “count” generations.
Other studies have used mitochondrial DNA, which women seemto pass down virtually unchanged from mother to daughter, toshow that the genetic “Eve” lived 143,000 years ago.
The latest study, published in the November issue of thejournal Nature Genetics, reconciles the two findings, and in theprocess the researchers came up with new tool for looking at howpeople are different from one another genetically.
They also added a great deal of detail to the family tree ofall men living today, information that can be used byhistorians, anthropologists and other researchers. “We can lookat the tree and see, ‘Oh, this section of the tree is whereAsians go.’ We can say, ‘Oh, here is a Japanese Y chromosome andthis is a Chinese Y chromosome,’” Underhill said.
Race Not Evident in Genetics
What the tree does not do, he stresses, is identifyso-called races. Geneticists have long agreed there is nogenetic basis to race — only to ethnic and geographic groups.
“People look at a very conspicuous trait like skin color andthey say, ‘Well, this person’s so different’ ... but that’s onlyskin deep,” Underhill said. “When you look at the level of the Ychromosome you find that, gee, there is very little differencebetween them. And skin color differences are strictly aconsequence of climate.”
But the differences, while tiny, are enough for experts suchas Underhill’s team to try to figure out how many generationsyou have to go back to find a single man who is related to allliving people today. “The history of our species is something onthe order of 4,000 or 5,000 generations,” Underhill said.
Women were good at passing on their genes, while some menwere less lucky. Underhill’s team found evidence of geneticbottlenecks that shortened the male genetic legacy.
What could explain them? Real-life scenarios from recordedhistory provide plenty of explanations.
Dominant Tribe Wins Women
“One tribe conquers another tribe. The dominant tribe, thesuccessful tribe, gets to mate with all the women — its ownwomen plus the women they conquered,” Underhill said.
Polygamy, a common practice, would also explain it. A fewdominant males get to marry and have children and the rest seetheir genes consigned to the rubbish heap of posterity.
Even nature itself can play a role. “I’m a man and if I getmarried and just by chance, a flip of the coin, I only havedaughters, that is a random chance event. It has nothing to dowith my being inferior or superior,” Underhill said.
But such a man would not pass on his Y chromosome and sowould chalk up a big zero in the “Adam” and “Eve” genetic stakes— although of course his other genes would live on.
Fossil records suggest that Homo sapiens, or modern humans,first appeared in Africa about 150,000 years ago, then moved outand spread across the world fairly quickly: perhaps 50,000 yearsago to Europe and as long as 60,000 years ago to Australasia.
Tracing our ancestors genetically can be almost as hard asusing a piece of skull or a tooth to date a fossil, butUnderhill and his team developed a new method of looking atchromosomes called denaturing high performance liquidchromatography. They can use it to compare the Y chromosome ofone man to another, something that used to be laborious.
“By using this new method, we could do it far, far faster,”Peter Oefner of Stanford University, who also worked on the study, said ina telephone interview. “When anatomically modern humans leftAfrica 45,000 to 60,000 years ago, they expanded rapidly acrossthe world. The reason I can say this is because there is a nicerelationship between [genetic] haplotype and geography.”
Genes Tell Histories
The genes reflect known history. “There was the Ice Age andso people couldn’t migrate much. So they stayed local andaccumulated all the changes that allow us basically to tracethem back,” Oefner said.
The same group reports in the latest issue of the journalScience that they used their method to determine that 95 percentof European men descend from about 10 “Adams” who in turn can betraced back to three different waves of migration.
The oldest male lineage they found dates back to the OldStone Age or Paleolithic period 35,000 to 40,000 years ago. Asecond lineage dates to about 22,000 years ago and is associatedwith the Gravettian culture, known for its Venus figurines andshell jewelry and for using mammoth bones to build homes.
The third group, about 20 percent of the men, seem to datefrom more recent times, having come to Europe between 15,000 and20,000 years ago. They were probably the first Neolithic farmerswho migrated from the Fertile Crescent in the Middle East.
The genetic comparison method has some modern-dayapplication. “This is also a powerful tool for forensic scienceand paternity testing,” Oefner said.
He said several companies had approached Stanford asking forpermission to use those markers in forensic test kits. Theuniversity is trying to decide what to do with any profits thatmight come from licensing the kits.
“Whose property are those markers?” Oefner asked. “If thereis money in this, there are things to be discussed about how toreimburse, especially all those indigent people that contributedtheir genetic material.”
Oefner thinks profits from licensing the technology might goto an international group that helps people without regard toculture, religious beliefs or politics such as French groupMedecins Sans Frontieries (Doctors Without Borders).