By MAGGIE FOX
Science may have caught up with the Bible, which says that Adam and Eve are the ancestors of all humans 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 lived tens of thousands of years apart.
How could this be?
Peter Underhill and colleagues at Stanford University in California have an explanation. “They had different molecular clocks,” Underhill said in a telephone interview. “Fewer men participated in reproduction than women did.”
Tracing Women to Earlier Time
His team, working with top geneticists across the United States, Europe, Israel and Africa, did a genetic analysis of DNA samples from the Y chromosomes of more than 1,000 men from 22 geographic areas and determined that their most recent common ancestor was a man who lived in Africa around 59,000 years ago.
Only men have Y chromosomes and researchers can look at gradual genetic mutations in them to “count” generations.
Other studies have used mitochondrial DNA, which women seem to pass down virtually unchanged from mother to daughter, to show that the genetic “Eve” lived 143,000 years ago.
The latest study, published in the November issue of the journal Nature Genetics, reconciles the two findings, and in the process the researchers came up with new tool for looking at how people are different from one another genetically.
They also added a great deal of detail to the family tree of all men living today, information that can be used by historians, anthropologists and other researchers. “We can look at the tree and see, ‘Oh, this section of the tree is where Asians go.’ We can say, ‘Oh, here is a Japanese Y chromosome and this is a Chinese Y chromosome,’” Underhill said.
Race Not Evident in Genetics
What the tree does not do, he stresses, is identify so-called races. Geneticists have long agreed there is no genetic basis to race — only to ethnic and geographic groups.
“People look at a very conspicuous trait like skin color and they say, ‘Well, this person’s so different’ … but that’s only skin deep,” Underhill said. “When you look at the level of the Y chromosome you find that, gee, there is very little difference between them. And skin color differences are strictly a consequence of climate.”
But the differences, while tiny, are enough for experts such as Underhill’s team to try to figure out how many generations you have to go back to find a single man who is related to all living people today. “The history of our species is something on the order of 4,000 or 5,000 generations,” Underhill said.
Women were good at passing on their genes, while some men were less lucky. Underhill’s team found evidence of genetic bottlenecks that shortened the male genetic legacy.
What could explain them? Real-life scenarios from recorded history provide plenty of explanations.
Dominant Tribe Wins Women
“One tribe conquers another tribe. The dominant tribe, the successful tribe, gets to mate with all the women — its own women plus the women they conquered,” Underhill said.
Polygamy, a common practice, would also explain it. A few dominant males get to marry and have children and the rest see their genes consigned to the rubbish heap of posterity.
Even nature itself can play a role. “I’m a man and if I get married and just by chance, a flip of the coin, I only have daughters, that is a random chance event. It has nothing to do with my being inferior or superior,” Underhill said.
But such a man would not pass on his Y chromosome and so would 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 out and spread across the world fairly quickly: perhaps 50,000 years ago to Europe and as long as 60,000 years ago to Australasia.
Tracing our ancestors genetically can be almost as hard as using a piece of skull or a tooth to date a fossil, but Underhill and his team developed a new method of looking at chromosomes called denaturing high performance liquid chromatography. They can use it to compare the Y chromosome of one 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 in a telephone interview. “When anatomically modern humans left Africa 45,000 to 60,000 years ago, they expanded rapidly across the world. The reason I can say this is because there is a nice relationship between [genetic] haplotype and geography.”
Genes Tell Histories
The genes reflect known history. “There was the Ice Age and so people couldn’t migrate much. So they stayed local and accumulated all the changes that allow us basically to trace them back,” Oefner said.
The same group reports in the latest issue of the journal Science that they used their method to determine that 95 percent of European men descend from about 10 “Adams” who in turn can be traced back to three different waves of migration.
The oldest male lineage they found dates back to the Old Stone Age or Paleolithic period 35,000 to 40,000 years ago. A second lineage dates to about 22,000 years ago and is associated with the Gravettian culture, known for its Venus figurines and shell jewelry and for using mammoth bones to build homes.
The third group, about 20 percent of the men, seem to date from more recent times, having come to Europe between 15,000 and 20,000 years ago. They were probably the first Neolithic farmers who migrated from the Fertile Crescent in the Middle East.
The genetic comparison method has some modern-day application. “This is also a powerful tool for forensic science and paternity testing,” Oefner said.
He said several companies had approached Stanford asking for permission to use those markers in forensic test kits. The university is trying to decide what to do with any profits that might come from licensing the kits.
“Whose property are those markers?” Oefner asked. “If there is money in this, there are things to be discussed about how to reimburse, especially all those indigent people that contributed their genetic material.”
Oefner thinks profits from licensing the technology might go to an international group that helps people without regard to culture, religious beliefs or politics such as French group Medecins Sans Frontieries (Doctors Without Borders).