Their mates were real Neanderthals

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Their mates were real Neanderthals

A genetic comparison of ancient bones and living people shows interbreeding

about 80,000 years ago, researchers say. Traces of the extinct hominid remain

today.

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The first modern humans to leave Africa about 80,000 years ago encountered

Neanderthal settlements in the Middle East and, on at least some occasions,

chose to make love instead of war, according to an international team of

scientists who have pieced together the genetic code of humanity's closest

relatives.

Traces of that ancient DNA live on in most human beings today, the researchers

report in Friday's edition of the journal Science.

The finding, which was made by analyzing DNA from Neanderthal bones and

comparing it with that of five living humans, appears to resolve a long-standing

mystery about the relationship between Neanderthals and Homo sapiens, who

coexisted in Europe and Western Asia for more than 10,000 years until

Neanderthals disappeared about 30,000 years ago.

" We can now say with absolute certainty that we've got these Neanderthal genes, "

said Hawks, an evolutionary geneticist at the University of Wisconsin who

was not involved in the study. " They're not 'them' anymore; they're 'us.' "

Svante Paabo, the geneticist at the Max Planck Institute for Evolutionary

Anthropology in Leipzig, Germany, who spearheaded the study, said he now sees

his ancestors in a new light. His initial research on a different type of DNA

that contains far less information had concluded — incorrectly, it turns out —

that Neanderthals have no genetic connection to people today.

Now, Paabo said, " I would more see them as a form of humans that were a bit more

different than people are from each other today. "

Most important, scientists said, knowing the precise structure of the

Neanderthal genome will help answer the fundamental biological question: What

makes us human?

Neanderthal DNA is 99.7% identical to that of people, according to the analysis,

which involved dozens of researchers. Something in the remaining 0.3% must make

us unique.

" It's not about understanding Neanderthals, " said genome biologist Ed Green, who

led the study as a research fellow in Paabo's lab and is now at UC Santa Cruz.

" It's understanding us. "

By lining up the Neanderthal genome with DNA from humans and chimpanzees, Green

and colleagues identified small changes that are unique to humans. Some were in

genes involved in energy metabolism, skeletal structure and brain development,

including four that are thought to contribute to conditions such as autism, Down

syndrome and schizophrenia.

The researchers constructed the Neanderthal genome from three bone fragments

found in Croatia's Vindija Cave. Using a sterile dentistry drill, the scientists

removed 400 milligrams of bone powder — an amount equivalent to the size of an

aspirin.

Extracting DNA from ancient bones was a dicey proposition.

For starters, 95% to 99% of the DNA the team found came from microbes that

colonized the bones after the Neanderthals died more than 38,000 years ago. To

address that problem, the scientists discarded DNA fragments with letter

combinations that were especially common in microbes.

In addition, the Neanderthal DNA was badly degraded, which caused sequencing

machines to misread some of the chemical letters in the sequence. The

researchers developed a computer program to correct those mistakes.

To conserve their limited supply of Neanderthal bones, they practiced their

techniques on bones from extinct cave bears first.

The researchers took special precautions to keep their own DNA out of the

Neanderthal samples. Workers wore full-body suits, including masks and gloves.

The air pressure inside the lab was kept high so that nothing could blow in

accidentally, and the room was irradiated after the researchers went home, Green

said.

After four years of work, the team identified 4 billion fragments of Neanderthal

DNA and organized them into a draft genome. The sequence is 60% complete.

" It is a very poor quality for a human genome, but it is outstanding for a

30,000-year-old extinct hominid, " said Eddy Rubin, who has sequenced samples of

Neanderthal DNA at Lawrence Berkeley National Laboratory but was not involved in

this study.

To look for evidence of gene flow between humans and Neanderthals, the

researchers sequenced the DNA of five people who live in Southern Africa,

Western Africa, France, China and Papua New Guinea. Because they didn't think

Neanderthal genes had passed to humans, they expected to find the same degree of

difference between the Neanderthal genome and all five people.

Instead, they discovered that the Neanderthal DNA was slightly more similar to

the three people living outside Africa. Even more surprising, the relationship

was just as strong for the individuals from China and Papua New Guinea as for

the person from France, who lives in the Neanderthals' old stamping grounds.

The simplest explanation is that a small group of humans met the Neanderthals

50,000 to 80,000 years ago after they left Africa but before they had spread

throughout Europe, Asia and beyond. The logical meeting place was the Middle

East, which connects northeast Africa to the Eurasian continent.

" The contact must have happened early for the Neanderthals' genes to have spread

so widely and uniformly, " said Henry Harpending, an anthropologist at the

University of Utah, who was not involved in the study.

The amount of mixing was small: Only 1% to 4% of the DNA in non-African humans

originated in Neanderthals, according to the study. The researchers said none of

that DNA is functional; in fact, the particular 1% to 4% is different in every

individual.

Interbreeding may well have continued in Europe, but that would be harder to

detect because both populations there were large and any small Neanderthal

contribution would be too dilute to see, Paabo said.