SYDNEY: Ancient DNA extracted from a 38,000-year-old Neanderthal bone is shedding light on crucial questions about human evolution.
The human and Neanderthal genomes are “from 99.5 to almost 99.9 per cent identical,” said Edward Rubin of Lawrence Berkeley National Laboratory in California, the leader of one of the two international teams who have sequenced large parts of its genetic code.
“[There are] 3 million base pair differences between Neanderthals and Homo sapiens, which is sort of like a drop in the bucket when one considers that the human genome is 3 billion base pairs.” Rubin said. “Chimpanzees, on the other hand, differ from humans [by] 30 to 50 million base pairs.”
The researchers are confident they should be able to work out the complete DNA sequence of our closest relatives within two years, revealing what makes us different from the Neanderthals.
So far, the DNA analysis suggests that Neanderthals and modern humans split from a common lineage about 500,000 years ago, and there was little, if any interbreeding of the two species.
Neanderthals were shorter and stouter than modern humans, but with larger brains. They inhabited Europe, central Asia and the Middle East for about 170,000 years before disappearing between 33,000 to 24,000 years ago, coinciding with the migration of modern humans out of Africa.
One of the greatest mysteries in the history of hominids is what actually happened to the Neanderthals. Some theories suggest that modern humans out-competed and replaced the Neanderthals. Others propose that interbreeding occurred, with Neanderthals being absorbed into the modern human population.
Rubin thinks the latter theory is unlikely: “While unable to definitely conclude that interbreeding between the two species of humans did not occur, analysis of the nuclear DNA from the Neanderthal suggests the low likelihood of it having occurred at any appreciable level,” he said.
This follows a report earlier this month of ancient hominid remains from Romania that had features of both modern humans and Neanderthals. The authors of that study considered the findings evidence that humans and Neanderthals interbred, disproving the theory that Neanderthals were simply replaced (read more).
The debate will be settled when the complete Neanderthal genome is sequenced, according Svante Pääbo of the Max Plank Institute for Evolutionary Anthropology in Germany, the leader of one of the research teams.
Ancient DNA from extinct species is notoriously difficult to sequence because it is usually highly fragmented and contaminated with DNA from other species. This has led some researchers to claim the task is impossible.
But when Pääbo tested the leg bone of a 38,000-year-old Neanderthal, he found it remarkably free of contaminants, and shared the DNA extracts with Rubin. The two researchers led teams that sequenced the genome using different methods, both publishing their results today and proving that ancient DNA can, in fact, be sequenced.
Pääbo and his colleagues sequenced more than one million base pairs using recent advances in DNA sequencing technology. The DNA was split into fragments (which meant that fragmentation due to age was not a problem) and was then reassembled by comparing these fragments with human sequences.
According to Pääbo’s team, Neanderthals and humans diverged from a common lineage between 465,000 and 569,000 years ago, with their best estimate at 516,000 years ago. The team publish their results today in the British journal Nature.
Rubin and his colleagues sequenced 65,000 base pairs by adapting methods used on organisms that are not readily grown in laboratories. They targeted the Neanderthal sequences most similar to human genes, recovering 29 of the 35 genes targeted in this way.
Rubin’s estimate of human and Neanderthal divergence – which largely agrees with Pääbo’s – is between 120,000 and 670,000 year ago, with their best estimate at 370,000 years ago. His team published their results today in the U.S. journal Science.
According to Pääbo, sequencing the entire Neanderthal genome will provide crucial insights into human evolution. “[Soon] we will be able to say more about the interaction between Neanderthals and modern humans,” he said. “Did Neanderthals contribute to the variation of us today [and] the reverse question: did early modern humans interbreed and contribute to genetic variation of Neanderthals?”