CHICAGO — An international group of scientists has completed the first rough draft of Neandertal’s genetic instruction manual. The genetic evidence suggests that humans and Neandertals are very similar, but that the two species probably didn’t interbreed.
Speaking by video teleconference from Leipzig, Germany, scientists led by Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology announced the achievement to reporters gathered at the annual meeting of the American Association for the Advancement of Science. Pääbo said that the team has decoded 3.7 billion bases of Neandertal DNA from a bone of a female Neandertal fossil discovered in Vindija cave in Croatia. That DNA represents about 63 percent of the total Neandertal genome.
“It’s a milestone,” says John Hawks, a paleoanthropologist at the University of Wisconsin–Madison. The announcement was not a surprise, and the team decoding the Neandertal genome has presented updates of its progress and some of its findings at scientific meetings, but Hawks and other scientists are looking forward to the public release of the Neandertal genome data, expected later this year.
Analysis of the genome reveals that humans and Neandertals share genetic roots stretching back at least 830,000 years. Neandertals, the species Homo neanderthalensis, were humans’ closest relatives, appearing about 300,000 years ago and living in Europe and parts of Asia until going extinct about 30,000 years ago.
Anatomically modern humans, the species known as Homo sapiens, first appeared in Africa about 250,000 to 200,000 years ago.
“In the beginning it was just another hominid species,” says Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Leipzig. About 100,000 to 50,000 years ago the human species began to spread out of Africa and around the world. “Eventually it reached the moon, and it will go to Mars,” Hublin says.
Comparing the genes of humans, Neandertals and apes may reveal the genetic changes that allowed humans to adapt to such a wide range of environments and eventually become the only living hominid species on the planet.
“This project really has more to do with our history than that of Neandertals,” Pääbo says.
Already the comparisons of the human, Neandertal and chimpanzee genomes have revealed evidence that natural selection recently played a hand in shaping the human genome. Neandertals are similar to humans across most of the genome — any given Neandertal was about as similar to a human as two humans are to each other. But the team discovered that in some parts of the genome, Neandertals had more in common with chimps. In those parts of the genome, humans underwent several changes that could hold the key to understanding what makes humans human.
“That tells us that what we [humans] share is small. It’s important; it made us human and is the result of a series of finite steps,” Hawks says.
Interbreeding between modern humans and Neandertals was probably minimal, Pääbo says. Neandertals contributed “very little, if anything,” to the genetic makeup of modern humans. “If there was a contribution, it was very small.”
Some researchers have suggested that a variant of a gene called microcephalin-1, which is associated with brain development in modern humans, may have come from Neandertals. But the new analysis reveals that Neandertals did not carry the same variant common to humans outside Africa. Instead, the Neandertals had a variant of the gene associated with other extinct human ancestors.
Also, most people in the world are lactose-intolerant — unable to digest milk as adults. But a variant of the lactase gene, which encodes an enzyme that breaks down the sugar lactose in milk, is common in people of northern European descent. The researchers examined the lactase gene in Neandertals to see if that trait might have been passed from Neandertals to modern humans. But Neandertals have the lactose-intolerant version of the gene.
“The Neandertal, as you would expect, was not able to drink milk after it was weaned,” Pääbo says.
But humans and Neandertals did share a version of the FOXP2 gene associated with speech in humans. The result may indicate that Neandertals could speak.
In addition to the Croatian specimen, the researchers have decoded about 2 million bases of DNA from 43,000-year-old bones from a Neandertal discovered in a cave in El Sidrón, Spain. Those bones were excavated under sterile conditions to minimize contamination of the Neandertal DNA with human DNA.
Russian scientists also contributed to the project with DNA from a 60,000- to 70,000-year-old bone from a Neandertal infant skeleton discovered in Mezmaiskaya cave in the northwestern foothills of the Caucasus mountains. And the LVR-LandesMuseum in Bonn, Germany, which houses the first Neandertal fossil discovered (in 1856 in the Neander Valley), allowed Pääbo’s team to extract DNA from that specimen. DNA from those specimens will allow scientists to assess genetic variation in Neandertals and better define the genetic makeup of the species.
Neandertals appear to have been less genetically diverse than humans are, Pääbo says. “That suggests to me that they had a long history of expanding from small populations just like we have, or that they went through a series of [population] bottlenecks,” he says.