By analyzing DNA they extracted from three Neanderthal bones over 40,000 years old discovered in a cave in Croatia, scientists from Europe and the US have revealed in intricate detail how humans are related to this long-extinct relative of ours.

You can read about the work behind the finding that between 1 and 4 per cent of the DNA of non-African humans came from Neanderthals in the 7 May issue of Science.

Neanderthals are the closest evolutionary relatives of modern day humans. They lived in large parts of Europe and western Asia before they became extinct about 30,000 years ago, although the reason for that is unclear.

Fossil records suggest that Neanderthals diverged from our ancestors between 270,000 and 440,000 years ago, and although clearly different physically, they are the closest anatomically to modern humans.

Probably better adapted to colder climates, they had bigger faces than ours, their chests were barrel-shaped and their arms and legs were shorter. Their brain was also a different shape (longer with perhaps smaller temporal lobes), but probably the same size as ours.

The fossil record also shows that modern humans and Neanderthals overlapped and lived in the same parts of the world, in Europe and Asia, for tens of thousands of years. So did the two species interbreed? And did they pool their genes? These were the questions that prompted this project.

The work was done in two phases: in the first phase a team of scientists at the Max Planck Institute in Leipzig, Germany, extracted the DNA from three Neanderthal bones found in in Vindija Cave, an archeological site in Croatia, and in the second phase, an international consortium analyzed the data to find out how Neanderthals are related genetically to modern humans.

A key part of the second phase was the discovery that Neanderthals interbred with ancestors of modern Europeans and Asians but not with Africans. HMS associate professor of genetics David Reich, who led this part of the project, told the media that:

“Europeans and East Asians today bear a closer relationship to Neanderthals than do Africans.”

“One to four percent of the genomes of non-Africans trace their ancestry to Neanderthals,” he explained, adding that:

“Europeans and Asians both have this signal, indicating that the gene flow occurred at least 45,000 years ago, before these two populations’ ancestors separated.”

The researchers at Max Planck have been working for the last ten years to try and produce DNA samples that are pure enough to sequence: many of those they prepared were contaminated with microbe DNA, or even DNA from the technician who handled them.

It was not until more recent technology allowed them to detect and filter out DNA from other sources that they were able to prepare samples of DNA pure enough for genomic analysis. With new methods they were able to reconstruct a genome comprising more than 4 billion base pairs of DNA taken from several cells. Altogether, this represented about 60 per cent of the Neanderthal genome: enough to pass on to the next phase.

For the second phase, Reich and colleagues compared the Neanderthal genome assembled by the Max Planck researchers to that of five present day humans from different parts of the world.

This is how they discovered that between 1 and 4 per cent of the genome of non-African humans is descended from Neanderthals, and furthermore, the groups all appear to be equally related to the now extinct “cousin”.

Speculating on why the genetic distribution is so uniform among these disparate non-African groups, the researchers suggested perhaps the interbreeding took place at the gateway of the migration that took place when humans left Africa to populate Europe and Asia in the upper Paleolithic period about 45,000 years ago.

Reich said although that was not the only explanation it was the most likely one. He also said there remains a “tremendous amount of yet untapped information” in the genome and he envisages they will be spending “many more decades getting closer to what it all means”.

Dr Daniel Lieberman, professor of human evolutionary biology at Harvard University, who was not involved with the study, said we should not be surprised to find that our ancestors interbred with Neanderthals. He said closely related species often interbreed:

“I do not think this in any way lessens the evidence that Neanderthals and modern humans were indeed separate species,” he commented.

The researchers said they think our small percentage of Neanderthal DNA is random and does not link to any specific traits, as far as they can tell.

“A Draft Sequence of the Neandertal Genome.”
Richard E. Green, Johannes Krause, Adrian W. Briggs, Tomislav Maricic, Udo Stenzel, Martin Kircher, Nick Patterson, Heng Li, Weiwei Zhai, Markus Hsi-Yang Fritz, Nancy F. Hansen, Eric Y. Durand, Anna-Sapfo Malaspinas, Jeffrey D. Jensen, Tomas Marques-Bonet,, Can Alkan Kay Prü̈fer, Matthias Meyer, Hernán A. Burbano, Jeffrey M. Good,, Rigo Schultz, Ayinuer Aximu-Petri, Anne Butthof, Barbara Höber, Barbara Höffner, Madlen Siegemund, Antje Weihmann, Chad Nusbaum, Eric S. Lander, Carsten Russ, Nathaniel Novod, Jason Affourtit, Michael Egholm, Christine Verna, Pavao Rudan, Dejana Brajkovic, Zeljko Kucan, Ivan Gusic, Vladimir B. Doronichev, Liubov V. Golovanova, Carles Lalueza-Fox, Marco de la Rasilla, Javier Fortea, Antonio Rosas, Ralf W. Schmitz,, Philip L. F. Johnson Evan E. Eichler Daniel Falush Ewan Birney James C. Mullikin, Montgomery Slatkin, Rasmus Nielsen, Janet Kelso, Michael Lachmann, David Reich, Svante Pääbo.
Science, Vol 328, Issue 5979, 7 May 2010.
DOI: 10.1126/science.1188021

Source: Harvard Medical School.

Written by: Catharine Paddock, PhD