August 14, 2012
Doubts About Link Between Neanderthals And Humans – Interbreeding Or Common Ancestry?
April Flowers for redOrbit.com - Your Universe Online
A new study from the University of Cambridge finds that the DNA similarities between Neanderthals and modern humans are more likely to have arisen from a shared common ancestor than from interbreeding.Previously, it has been suggested that the shared parts of the genome sequence between the two populations was the result of interbreeding, but the new research, published this week in the journal Proceedings of the National Academy of Sciences, proposes a different explanation.
The origin of modern humans is a hotly debated topic with four main theories arising to explain the evolution of Homo sapiens. Before this new research, two of the theories had taken precedence. First, that of a multiregional evolution (the first humans arose about two million years ago during the Pleistocene in Africa and human evolution has been within a single, continuous human species) and second, that of a recent African origin (that modern humans arose in Africa around 100 — 200,000 years ago to replace older human forms with interbreeding.)
All four theories argue for an African origin, but the most important distinction between them is whether or not interbreeding, or hybridization, between ancient and modern humans took place.
The Cambridge research team, including evolutionary biologists Dr. Anders Eriksson and Dr. Andrea Manica, used computer simulations to reassess the strength of evidence supporting hybridization events.
The amount of DNA shared between modern Eurasian humans and Neanderthals — estimated between 1 -4% - can be explained if both arose from a geographically isolated population, most likely in North Africa, which shared a common ancestor around 300-350 thousand years ago.
Tribes of modern humans, migrating out of Africa around 60-70,000 years ago, took that genetic similarity with them.
In contrast to this study, previous ancient DNA studies of Neanderthal remains have shown that their genomes harbor polymorphisms, or genetic signatures, that are also seen in the genomes of modern Europeans, East Asians and Oceanians. They are not found, however, in modern African populations.
These new findings challenge previously held views based on several lines of evidence that modern humans had replaced the Neanderthals with little or no gene flow occurring between them. Gene flow, also called migration, is any movement of genes from one population to another.
Gene flow can include different types of events, such as pollen blowing to a new location or people moving to a new city or country. If carried to a population where those genes did not previously exist, gene flow can be a very important source of genetic variation.
Some evolutionary biologists argue that this genetic similarity in the Neanderthal genome arose through hybridization between the ancestors of modern non-Africans and Neanderthals who were already resident in Europe and western Asia.
The new study has detractors, however, reports BBC News. Professor David Reich, from Harvard University, is an exponent of the hybridization theory, and he is not convinced that the data out of Cambridge represents a powerful argument against interbreeding.
Using methods that are able to differentiate between genetic similarity caused by gene flow via hybridization v. shared ancestry, he argues that, "the patterns observed [in our analyses] are exactly what one would expect from recent gene flow" - a view shared by his collaborator Professor Svante Paabo from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Prof Reich went on to say that their data shows that Neanderthals and non-Africans last exchanged genetic material 47-65,000 years ago.