martes, 13 de marzo de 2012

Genetic Studies of Modern Populations Show Varying Neandertal Ancestry

Research is revealing evidence that sheds new light on the genetic relationship between the ancient Neandertals and modern humans.
The complex world of human genetics research speaks a language unfamiliar to most of us, but it has opened up a new window on our understanding of the dynamics of ancient populations; and few areas of research have been more tantalizing than that surrounding the questions of how modern humans are related to the Neandertals, an ancient species of human whose morphology or physical characteristics disappeared from the human fossil record roughly 30,000 years ago. The most recent studies have provided evidence about when the Neandertal (Homo neandertalensis) and modern human populations (Homo sapiens) first diverged from a common ancestral population. They have also suggested that Neandertals and ancient modern humans interbred, and that some distinct modern populations have more Neandertal ancestry than others.

In a 2010 benchmark study conducted by a consortium of scientists and institutions, researchers compared and analyzed a Neandertal genome constructed from samples taken from the bones of three Neandertal individuals excavated at the Vindija Cave in Croatia. A genome is an organism's complete hereditary information as encoded encoded in DNA. They compared the genome with modern human genomes from a sampling of present-day human groups from different parts of the world. What they found was a number of genetic variants in regions along the genome that both Neandertals and modern humans shared as a result of positive natural selection, "including genes involved in metabolism and in cognitive and skeletal development". [1]
The evidence suggested some additional conclusions. One of them deals with the long-standing debate about when modern humans and Neandertals diverged in the time-line of evolution. Examination of the new data now indicates that the split took place between 270,000 and 440,000 years ago, "a date that is compatible with some interpretations of the paleontological and archaeological record" and a common ancestor that lived within the last 500,000 years. [1]

Equally significant is the answer they found regarding how the Neandertal genome variants ended up in the modern human genome in the first place.

"A challenge in detecting signals of gene flow between Neandertals and modern human ancestors," state the study authors in the report, "is that the two groups share common ancestors within the last 500,000 years, which is no deeper than the nuclear DNA sequence variation within present-day humans. Thus, even if no gene flow occurred, in many segments of the genome, Neandertals are expected to be more closely related to some present-day humans than they are to each other. However, if Neandertals are, on average across many independent regions of the genome, more closely related to present-day humans in certain parts of the world than in others, this would strongly suggest that Neandertals exchanged parts of their genome with the ancestors of these groups." [1]

In other words, if there are differences in the degree to which different geographically dispersed present-day population groups show Neandertal ancestry, this would suggest that Neandertals and ancient modern human ancestors interbred.

*We performed this test using eight present-day humans: two European Americans (CEU), two East Asians (ASN), and four West Africans (YRI) We find that the Neandertals are equally close to Europeans and East Asians....... However, the Neandertals are significantly closer to non-Africans than to Africans." [1]

The researchers' best explanation for these findings is that the Neandertals exchanged genes with the ancestors of non-Africans. Eurasian Neandertals interbred with ancient modern humans. But, the study authors continue, "the actual amount of interbreeding between Neandertals and modern humans may have been very limited, given that it contributed only 1 to 4% of the genome of present-day non-Africans". [1]

What is more, they were able to determine the relative time in which the mixing began:

A striking observation is that Neandertals are as closely related to a Chinese and Papuan individual as to a French individual, even though morphologically recognizable Neandertals exist only in the fossil record of Europe and western Asia. Thus, the gene flow between Neandertals and modern humans that we detect most likely occurred before the divergence of Europeans, East Asians, and Papuans. This may be explained by mixing of early modern humans ancestral to present-day non-Africans with Neandertals in the Middle East before their expansion into Eurasia. Such a scenario is compatible with the archaeological record, which shows that modern humans appeared in the Middle East before 100,000 years ago whereas the Neandertals existed in the same region after this time, probably until 50,000 years ago. [1]

Any canoodling, then, may have actually started before the two species encountered each other in what is now present-day Europe or West Asia.

The picture becomes more complicated with later studies, which have shown some interesting new details. John Hawks, Associate Professor of Anthropology at the University of Wisconsin—Madison, reports the results of a study of the comparison of East Asian region samples (Japanese, Han Chinese in Beijing, and Han Chinese originating in South China) with European region samples (Tuscans, British, Finn, CEU, and Spanish) taken from the 1000 Genomes Project in his weblog entitled The Malapa Soft Tissue Project. Here he concludes that "the Europeans average a bit more Neandertal than Asians", suggesting that "Europeans may have mixed with Neandertals as they moved into Europe, constituting a second process of population mixture beyond that shared by European and Asian ancestors". [2] More interesting still were the differences detected among the samples within each of the two regions. Within the East Asian region, the North China population was found to have more Neandertal indicators than the South China, and within the European region, the southern populations more than the northern, with the Tuscans having "the highest level of Neandertal similarity of any of the 1000 Genomes Project samples". [2] Hawks relates the results of research on African populations, as well, which also show variability. He points to the Yoruba people, a West African population, having significantly more Neandertal genome similarity than the Luhya, an East African population. "We now know from examination of genetic variation within Africa today," states Hawks about the possible implications, "that some of today's diversity can be traced to ancient population structure in Middle Pleistocene African populations. For example, Neandertals could be more closely related to some African populations than others today because Neandertals actually exchanged genes with some ancient African populations. Or Neandertals might have sprung from one African population among many who lived 250,000 years ago..........As we combine the archaic genome data with our growing picture of diverse lineages in Africa today, we may discover ancient populations that are not apparent archaeologically. Again, genetics is giving us a totally new picture of the diversity and population dynamics of ancient people." [2]

Genome research continues to provide new findings through an expanding source of shared data, affording new details. Looking forward, Hawks asks the next question: "Which Neandertal-derived variants are shared between regions, and which are unique to one region?......Now, we have sequences capable of telling us much more." [2]


[1] Green, Richard E.,, A Draft Sequence of the Neandertal Genome, Science 7 May 2010, Vol. 328 no. 5979 pp. 710-722 DOI: 10.1126/science.1188021.

[2] John Hawks, The Malapa Soft Tissue Project, Weblog at

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