Plenary lecture III: branching processes and the problem of admixture of neanderthal mitochondrial DNA to Archaic H. Sapiens
Authors: Krzysztof A. Cyran
Venue: Proceedings of the 2nd conference on European computing conference (2008)

A lot of relevant discoveries have been recently made in the area of origin of modern humans. The first goal of the lecture is to review some known facts about the relationship between populations of H. neanderthalensis and Upper Paleolithic H. sapiens. The second goal is to infer the upper limit of hypothetical Neanderthal mitochondrial DNA (mtDNA) admixture, consistent with the testimony of sequences genotyped from contemporary modern humans and fossils of Neanderthals and archaic H. sapiens living at the same time in Europe. Because the PCR amplification of mtDNA present in one cell in multiple copies is relatively easy, the mtDNA-based inferences have became an important source of knowledge about origin of H. sapiens. Therefore, successful sequencing of the mtDNA from Neanderthal fossils can be considered as the milestone in revealing our evolutionary paths. For example, until recently, the estimation of the mtDNA mutation rate could rely only on human-chimpanzee divergence data. However, due to relatively long time to this divergence, all estimates of this time were very inaccurate ranging from 4 to 9 million years. Consequently, estimated mutation rate could not be accurate and it is also true for mitochondrial Eve dating. Situation changed after 1997, when for the first time the mtDNA from H. neanderthalensis dated to be alive about 40,000 years ago was sequenced. However, only fewer than 400 base pairs were sequenced, hence any estimates based on this data were not very reliable. The next successful sequencings of Neanderthal mtDNA in 1999 and 2000 confirmed the accuracy of the first experiment. In 2004 the four additional Neanderthal fossils yielded mtDNA sequences together with five early modern humans fossils and the results were corroborating with previous sequencing efforts. What is also important, some fossils sequenced by Serre et al. in 2004, contained examples named Vandija 77, Vandija 80, Mladec 25c and Mladec 2, considered by multiregionalists as "transitional" forms between Neanderthals and archaic modern humans. Yet the mtDNA proved to be of Neanderthal type for Vandija fossils considered as Neanderthals, and of modern human type for Mladec fossils, considered as modern humans. This is exactly what is expected by recent out-of-Africa model of modern human evolution. Serre and his colleagues, apart from reporting these results, tried to estimate the upper limit of possible Neanderthal admixture to Upper Paleolithic H. sapiens, consistent with mtDNA record. The numerical value of the estimate equal to 25 percent is given only for the simplest case of population with constant size, known, however, to be unrealistic. In the lecture it will be shown how to estimate similar limit using branching processes and computer simulations methodology. Obtained results further reduce the maximum hypothetical Neanderthal mtDNA admixture to early modern humans gene pool to about 15 percent. Remarkably, branching processes have been recently used also for inferring the age of the primate last common ancestor, basing on archeological stratification and the number of species known to be alive in a given period. Finally, the consequences of the maximum admixture of 15% for two competing theories of modern human origin will be discussed.