Did Neandertals and modern humans interbreed?

Ever since William King proposed the taxonomic designation Homo neanderthalensis in 1864, there has been intense debate as to whether Neandertals represent a distinct species from us. Species, as defined by the biological species concept, are populations of organisms that can potentially interbreed and have fertile offspring. It is believed that the lineage leading to Neandertals and modern humans split sometime around 500,000 years ago. For most of their existence Neandertals and early modern humans were geographically isolated (and by extension reproductively isolated) from one another. The big question is whether they could have produced viable offspring when they met.
Today, most researchers acknowledge that some sexual encounters could have occurred between Neandertals and modern humans. The more interesting question is how common were these encounters and did they leave their mark on the modern gene pool. Undoubtedly, modern humans and Neandertals would have recognised each other as fellow humans but this does not mean that they would have acted humanely to each another. Countless social and psychological studies have shown humans to have a very strong “us versus them” mentality, that no doubt also existed in our ancestors. It is unlikely that modern humans and Neandertals had an easy relationship. Most sexual encounters that took place between the two were likely opportunistic and probably involved enslavement and rape.

The morphological evidence

Palaeoanthropologists generally have little problem seperating Neandertals and modern humans based on their gross morphologies. However, some of the earliest modern humans from central Europe have traits that have been seen as evidence for continuity between them and Neandertals. These fossils, particularly those from Peştera cu Oase in Romania and Mladeč in the Czech Republic, have been touted as exemplars for modern-Neandertal admixture. These specimens show traits that are seen in high frequencies in Neandertals, such as bunning of the occipital and the presence of a suprainiac fossa.

However, many researchers have questioned whether these traits are in fact distinctly Neandertal. For instance, the form of the occipital seems to be different in early Upper Palaeolithic populations, leading many to favour the term hemibun to describe the shape of the occipital in early Europeans. Lieberman and colleagues has gone as far as to suggest that the buns seen in these two groups are not homologous. Similarly, it has been argued that the shape of the suprainiac fossa is distinct in early modern Europeans compared to Neandertals.

A palpable difficulty in assessing proposed Neandertal traits in early modern humans is that both groups shared similar niches and some traits may be the result of lifetime behavioural adaptations or convergent evolution. Indeed, the shared robustness of these early humans is likely due to the higher physical activities of these Late Pleistocene groups than during later period.

The genetic evidence

Mitochondrial DNA (mtDNA) has some characteristics that make it ideal for analyses of ancient specimens. MtDNA is found in abundance – cells can have thousands of copies of mtDNA, while only containing two copies of nuclear DNA. Moreover, its structure and location within the cell make it more resistant to decay. All the studies of Neandertal mtDNA to date cluster outside the range for modern human mtDNA variation. However, the mitochondria contain only a small part of the total DNA that make up a genome. The possibility that Neandertal genes could show up somewhere else in the genome cannot be ruled out.

The recent announcement by Svante Pääbo that he is sure that Neandertals and modern humans had sex is quite a bold pronouncement coming from a scientist. It raises the question of whether this ascertain is based on some hard evidence they found while sequencing the Neandertal genome. It is possible that if there was some Neandertal genes passed on to the first moderns in Europe, they could have got eliminated from the subsequent gene pool as population sizes fluctuated during the more severe climatic episodes. A more likely scenario is that Pääbo’s team found evidence of modern introgression in the Neandertal genome. In all likelihood the incoming modern humans were more numerous than the Neandertals, thereby absorbing the endemic populations through genetic swamping.

References

Caspari RE. 1991. The evolution of the posterior cranial vault in the central European Upper Pleistocene. PhD dissertation. Ann Arbor, MI: University of Michigan.

King, W., 1864. The reputed fossil man of Neanderthal. Quarterly Journal of Science 1, 88–97.

Krings et al. 1997. Neandertal DNA sequences and the origin of modern humans. Cell vol. 90 (1) pp. 19-30.

Krings M, Capelli C, Tschentscher F, et al. 2000. A view of Neandertal genetic diversity. Nat Genet 26, 144–146.

Lieberman et al. 2000. Basicranial influence on overall cranial shape. J. Hum. Evol. vol. 38 (2) pp. 291-315.

Nara MT. 1994. Etude de la variabilité de certainscaractères métriques et morphologiques des Néandertaliens. Bordeaux: Thèse de Docteur.

Pääbo S, Poinar H, Serre D, et al. 2004. Genetic analyses from ancient DNA. Ann Rev Genet 38, 645–679.

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Above photos modified from originals by erix! and fangleman under creative commons license.

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