Four models circulate within anthropology to explain the origin of Homo sapiens: Out-of-Africa (humans evolved in Africa, some populations migrated out and replaced other hominin species without significant hybridisation), African Hybridisation and Replacement (when human populations migrated out of Africa, some hybridisation with other hominins occurred), Multiregional (humans evolved all over the world but maintained gene flow to keep us one single species) and, Assimilation (humans evolved in Africa but populations migrating out assimilated with other hominins rather than replacing them; Eriksson & Manica 2012: 13956). Mitochondrial DNA and Y chromosome studies have been able to link back all humans to a female and male ancestor in Africa (Neves & Serva 2012: 6), while at the same time showing that African populations are more diverse genetically than non-Africans due to bottlenecking when African populations migrated out (Eriksson & Manica 2012: 13956). This information disagrees with a multiregional model. What has become apparent in the last couple of years is that hybridisation of hominin species may have occurred, with Neanderthals and humans mating in the Middle East before humans migrated on towards Eurasia. This is based upon Neanderthals being more closely related to non-Africans equally but not to Africans (based upon three European Neanderthals and five living humans), hence the admixing would have had to occur before the divergence of the non-African lineages. Only sharing 1-4% of Neanderthal DNA with us (Green et al. 2010: 721) could mean that two hominin populations would only be mating every 12th generation (or a maximum of every 77th generation; Neves & Serva 2012: 2). Although Green et al. (2010: 721) postulate that the small percentage could be because more traces of hominin gene flow has become obscured by other factors.
Although Green et al. (2010: 711) did a thorough job of investigating the Neanderthal genome, they do posit that belief that interbreeding did occur can carry bias when experimenting. In Eriksson & Manica’s (2012: 13958) paper, they argue that their model is compatible in scenarios where no hybridisation occurred (Limitations on “…looking for polymorphisms that are only shared between a single ancient genome and a restricted group of modern populations”), that a migrating northern African human population would have more in common to Neanderthals than Southern Africans because migrating hominin populations ancestral to Neanderthals and humans travelled also from northern Africa to become Neanderthals (Marshal 2012). Obviously, Neanderthals and humans will share similarities from sharing a last common ancestor, but whether Eriksson & Manica’s (2012) model is valid is also under contention. For now, it is inconclusive whether there was interbreeding, although the scale does seem to be tipped towards hybridisation. More research will need to be performed however until there is more conclusive evidence. And does it matter? Yes. If a testable theory is brought to light in any scientific domain, it is worth examining it. If Neanderthals did interbreed with humans, then there is a lot more to discover about species relationships and Neanderthal life than originally thought.
Bibliography:
- Eriksson, A. & Manica, A. (2012). Effect of ancient population structure on the degree of polymorphism shared between modern human populations and ancient hominins. Proceedings of the National Academy of Sciences 109
- Green, R. E., Krause, J., Briggs, A. W., Maricic, T., Stenzel, U., Kircher, M., Patterson, N., Li, H., Zhai, W., Fritz, M. H. Y., Hansen, N. F., Durand, E. Y., Malaspinas, A. S., Jensen, J. D., Marques-Bonet, T., Alkan, C., Prüfer, K., Meyer, M., Burbano, H. A., Good, J. M., Scultz, R., Aximu-Petri, A., Butthof, A., Höber, B., Höffner, B., Siegemund, M., Weihmann, A., Nusbaum, C., Lander, E. s., Russ, C., Novod, N., Affourtit, J., Egholm, M., Verna, C., Rudan, P., Brajkovic, D., Kucan, Z., Gušic, I., Doronichev, V. B., Golovanova, L. V., Lalueza-Fox, C., De la Rasilla, M., Fortea, J., Rosas, A., Schmitz, R. W., Johnson, P. L. F., Eichler, E. E., Falush, D., Birney, E., Mullikin, J. C., Slatkin, M., Nielsen, R., Kelso, J., Lachmann, M., Reich, D. & Pääbo, S. (2010). A draft sequence of the Neanderthal genome. Science 328
- Marshall, M. (2012). Human and Neanderthal interbreeding questioned. Found electronically at http://www.newscientist.com/article/dn22168-human-and-neanderthal-interbreeding-questioned.html, accessed on 27th February 2013, 2:40pm
- Neves, A. & Serva, M. (2012). Extremely rare interbreeding events can explain Neanderthal DNA in living humans. PLoS ONE 7 (10)
Pictures:
Main image from: http://cdn4.sci-news.com/images/enlarge/image_1837e-Neanderthal.jpg
Namuhyou 
To prove that sapiens cannot mate with Neanderthal is in a best case scenario, there are 202 different Mitochondrial base pairs between them and us. Within all of natural speciation, this difference is too extreme to support a viable offspring. Between a Bonobo and Common chimpanzee there are only 75-130 different mitochondrial base pairs and they cannot produce a viable offspring, so why should we believe that 202 differences could and did? (And 202 was the best case scenario.)
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Mmm I am also sceptical of the mating between the two species, more on just the basis that we share a common ancestor and were subject to similar/ same environment (for Eurasians). I find this subject hard to ponder however, when media coverage continually bombards us with the mating event as fact. Thank you for commenting ^^
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