Saturday, 9 February 2013

BH-1 hominin mandible from Serbia suggests Neanderthals evolved in isolation in Western Europe

BH-1 is a left fragment of a human mandible (lower jawbone), complete with all three molar teeth. It was recovered in 2005 at Mala Balanica cave, Serbia, along with a number of quartz artefacts. Originally estimated to be around 115,000 years old (Roksandic, et al., 2011), it is now believed to be at least 400,000 years old. Newly obtained ages, based on electron spin resonance combined with uranium series isotopic analysis, and infrared/post-infrared luminescence dating, yielded a minimum age of between 397,000 to 525,000 years old. This date makes BH-1 one of the oldest hominins in Europe, and the most easterly European hominin of the Middle Pleistocene (Rink, Mercier, Mihailovic, Morley, Thompson, & Roksandic, 2013).

Middle Pleistocene hominins from the period 600,000 to around 200,000 years ago are conventionally lumped together as Homo heidelbergensis (or Archaic Homo sapiens). By the end of this period, the Neanderthals had emerged from the European populations and modern Homo sapiens from the African populations. In fact, the reality of the situation is far from understood and was almost certainly far more complicated.

The problem with Homo heidelbergensis is as Archaic Homo sapiens it came to be used as a kind of ‘wastebasket category’ for anything that wasn’t Homo erectus, a Neanderthal, or a modern human (Cameron & Groves, 2004). Consequently, it tends to be defined in terms of features intermediate between Homo erectus and later humans rather than unique traits, which are a prerequisite for properly defining a species (Harvati, 2007). The situation has been referred to by anthropologist Phillip Rightmire (1998) as the ‘muddle in the middle’ and there is much debate as to whether Homo heidelbergensis is indeed a single species.

BH-1 is a potentially important piece in the jigsaw. It differs significantly from European hominins generally classified as Homo heidelbergensis. It shows a complete lack of the incipient Neanderthal traits that are present for most Western European hominins of the Middle Pleistocene. Instead, it shows primitive Homo erectus-like traits (Roksandic, et al., 2011).

This data suggests that the Neanderthals may have arisen solely in Western Europe, only later spreading to Southeast Europe and Southwest Asia. During glacial periods, Western Europe was cut off from the rest of Eurasia, and the distinctive morphology of the Neanderthals may have evolved in isolation. The process may have been driven by genetic drift impacting on small isolated proto-Neanderthal populations, rather than the effects of Darwinian natural selection (Weaver, 2009). Genetic drift refers to random changes in the relative frequency in which an allele occurs in a population. In small populations, over a number of generations, the effect can result in some alleles becoming fixed and others disappearing altogether, even if the prevailing alleles confer no particular selective advantage on their possessors. An analogy for genetic drift is seen in small isolated villages where everybody ends up with the same surname. If for example Mr and Mrs Smith are the only Smiths in the village and they have only daughters, then the surname Smith will disappear from the next generation. Over enough generations, the villagers will ‘drift’ to just one surname.

In contrast to Western Europe, the Balkan Peninsula was never isolated, and early humans there remained biologically similar to those from Southwest Asia. Accordingly, the population inhabiting the Balkan Peninsula could have retained a number of primitive non-Neanderthal traits, without precluding morphological changes associated with increased brain size and tooth reduction observed in Middle Pleistocene populations throughout Eurasia and Africa (Rink, Mercier, Mihailovic, Morley, Thompson, & Roksandic, 2013).

Rink et al (2013) New Radiometric Ages for the BH-1 Hominin from Balanica (Serbia): Implications for Understanding the Role of the Balkans in Middle Pleistocene Human Evolution may be downloaded from the open-access PLoS One website.

Cameron, D., & Groves, C. (2004). Bones, Stones and Molecules: “Out of Africa” and Human Origins. Elsevier Academic Press.

Harvati, K. (2007). 100 years of Homo heidelbergensis - life and times of a controversial taxon. Mitteilungen der Gesellschaft für Urgeschichte, 16, 85-94.

Rightmire, P. (1998). Human Evolution in the Middle Pleistocene: The Role of Homo heidelbergensis. Evolutionary Anthropology, 6(6), 218-227.

Rink, W., Mercier, N., Mihailovic, D., Morley, M., Thompson, J., & Roksandic, M. (2013). New Radiometric Ages for the BH-1 Hominin from Balanica (Serbia): Implications for Understanding the Role of the Balkans in Middle Pleistocene Human Evolution. PLoS One, 8(2).

Roksandic, M., Mihailovic, D., Mercier, N., Dimitrijevic, V., Morley, M., Rakocevic, Z., et al. (2011). A human mandible (BH-1) from the Pleistocene deposits of Mala Balanica cave (Sicevo Gorge, Nis, Serbia). Journal of Human Evolution, 61(2), 186-196.

Weaver, T. (2009). The meaning of Neandertal skeletal morphology. PNAS, 106(38), 16028–16033.

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