Tuesday, 29 September 2009

Out of Europe?

Three weeks ago, I criticised the “quality” papers (the “Daily Telegraph” was an exception) for running a story based on a two-year-old paper about Homo georgicus as fresh news and for hyperbolic headlines about “rewriting the history of man”. True to form, the media have now completely missed out on an article published in Nature which does – if its conclusions are correct – have a substantial bearing on how we view the path of human evolution. The paper appeared in the 3 September edition of the journal, but due to a postal service that would be considered a disgrace in Somalia, it has only just reached me.

The paper entitled “The oldest hand-axes” in Europe is by Gary R. Scott and Luis Gibert of the Berkeley Geochronology Center, Berkeley, CA. It reports the re-dating of Acheulean hand-axes from Solana del Zamborino and Estrecho del Quipar, both in south eastern Spain. Both sites were previously considered to be among the younger Acheulean sites on the Iberian Peninsula. An age of approx 200ky assumed for Solana was based on its well-developed Acheulean technology. However Scott & Gibert reported revised dates of 0.9mya and 0.76mya for Solana del Zamborino and Estrecho del Quipar respectively, based on magnetic polarity considerations (Scott & Gilbert, 2009).

The oldest Acheulean hand-axes are around 1.65my old, from West Turkana in Kenya and by 1.4mya their usage was fairly widespread in Africa so on the face of it the fact that hand-axes had reached Spain by 0.9mya is hardly earth-shattering, albeit interesting. However while Acheulean hand-axes were once described as displaying a “variable sameness” that strikes “even enthusiasts as monotonous”, later hand-axes do appear to be more refined than earlier ones, which tend to be much thicker, less extensively trimmed and less symmetrical. The “evolved Mode 2” technology is not seen in the archaeological record in Africa until after 600,000 years ago, 300,000 years after its appearance in Spain.

The significance of this is that while even the earlier hand-axes probably represented a cognitive advance over that required to produce the early Oldowan tools, the later hand-axes possess three-dimensional symmetry that may imply a further cognitive advance as the tool would have had to be viewed and rotated through the mind’s eye while it yet remained a block of un-worked stone (Klein, 2005). The date of 600,000 years ago coincides with the first appearance in the fossil record of Homo heidelbergensis, which while slightly smaller-brained than a modern human, was considerably better endowed in that department than its supposed ancestor, Homo erectus. Homo heidelbergensis (or archaic Homo sapiens as it was known until fairly recently) is believed to be the common ancestor of both modern humans and the Neanderthals. Its bigger brain may have helped it to master the harsh conditions in Europe, which was periodically affected by episodes of glaciation. It could also have been responsible for the refinement in hand-axe making technology.

One problem is that there is no obvious evolutionary cause for this expansion in brain size if we accept, as is generally assumed, that Homo heidelbergensis evolved in Africa. The earliest human species, Homo habilis, possessed a larger brain than the australopithecines it is supposed to have evolved from. Its appearance coincides with the start of the present series of ice ages, 2.6 million years ago. Similarly Homo erectus – which also represented a cognitive step-up from its predecessor – appeared around 1.8-1.7mya during a further deterioration in the climate. But no such event seems to signal the appearance of Homo heidelbergensis.

While it would be a mistake to think there is agreement on the matter, a widely-held view is that Homo heidelbergensis evolved in Africa and migrated into Europe. The European deme eventually became the Neanderthals while the stay-at-home African deme eventually became Homo sapiens. Homo heidelbergensis was not the first human species to colonise Europe, but it was the first to stay there and reach more northerly places such as England and Germany. Previous migrants seem to have eventually been extinguished such as Homo antecessor, known from Level TD6 of the Gran Dolina cave, Atapuerca, Spain or back-migrants from Asia, en route back into Africa such as Homo cepranensis, known from a single skull found near Ceprano, Italy. The technology of the TD6 people was pre-Acheulean, that of the Ceprano people unknown. Both “species” were probably Homo erectus (broadly defined) and they were in Europe at about the same time as the makers of the Solana tools.

If it is the case that a) the re-dating is correct and, b) the “2g Acheulian” Solana tools were beyond the capabilities of Homo erectus then another human species must have made them. One possibility is Homo antecessor, but the problem is that while this species is said to have been larger-brained than “standard” Homo erectus (Bermudez de Castro, Arsuaga, Carbonell, Rosas, Martınez, & Mosquera, 1997) it lacked even the older-style Acheulean tools (Carbonell, et al., 2008), though we cannot rule out the possibility that these might at some stage come to light.

A more radical possibility is that a Homo erectus group equipped with Acheulean technology migrated into Europe and – unlike other migrants – managed to adapt to the harsher conditions. These provided the selective pressure for brain expansion and the migrant population eventually evolved into Homo heidelbergensis which then ranged far and wide throughout Eurasia, eventually evolving into the Neanderthals. But one group moved back into Africa and eventually became Homo sapiens. Thus the immediate ancestors of modern humans were actually a species of European origin rather than African.

© Christopher Seddon 2009
Bermudez de Castro, J., Arsuaga, J., Carbonell, E., Rosas, A., Martınez, I., & Mosquera, M. (1997). A Hominid from the Lower Pleistocene of Atapuerca, Spain: Possible Ancestor to Neandertals and Modern Humans. Science , 276, 1392-1395.
Carbonell, E., Bermudez de Castro, J., Pares, J., Perez-Gonzalez, A., Cuenca-Bescos, G., Olle, A., et al. (2008). The first hominin of Europe. Nature , 452, 465-470.
Klein, R. (2005). Hominin Dispersals in the Old World. In C. Scarre, The Human Past (pp. 84-123).
Scott, G., & Gibert, L. (2009). The oldest hand-axes in Europe. Nature , 461, 82-85.

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