Much of the interpretation of the growing database of human genetic variability has so far focused on migration out of Africa and across the habitable continents. To some extent the largest variability, of Africans themselves, has been undersampled, but a multinational team of Africans and non-Africans has now begun to redress the balance (Tishkoff and 24 others 2009. The genetic structure and history of Africans and African Americans. Science, v. 324, p. 1025-1043) partly to study genetically-linked epidemiology and partly anthropology. The study centres on African’s own ideas about their identity/ethnicity as well as documented cultural and linguistic division, and covers 3194 individuals from 121 populations in the continent, African-American populations in 4 US cities and 60 other populations from outside Africa. The team expands knowledge tremendously, as expressed by the many intricate diagrams. They use the statistical method of Bayesian clustering to tease out the ancestral bases for the genetic patterns preserved by Africans, which appear to be based on 14 major ancestral groups that mostly tally with cultural and linguistic divisions. Overall, the picture is one of repeated mixing of populations through migrations within the continent, many within historic times such as the shift of West Africans south-eastwards, but also much earlier movements such as the ancestors of the San people of southern Africa. These remaining gatherer-hunter people together with central African pygmies and the Hadza and Sandawe of Tanzania share ancestry and also, except for pygmies, language that involves click-sounds – the pygmies abandoned their original language in favour of that of the groups that now surround them in the Equatorial rain forests. Of the three groups, the Hadza most maintain the genetic structure of the earliest ancestors on the continent, but all three shared a common ancestor about 35 Ka ago. Interestingly, comparison with people outside Africa confirms earlier studies that indicated a source population for the out-of-Africa migration in East Africa close to the Red Sea. The paper is necessarily condensed and so difficult to follow, but clearly opens up great vistas in understanding intricacies at which anthropologists have previously only guessed. Like the physical landscape of Africa, that of its population reflects the range of factors that have shaped human evolution and hence a great deal of its destiny.
See also: Gibbons, A. 2009. African’s deep genetic roots reveal their evolutionary story. Science, v. 324, p. 575.
Very old human footprints in Mexico?
In 2006 palaeoanthropologists in the Americas, already at loggerheads about evidence for pre-Clovis (pre 13 ka) colonisation, were rocked to their boots. A team from Liverpool John Moores University, Bournemouth University and the Mexican Geophysics Institute claimed to have found human footprints more than 40 ka old in a volcanic ash deposit (Gonzalez, S. et al. 2006. Human footprints in Central Mexico older than 40,000 years. Quaternary Science Reviews, v. 25, p. 201-222). The extensive site exposed by quarrying carries many apparent footprints, both human and non-human. Moreover, some of the prints are in convincing-looking trackways. The very old date was obtained by optically stimulated luminescence dating of quartz-grains that measures the time since the grains were last exposed to sunlight or thermal baking. Were it not for that result probably little fuss would have been made. Now this remarkable find is under serious challenge (Feinberg, J.M. et al. 2009. Age constrains on alleged ‘footprints’ in the Xa;nene Tuff near Puebla, Mexico. Geology, v. 37, p. 267-270). This US-Mexican team applied Ar-Ar dating to the ash and found an age of about 1.3 Ma, confirmed by its association with reversed magnetic polarity in the deposit – at 40 ka the geomagnetic field was as it is today. On that basis, Feinberg and colleagues claim to have refuted the identification of human footprints, and claim that they are merely quarrying marks degraded by later weathering. The Xalnene Tuff in which the footprints were found was deposited in a lake that has been periodically filled and dried out. If the disputed features can be shown irrefutably to be footprints, then there are only two possibilities: either they date from a 40 ka lowstand when the tuff was rewetted and soft, or they are of Homo erectus who somehow found their way to the Americas after leaving Africa around 1.7 Ma ago and crossed the drying lake bed shortly after the tuff was ejected from a nearby volcano.
Bones of at least 6 or 7 small people have turned up in the now famous Liang Bua cave on the island of Flores, Indonesia. Their stratigraphic positions span the period from 95 to 17 ka. There have been numerous claims that they do not represent a dwarfed human species – i.e. Homo floresiensis – but individuals who suffered from some form of pathological condition. The strongest evidence supporting that sceptical view is that the one near-complete skull does not fall on the well-established brain –body-size distribution that covers many species: it seems too small for either a normal pigmy modern human or a similarly diminutive H. erectus. Now crucial new anatomical evidence seems set to swing the balance. (Jungers, W.L. et al. 2009. The foot of Homo floresiensis. Nature, v. 459, p. 81-84; Weston, E.N. & Lister A.M. 2009. Insular dwarfism in hippos and a model for brain size reduction in Homo floresiensis. Nature, v. 459, p. 85-88). The foot bones of the most recent and most complete specimen are not like those of humans but more ape-like, although they show clear evidence of bipedalism. Interestingly, they seem to be more primitive than those of H. erectus, raising the possibility of an undocumented dispersal of perhaps from Africa into Eurasia as an ultimate ancestor. Curiously, the foot is disproportionately long compared with the rest of the skeleton; another bonus for ‘hobbit’ fans. Not having a snout, H. floresiensis certainly was no ape, indeed the skull is best expressed as a scaled-down version of either H. erectus or H. habilis. As to extremely small brain size in relation to the body size of H. floresiensis, insular dwarfism of fossil hippos in Madagascar provides a useful analogue, as Weston and Lister suggest. In adulthood they also have disproportionately small brains. As with many puzzles in human evolution, the stir caused by these new discoveries maintains H. floresiensis as a ‘hot topic’ and further excavations are inevitable – Flores has plenty of caves, as do many islands in the Indonesian chain.
See also: Lieberman, D.E. 2009. H. floresiensis from head to toe. Nature, v. 459, p. 41-42.