The Sterkfontein Caves near Johannesburg in South Africa have provided some of the best preserved hominid remains, because they are enveloped in chemically precipitated cement. Fossils are also much more plentiful than at other sites, and the caves have yielded about 500 specimens. However, unlike sites in bedded sediments interleaved with volcanic horizons, cave deposits are difficult to date accurately. Up to now, correlation of other fossil animals in the breccias that encase Sterkfontein hominids with those at more amenable sites, together with dating based on palaeomagnetic reversals, have been hotly disputed. A new technique based on the radioactive decay of isotopes that cosmic-ray bombardment induces in quartz grains promises to resolve the paradox of wonderful fossils that cannot be dated. While quartz grains are at the surface, in alluvium or the debris on slopes, cosmic rays produce radioactive aluminium and beryllium isotopes in a fixed proportion. The longer the exposure time, the more radioactive isotopes are produced. But if such irradiated grains are buried, the isotopes decay away, because they are protected by overlying material. Detrital sediments enter cave systems very quickly, so they are near-ideal for the use of cosmogenic dating. Of the two most-used isotopes, 26Al decays quicker than 10Be. So, the 26Al/10Be ratio decreases with time and gives a measure of how long the sediment has been buried. Results from Sterkfontein (Partridge, T.C. et al. 2003. Lower Pliocene hominid remains from Sterkfontein. Science, v. 300, p. 607-612) show that the stratigraphically lowest fossils are much older than previously thought; around 4 Ma.. Previous age estimates suggested that the oldest Sterkfontein hominids lived around the same time as Australopithecus afarensis, of which the famous “Lucy” skeleton was an Ethiopian member. Four million years ago A. anamensis would have been a contemporary, yet the hominids at Sterkfontein seem quite different anatomically. Maybe there were two species in Pliocene Africa, one East African and the other a southern one. In fact, there are hints that perhaps two species of australopithecines, along with a more robust paranthropoid may have been washed into the caves. There are two problems though: cosmogenic dating is notoriously imprecise (the age reported is 4.2±0.3 Ma), and Sterkfontein has such excellent preservation that the number of specimens outweighs those from elsewhere – comparisons are not easy!
Tracking migrations with language
One of the first surprises that arose when genetic relatedness among living people and the estimated time of their separation began to encompass global populations was how well the genetic patterns matched with the distribution of the world’s languages. When populations move they not only carry their genetic heritage but their languages. Probably the greatest migrations in human evolution took place at the end of the last Ice Age, and so it might seem that plotting language distribution ought to chart the paths these wandering people took. Jared Diamond and Peter Bellwood (Diamond, J. & Bellwood, P. 2003. Farmers and their languages: the first expansions. Science, v. 300, p. 597-603) have reviewed just how complex such a task will be. Genes and language can tell only part of the story, because people carry skills and culture too. The two dominant cultures around 11 000 years ago were the age-old ways of the hunter-gatherer and the new agriculture and animal husbandry. There are at least five possibilities involved. Genes, language and lifestyle could mix between both groups when they came into contact. Hunters might take up farming but keep their identity. Hunters were as likely to shift as farmers when climate belts changed. Powerful incomers might impose their language but not their genes. When one group moved, another might take its place. Bearing in mind these caveats, Diamond and Bellwood review the main patterns of linguistic groups, using excellent graphics.