Hominid evolution: a line or a bush?

From the late 19th century it has been clear that two species of our genus Homo inhabited Europe and the Middle East: modern humans and Neanderthals. Recent partial sequences of Neanderthal genetic material, compared with the human genome, confirm that the two did not interbreed; at least, no trace of Neanderthal genetics remains in that of modern humans. The discovery in Indonesia that fully modern immigrants occupied the same territory as Homo erectus from 70 to 20 thousand years ago adds more weight to the hypothesis of multiple occupancy of the world by different kinds of humans until recent times. The astonishing discovery in 2003 of the remains of tiny hominids (Homo floresiensis) on Flores whose occupancy lasted from at least 840 ka to as recent as 12 ka (see The little people of Flores, Indonesia, November 2004 issue of EPN) confirms mixed occupancy late in hominid evolution. That includes several different representatives of Homohabilis, eragster and erectus – and also paranthropoids in Africa around 2 Ma years ago. As regards Homo, this cohabitation, especially that in Africa, supports two hypotheses: that our lineage was bush-like and involved separate extinctions and sudden appearances of new species (cladogenesis), or that the great variability in physiognomy (polymorphy) of modern humans extended back for a considerable time. The second is the view of Jonathan Kingdon, who believes insufficient hominid fossils have been collected to rule out polymorphism among tool-using and tool-creating beings. The idea of a single lineage since the first appearance of bipedal apes that led unerringly through gradual changes to modern humans (phyletic evolution) has been largely discarded. For at least part of the 6-7 Ma hominid record, that abandonment of phyletic evolution may have to be reconsidered, following a report of remarkably productive excavations in the Awash Valley of NE Ethiopia (White, T.D. and 21 others 2006. Asa Issie, Aramis and the origin of Australopithecus. Nature, v. 440, p. 883-889).

The Middle Awash is the single most productive area for hominid remains and other fossils that help establish changes in their environment. That is so because of consistent collecting for more than two decades by a multinational team, co-led by Ethiopian and US palaeoanthropologists, from a sequence of flood plain sediments over 1 km thick, liberally interlayered with dateable volcanic horizons. Its middle parts record three species, Ardepithecus ramidus, Australopithecus anamensis and Australopithecus afarensis (of which ‘Lucy’ was a member), in an age range from 4.42 to 3.88 Ma. White and the other members of the team have unearthed 30 new fossils of all three species, but, so far, no examples of more than one in a particular thickness of sediments. Of course, ‘absence of evidence is not evidence of absence’, but this massive addition to the Pliocene hominid record is a challenge to the prevailing hypothesis of cladogenesis – Steven J. Gould’s idea of punctuated equilibrium, in which species arise by sudden appearance of new characteristics from earlier ancestors. Its test is whether or not ancestral species co-exist with new species for a time. In the Middle Awash, it seems that they do not, even though the critical 300 m of sediments represents only 200 thousand years.

The three species, and their predecessor Ardepithecus ramidus kadabba (5.5-5.8 Ma), show variations in their teeth, with Ar. r. kadabba and Ar. ramidus sharing some similarities, and Au. anamensis and Au. afarensis others. The shift between the two sets of common dentition can be explained by either gradual changes in a single lineage over about 2.5 to 3.0 Ma, or a sudden speciation event, perhaps around 4.5 Ma. The lack of overlap favours the first hypothesis. Complicating factors are rife, however, for there may have been migrations (Ar. Ramidus is known from far to the south in Kenya), and yet more evidence will undubtedly be found from the vast amount of sediment of this age in the Afar Depression.

See also: Dalton, R. 2006. Feel it in your bones. Nature, v. 440, p. 1100-1101.

Palaeodentistry

Those of a nervous disposition should not read this item.

A 7500 to 9000 year-old Neolithic graveyard in Pakistan has yielded remains of about 300 people who cultivated wheat, barley and cotton, and herded cattle. There is nothing remarkable in that, except that nine individuals have teeth that have clearly been drilled neatly (Coppa, A. et al. 2006. Nature, v. 440, p. 755). The holes are between 1-3 mm in diameter and up to 3.5 mm deep, and would have exposed sensitive parts of the tooth. In excavations of the nearby village of Merhgarh are found tiny flint drill heads associated with beads of various ornamental materials. The drills are of the same size as the tooth holes. Quite probably, miniature bow-drills tipped with flint would have been used by Neolithic dentists for at least 1500 years – there is no evidence for tooth drilling from younger cemeteries in the area, despite abundant evidence of dental decay. Experiments show that such drills would take less than a minute to produce the neat holes, probably wielded by jewellers rather than dentists.

Asian Homo erectus skilled in tool making

The 1.8 Ma emigrants from Africa who first populated the Far East have not been regarded as having been especially inventive. While their ‘cousins’ in Africa developed the aesthetically stunning bi-face axe about 1.6 to 1.4 Ma ago (the first instance of visualising a finished object within a rough piece of raw material), H. erectus in East Asia is associated with the most primitive stone tools made by simply breaking flinty stones. That seemed to have been the extent of their stone-using skills up to their final demise about 20 thousand years ago –not a lot of progress in 1.8 million years. A report in March at the Indo-Pacific Prehistory Association Congress (Manila) of yet to be published work by Harry Widianto of Indonesia’s National centre of Archaeology may force a revision of this less than charitable view of early Asians (Stone, R. 2006. Java Man’s first tools. Science, v. 312, p. 361). In the Solo district of Java, made famous by Renée Dubois who found the first fossils of H. erectus there, a wealth of finely worked flake tools has been discovered in sediments that are about 1.6 Ma old. Most are small and made from blood-red to beige, translucent chalcedony. It seems that necessity was the mother of invention in this case, because suitable materials for sharp tools are very scarce in Java.

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