Bonanza time for Bonzo

The big news of July was without doubt from the palaeoanthropologists; a report on finds at the 1.75 Ma Dmanisi site in Georgia (Vekua, A. and 11 others 2002.  A new skull from Dmanisi, Georgia.  Science, v. 297, p. 85-89.), and the unveiling of a hominid-like skull from Chad dated at 7 Ma (Brunet, M. and 37 others 2002.  A new hominid from the Upper Miocene of Chad, central Africa.  Nature, v.  4418, p. 145-151).  Both threw the issues of human origins, evolution and migration back into the arena of debate.

Time and Newsweek, and once upon a time Life magazine often figure celebrities of the week or month on their covers.  Nature entered the celebrity cult on 11 July with a front-page photo of the magnificent cranium of Sahelanthropus tchadensis’ holotype found and analysed by a vast team from France, Chad, USA, Switzerland and Spain.  The skull is from Upper Miocene sediments around Lake Chad, dated from their varied fauna which is very like that of similar sediments in Kenya.  Its hominid credentials stem from the skull’s face, jaw and teeth, but it is odd.  From the back, it resembles a chimp, and so does the capacity of its brain case.  From the front, it bears close resemblance to an advanced Australopithecine.  Yet no limb bones have been recovered so far, and the attachment point of the skull to its backbone is not mentioned.  Both features would be needed to prove upright gait.  Undeterred, the authors and many commentators are convinced that it is the oldest human ancestor, from the very limit in time at which modern genetic analyses suggest that the human “bush” of descent parted from that which led to modern chimpanzees.  Bernard Wood of George Washington University (Wood, B. 2002.  Hominid revelations from Chad.  Nature, v.  418, p. 133-135) discusses Sahelanthropus’ significance to human evolution, implying that it poses problems for both the linear model of descent from a single emergence of basic human anatomy and the “untidy” model, to which he subscribes – adaptive radiation to changed circumstances that occurred more than once.  In the “untidy” model, even an excellent-looking candidate for the first in the line may not have been ancestral to us.

Palaeoanthropologists have never been as well-endowed with bones as they are with funds, and one detects hints of the protectiveness that has long plagued the discipline.  The finders of the previous candidate for the first hominid – Brigite Senut and Michael Pickford of the Natural History Museum in Paris (Taking stock of hominid evolution, Earth Pages News, March 2002) who found Orrorin tugenensis, in 5.72 to 5.88 Ma sediments of the Tugen Hills in the Kenyan Rift – claim that Sahelanthropus is merely an ancestral gorilla, citing the creature’s large canines.  Without a pelvis or footbones to back up the hominid claim, they could well be right.  However, the good news is that East Africa has lost its primacy as the source of fossils bearing on human evolution.  Being 1500 km west of the nearest previous site, and unrelated to the East African Rift system.  The new sites in Chad open up a vast area for future searches of potentially fruitful Miocene sediments, that are neither abundant nor complete in the Rift (its formation is post-Miocene).

Georgia in the former Soviet Union has grown in significance since the first reports of very old human remains near Dmanisi, a decade ago.  The site is well preserved, contains abundant mammalian remains, and the containing strata overlie a 1.85 Ma basalt.  With supplementary palaeomagnetic stratigraphy, Abesalom Vekua and his colleagues from several Georgian institutions, the USA, Spain and Switzerland have narrowed the age of the site to 1.75 Ma.  Their new find is a superbly preserved  skull, together with a lower jaw, following earlier discoveries of two other cranial fossils.  The site is well endowed with stone artefacts, similar to those of the Oldowan culture of East Africa. 

The new skull has a smaller brain capacity than co-eval H. ergaster or H. erectus in Africa, and bears some resemblance to the earliest species of human, H. habilis, although the authors prefer not to muddy the waters with yet another species of Homo.  However, had this skull been found first, they might well have gone for H. habilis, and in the paper suggest that it and the others may have descended from habilines that left Africa some time before they were preserved.  As with Sahelanthropus, no limb bones have been found at Dmansi so far.  The three fossils are not identical, and another important possibility is that these humans, like us, were polymorphic, though this needs to be tempered with the possibility of differences between males and females, or that the smallest may have been adolescent.  Others have jumped on the differences to suggest that more than one species are represented.  Here we see the problem of meagre evidence, so that anatomy alone permits either “lumping” or “splitting”.  Jonathan Kingdon, in his book Self made man and his undoing (1993, Simon and Schuster) raised the issue of polymorphism, so characteristic of modern humans, to the consternation of most palaeoanthropologists, who remain largely silent on its implications for the whole issue of human classification.

There is no doubt that early humans with primitive tools were able to expand out of Africa as early as 1.75 Ma ago.  They were not well-endowed with brain power, and they were little people – they did not stride purposefully into the wide, blue yonder.  That they reached Georgia, of all places, is extremely odd, because a direct route from Africa is barred by the Caucasus mountain range, and the deserts of Syria and Iraq.  They might have tramped around the coast of Asia Minor, following the Dardanelles to the Black Sea coast and then into the Georgian plains.  A more extreme possibility is that first they crossed the Straits of Bab el Mandab (closed at the time) and, in Kingdon’s words, “standloped” to east Asia and the backtracked along the northern flanks of the great mountains of Asia to reach the steppes.  Finds of Oldowan artefacts and meagre human remains in China also provide ages around 1.8 Ma.

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