Ancestral lines squashed?

Many of the famous finds of hominid crania, on which ideas of human descent hang, consist of small fragments that have to be glued together to reconstruct their form.  The basic work of palaeoanthropology is very like doing a 1000-piece jigsaw puzzle, but in three dimensions.  Tim White, one of the pioneers of modern studies of hominin fossils, is now worried that the fragmentation of bone is connected with distortion during burial (White, T. 2003.  Early hominids – diversity or distortion.  Science, v. 299, p. 1994-1997).  His own studies of fossil pigs present a disturbing pattern of post-mortem distortion that spurred earlier workers to subdivide them “exuberantly”.  There are even “flat-headed flat pigs” and “narrow pigs” (literally, from their given Linnean names), but they are now known to be mechanically distorted remains of a single early pig.  Hominid crania viewed in this light, and there are nowhere near as many as those of pigs, are a mess.  White gives one example, Kenyanthropus platyops (“flat face”), which may well be a distorted and quite ordinary Australopithecus afarensis.  Combined with the shape variation within living species, notably humans but also among bonobo chimpanzees, distortion throws the bushy tree of human descent into considerable doubt, just as Jonathon Kingdon predicted 10 years ago in his book Self-Made Man and His Undoing.  There are so few hominid remains, and most are a mess, that it seems impossible to decide whether many hominin species existed together at any one time in the Late Miocene to Early Pleistocene, or that just a few (even one?) spread to many different habitats across the face of Africa; something of a bombshell for those who make a tidy living from skull-hunting and hominin cladistics.

Walking with Slade

Imagine, if you will, the Pliocene savannah of East Africa and a band of upright apes (Australopithecus afarensis), each (even the females) with the trademark sideburns of Noddy Holder.  Imagine too that peeping from the bush is a voyeuristic obstetrician who resembles Groucho Marx, drinking a hot beverage (Cuppasoup?) from a flask, and trying ever so hard to get one over on Whispering David (Attenborough).  There is a story here, because one of the apes is Lucy, who gets clobbered in Pliocene Slade’s fracas with a rival band (Staus Quo?), her infant falling into the long grass.  Her sister rescues the child, and all is well on the long road to humanity.  That was the first episode of the BBC’s Walking With Cavemen, the third series aimed at popularizing palaeontology, which began with Walking with Dinosaurs.  All three owe as much to Bambi and Dumbo as they do to computer animation and modern research, despite the best efforts of the numerous scientific advisors.  I saw the trailer for the next episode, concerning Homo ergaster – quite apt, because that was “Action Man”, that was.  Not only were they white with tangled grey locks, but despite the brow ridges it was hard to conceal the fact that they were Pan’s People and the Chippendales striding purposefully across a salt pan.  Did even female H. ergasters have 6-packs?  Physically arousing it may have been, again leaving out the brow ridges, the bad barnets and table manners, but I thought, “Tripe”, and watched the footy the following week.  (Note: “barnet” – rhyming slang for hair, from Barnet Fair).

A genetic key to human evolution?

It will not be too long before the publication of the chimpanzee genome.  Because chimps are our closest relatives, and we shared an ape ancestor about 5 to 7 Ma ago, there is bound to be a media hullabaloo (and agitation among creationists) on the day of the release.  At first sight, a comparison of human and chimpanzee genomes might seem to offer plain clues about the genetic side of our co-evolution, but evolutionary biologists are not so optimistic about an imminent breakthrough (Carroll, S.B. 2003.  Genetics and the making of Homo sapiensNature, v. 422, p. 849-857).  Their hesitancy stems from a matter of arithmetic and the sheer volume of work that needs to be done, as well as because of gross uncertainties about how genes relate to the important traits of humans and their differences from closely related apes.  The human genome consists of about 3 billion base pairs and the gross difference from that of chimpanzees is about 1.2% (incidentally, it is likely that all mammals, from mice to men, share around 80% of their genes).  Assuming that this difference is split 50:50 between the results of evolution towards us and towards chimps over the last 5 to 7 Ma, the divergence from the genotype of our shared ancestor in the human genome should amount to about 16 million new base pairs.  Some of them may be “chaff”, but the genetic side of human evolution is buried in this massive area of potential work.  Maybe around 200 000 are tied to evolved changes in protein production, that could be the key candidates for research.  Although there have been claims for genes that control this or that side of humanness, properly tying down traits to genes will be an awesome task.

The differences between chimpanzees and humans manifest themselves in anatomy and behaviour, and a huge body of knowledge on both has grown in the last two centuries.  So biologists know pretty well what they are looking for in terms of interesting genotype-phenotype links.  However, a chart of those parts of the genome that account for the differences, whenever that becomes a believable reality, really does not help with the hows and whens of the course taken by evolution over several million years.  They rely on the fossil record.  Astonishingly, chimpanzee fossils are almost totally unknown, especially in the early part of their phylogeny.  Even by the most optimistic account, the record of our predecessors is patchy and only a handful of near-complete skeletons are known from before about 500 ka.  Carroll uses the most “bushy” version of hominin cladistics claimed by palaeoanthropologists, with 19 species, to illustrate the current status of hominin descent.  White’s view of the uncertainties (Ancestral lines squashed?, earlier in this issue) makes the crucial connections before about half a million years ago extremely flimsy.  But, there will undoubtedly be a huge growth in human evolutionary studies, once the key chimpanzee data become available.  Of course there will be a massive media hype as well, and all manner of outlandish claims.  But maybe also more funds for palaeontology will stem from the potential to link the evidence from today’s graspable realities with the exciting though puzzling anatomical record since the late Miocene.

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