Implications of a mismatch between hominin genes and bones

Finds in Kenya, Ethiopia and Chad during the first few years of the 21st century suggest that bipedal hominins, perhaps on the human clade, emerged as long ago as 7 Ma. Even using the previously accepted molecular-clock age for separation of chimpanzees and hominins, this is dangerously close to the time of the last common ancestor of both (5-10 Ma). Results from comparison of more detailed chimp and human genomics (Paterson, N. et al. 2006. Genetic evidence for complex speciation of humans and chimpanzees. Nature, doi:10.1038/nature04789, online) throw up a bewildering series of possibilities. On Patterson et al’s reckoning, our descent split from that of our nearest relatives no more than 6.3 Ma ago and perhaps as recently as 5.4 Ma, implying an overlap between tangible evidence and that based on DNA. Of even greater concern is the fact that human and chimp X-chromosomes are more similar than the rest, and seem to have diverged even later. One way in which this greater similarity could have arisen is if natural selection had been operating more strongly on X-chromosome genes, which studies of other related species show to have stemmed from hybridisation. Genes found on X-chromosomes that make hybrids less fertile can create strong selection pressures on this chromosome. An explanation that takes into account the young date of apparent splitting and strong selection operating on X-chromosomes is that the actual speciation(s) did take place before the time when the oldest hominin fossils were preserved, but that there was common interbreeding between the two closely related lines. 

Understandably, palaeoanthropologists and geneticists are arguing heatedly, but failing to recognise the great differences between fossils and extant genetic evidence: each is bound to tell a different part of the story. Yet another is the ecology connected to either lineage, the end point being a regional separation into creatures of forest and open savannah, separated by considerable distances in Africa – basically west and east of the East African Rift system. Before that climatic and vegetation-cover schism what would there have been to stop a great many branchings from either lineage of very closely related animals? The rarity of fossils from either may leave the true relationships early in the history of both clades completely impenetrable. One thing is for sure, although chimps and humans today do make close friendships, that is as far as it goes…

See also: Holmes, B. 2006. Did humans and chimps once merge?. New Scientist, v. 190 20 May 2006, p. 14. Pennisi, E. 2006. Genomes throw kinks in timing of chimp-human split. Science, v. 312, p. 985-986.

Hobbit matters

Debate about the significance of the tiny hominid fossils from the Indonesian island of Flores (H. floresiensis) continues to escalate. The remains are sufficiently complete for analysis of other things than size and morphology of skull and brain. It seems that the shoulder structure is different from that of modern humans, but more similar to that of full-sized H. erectus (see Culotta, E. 2006. How the hobbit shrugged: tiny hominid’s story take a new turn. Science, v. 312, p. 983-984). In ourselves, when standing straight, our inner elbows face slightly forwards so that we can work with both hands in front of the body. The necessary twist in the humerus is somewhat less in H. floresiensis, and by itself that would inhibit being able to make tools. However, the shoulder bones of the fossil articulate differently with the hobbit humerus so that a hunched posture would allow intricate work, but not an overarm throwing action. Much the same features characterise the well-preserved upper bodies of H. erectus fossils from Africa and Georgia. Incidentally, like J.R.R Tolkien’s fictional Hobbit, H. floresiensis also had disproportionately large feet.

It seems inescapable that H. floresiensis did make tools. As well as the 90-12 ka artefacts found in the Liang Bua cave with the hominid remains, which some have reckoned to be too complex for the small people to have made the, large numbers of similarly sophisticated stone tools have been found at other sites in Flores. These occur with similar prey species, but not hominid remains, from as long ago as 800 ka; a time at which only H. erectus was present in the Indonesian archipelago (Brumm, A. et al. 2006. Early stone technology on Flores and its implications for Homo floresiensis. Nature. V. 441, p. 624-628).

The minute size of H. floresiensis, with a brain capacity of a mere 400 cm3, continues to cause some researchers to doubts that the fossils – in fact 9 sets of remains from Luing Bua – were other than congenitally deformed modern humans: microcephalics. Anatomist Robert Martin of the Chicago Field Museum of Natural History (see www.sciencemag.org/cgi/content/full/312/5776/999b) used scaling factors of other dwarfed mammals from island faunas to model the body versus brain size to be expected for similarly dwarfed hominids that might arise from isolated H. erectus. He calculated that the 400 cm3 brain of H. floresiensis should be associated with a creature with around 11 kg body mass: about the size of small monkey. But that conflicts with the fact that the famous skull shows no signs of other deformities associated with microcephaly (See Culotta, E. 2006. How the hobbit shrugged: tiny hominid’s story take a new turn. Science, v. 312, p. 983-984).

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