Weak jaws allow bigger brains

There is no topic in the geosciences that is more interdisciplinary than that of human origins.  Geologists, anthropologists (social as well as physical), archaeologists, geochemists, linguists, geneticists, dentists, specialists in nutrition and even novelists (for example Jean M. Auel) contribute.  Everyone is interested, and so everyone not only wants to have a say, but somehow to be involved.  Again and again in the pages, it becomes clear that bones and artefacts can no longer make major breaks through.  The Out of Africa hypothesis, although suggested by Charles Darwin and many palaeoanthropologists since, became widely accepted (though not completely) after the evidence for relatedness emerged from comparisons of mitochondrial DNA from women throughout the world.  That showed clear signs of a last common ancestor for all human groups around 200 thousand years ago, to whom modern Africans were most related.  At the end of March 2004 geneticists have again come up with something startling, but this time not guessed at before.

The first beings to whom the generic name Homo seems appropriate appear in the hominid fossil record about 2.0 million years ago.  Apart from evidence for bipedality and their association with rudimentary, but nonetheless deliberately made stone tools, the earliest humans are marked by the fragility and roundness of their skulls.  Many specialists have argued that “gracile” crania are an evolutionary pre-requisite for the growth of brain capacity – they can expand for a long period during development, before becoming completely ossified in adulthood.  The predecessors of these early humans (australopithecines) and their close companions in the African savannahs (paranthropoids) had smaller brain capacity and also very bony heads.  In the case of the paranthropoids, undoubtedly as closely related to earlier hominids as the first tool-making humans were, they survived as a group for another million years but never expanded their brains, nor presumably their intellects.  Bone-headed hominids had one feature in common with all earlier apes, and with the genera that survive today; powerful jaws and muscles that drive them.  To some degree or other they all have crests on top of their skulls, which provide the seats for these big jaw muscles.  Wielding awesome biting power requires skull strength, and therefore bulky bone.  That encumbers any possibility for expansion of the internal brain cavity, and also drives their bearing species into tight feeding habits.

A team of geneticists, anatomists, developmental biologists and plastic surgeons from the University of Pennsylvania and the Children’s’ Hospital of Philadelphia have studied one gene sequence of several that encode for a type of protein (myosin heavy chain) associated with the powerhouse muscles that are attached directly to bone, such as those which drive jaws (Stedman, H.H. and 9 others 2004.  Myosin gene mutation correlates with anatomical changes in the human lineage.  Nature, v. 428, p. 415-418).  Their investigation began with an interest in muscular dystrophy and possible underlying factors.  Specifically, the most interesting gene (MYH16) is expressed in primate jaw muscles.  The human gene contains a mutation that prevents the accumulation of the protein in our jaw muscles, so they cannot be as strong as those of other primates and mammals in general, in which the gene functions as it should.  By analysing MYH16 and related gene sequences in humans from widely separated populations, the researchers showed that the mutation in MYH16 diverged earlier than those in other MYH-related genes.  To estimate the time of that divergence involved detailed analysis of the mutations in other living species – dogs, macaque monkeys, oran-utans and chimpanzees.  This showed that MYH16 evolved under Darwinian selection, conferring fitness advantage, in the ancestral lineages leading to each species, whereas in humans there was no selective constraint.  Under the second condition, it can be assumed that any evolutionarily neutral changes took place at a constant rate.  Calculations suggest that in the human lineage, the mutation appeared 2.4±0.3 Ma ago.  That coincides with the earliest appearance of tools and a little earlier than the first remains of early Homo fossils.  The conclusion could be one of several: lost of biting power created conditions for expansion of a lighter skull; a changed diet to include more meat reduced the need for strong jaws, so that the mutation did not have a deleterious effect; or hands freed by walking upright did a lot of the work that other primates can only accomplish with their mouths.  Whichever, once established without decreasing fitness, the road to enlarged brains and fuller consciousness was opened by a chance event.

See also:  Ananthaswami, A. 2004.  less bite, more brain.  New Scientist, 27 March 2004, p. 7;  Currie, P. 2004.  Muscling in on hominid evolution.  Nature, v. 428, p. 373-374

Dental records of earliest hominids

Conditions on land are not as conducive to preservation of fossil remains as those on the sea floor.  When an animal dies it is generally eaten, what is left rots and is gnawed, and the action of wind and water breaks up the skeleton and transports it, and only this debris is preserved if it is buried by sediment.  The best chance of preservation is if the animal falls in a lake or bog, or in the case of fully modern humans if it is deliberately buried.  The so-called Turkana Boy (H. erectus) is an almost complete skeleton, because he did end up, uneaten, in a swamp.  Sturdy, large animals and those small and light enough to be quickly washed to burial stand the best chance of appearing as complete fossils.  Primates are medium-sized and lightweight, and that presents palaeoanthropologists with their single biggest problem, incompleteness of most fossils that they find.  In the depths of the Afar Depression of Ethiopia and Eritrea, which is the most productive area for hominid specialists, conditions from the early Miocene were not the best for preservation.  While the depression developed by extensional tectonics, its flanks rose to form the mighty Ethiopian escarpment from which torrents flowed seasonally.  High-energy streams clearly will break up any articulated skeleton and batter what is left before they end up in gravels and sands on the floor of the depression.  So it is a credit to the patience, experience and sheer visual acuity of those who work there that they can piece together the earliest parts of the human story.  Yohannes Haile-Selassie, Gen Suwa and Tim White have pushed back and detailed our record further than any other group, thanks in part to the richness of the Miocene to Recent Middle Awash sedimentary and volcanic sequence with which they work.  In 2001 Haile-Selassie discovered the earliest Afar hominid so far (see Taking stock of hominid evolution, March 2002 issue of EPN), Ardepithecus ramidus kadabba dated between 5.2 and 5.8 Ma.  In age it roughly matches Sahelanthropus and Orrorin from Chad and Kenya.  Only a leg bone from Orrorin gives some indication that it was bipedal, but all show cranial features that mark them out as probable hominids.  Of all the body parts of any animal, the teeth are the most likely to survive with little change.  Because our closest living relative are chimps, comparing early teeth with theirs, as well as with those of later hominids, is about the best that can be done to seek relatedness.  The three notable workers on Awash hominds have now reported their results (Haile-Selassie, J. et al. 2004.  Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution.  Science, v. 303, p. 1503-1505), which suggest the earlier find is a distinct species A. kadabba.  Putting together upper and lower canines and adjacent premolars shows a close resemblance to those of modern chimps.  However, it requires detailed measurements of the tooth shapes to check if the resemblance is more than superficial, and it is not.  All extinct and modern apes show signs of automatic honing of their canines, whereas hominids do not.  Not only A. kadabba but Orrorin and Sahelanthropus too, show no sign of canine honing.  That points to early members of human evolution.  Yet, the three show such close similarity that it is hard to support the idea that they are from anatomically different genera, despite their occurrence thousands of kilometres apart.  It is that close resemblance (and in other features as well) that re-opens the long debate between a complex, messy “bush” of human descent made up of many contemporary, different creatures, and one of a single line of descent.  Dental features are not enough to decide between the two.

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