We may owe our superior intelligence to weak jaw muscles.
A mutation 2.4 million years ago could have left us unable to produce one of the main proteins in primate jaw muscles, the team reports in this week's Nature. Lacking the constraints of a bulky chewing apparatus, the human skull may have been free to grow, the researchers say.
nature.com
This evidence of the first evolutionary steps that transformed the chimpanzee great grandparents into humans also illustrates a key distinction in current evolutionary thought and Darwin's original writings.
In Darwin's theory of Natural Selection there is a noted emphasis on the variations in offspring to take advantage of a new opportunity. This has sometimes been criticised as the Hopeful Monster pattern of mutation. Evolution is not an anticipatory process, and cannot target an opportunity. It really doesn't matter if there is an abundance of fruit in the trees for a creature than can't climb, or untapped acres of grass for a creature that can't digest fiber. Evolution is really a process of constraints.
The theory of Punctuated Equilibrium recognizes the process of variation around constraints. Some speculation: Organisms often go for very long periods of times with little change, this is their equilibrium phase. The organisms have reached a balance where they are pretty well adapted to their environment. Variations are constrained by the past success of close relatives who are in that sweet spot of compromise between traits. Any significant variation would compete directly with these well adapted relatives.
In the case of the chimpanzee ancestors of 2 or 3 million years ago there was a constraint in jaw muscles and the resulting food supply. Through variation some chimps would have weaker jaws, and some chimps would have larger brains. The weak jawed chimps would find it hard to chew the nuts and roots. The big brained chimps would develop headaches from the tight muscles attached around their skull. The sucessful chimps were those who got nice strong jaws and just enough brainsize to avoid the migranes.
Why then would some chimps set off on the path to a new species with a mutation that had most likely been a debilitating birth defect to many earlier generations of chimps? The answer must be inferred. When this chimp was born with the mutation there was a compensating constraint removed. Most likely it was a question of food supply. There must have been a reasonable abundant supply of food which did not require the heavy chewing muscles.
This was not an opportunity. The chimp still deviated from the norm, and was most likely at some disadvantage in locating it's restricted diet when hard nuts and roots were more plentiful. Other chimps could, and probably did take advantage of this new convienience food so this chimp was not afforded a special advantage. This chimp could survive because a constraint which had restricted earlier generations of weak jawed chimps had been removed.
Surviving is the key to a new generation, and therefore to a new species, but this chimp and others with the weak jaw mutation had something more. Future offspring might also get the big brain mutation. This time the bigger brained chimps would not get the headaches which had proved a disadvantage to prior repositories of the gene. Another constraint had been removed. It is a cascade of elimination of constraints that leads to the rapid phase of evolution , the punctuated phase. Eventually big brained chimps could compensate for their food constraint by the use of tools and processing so that a weak jaw was never again a significant constraint. The path to a new species is just a few and relatively rapid punctuated steps away of generation after generation of equilibrium.
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