The answer appears to be a lot of mutations in a lot of genes. We've done a rough calculation that the evolution of the human brain probably involves hundreds if not thousands of mutations in perhaps hundreds or thousands of genes -- and even that is a conservative estimate."
The numbers here should be considered in the correct context.
There are right now something between 10 billion and 50 billion new mutations in the human population. That is roughly the number of mutations that the living human population has.
I think we can put an upper limit on the number neede too rather than use a "conservative estimate". If we take humans as being 98% the same as chimps in their genomes and say all of that 2% is for the brain (obviously to high) we get 2% of 3 x 10**9 base pairs different (I think) which is at the very highest about 60 million differences.
This took about 6 million years (to make the numbers easy and ignoring that chimps have been changing too) that means we need 10 useful mutations a year in a population of a few 1,000 individuals. This is at the very worst and is waaay over a realistic number.
If you really want to get a feel for how possible it is you should look into the genetics that we are beginning to get a handle on.
From "Genome" by Matt Ridley
This is discussing different personalities, in particular those who are more adventure seeking and those who are not.
quote:
The first genetic difference turned up... in the D4DR gene on chromosome 11. D4DR has a variable repeat sequence in the middle, a minisatillite phrase 48 letters in length repeated between two and 11 times. Most of us have four or seven copies of the sequence, but some people have two, three, five, six, eight, nine, ten or eleven. The larger the number of repeats, the more ineffective is the dopamine receptor at capturing dopamine. A 'long' D4DR gene implies a low responsiveness to dopamine in certain parts of the brain, whereas a 'short' D4DR gene implies a high responsiveness.
Hamer... wanted to know if people with the long gene had different personalities from people with the short gene.
...
-- people (in a small sample) with either one or two long copies of the gene... were distinctly more novelty seeking than people with two short copies of the gene.
In addition, see
Message 1
The size of the brain is influenced (Ridley warns against assuming this one gene is the whole answer) by the number of repeats of of a 75 letter sequence. If this kind of thing applies in other places then a reasonably simple mutations (like a mistaken extra copy of 75 letters) can have profound influences.
The idea that mutations can not supply the changes needed to give a larger more complex brain is made to look pretty shakey by such discoveries.
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