Trends in Genetics references this paper -
Our Fragile Intellect.(download) What is interesting about this is that we peaked 2000-6000 years ago and now are devolving. Or… we started out pristine with the preternatural gifts of bodily immortality and freedom from sickness and have lost it.
Abstract
New developments in genetics, anthropology, and neurobiology predict that a very large number of genes underlie our intellectual and emotional abilities, making these abilities genetically surprisingly fragile.
“I would wager that if an average citizen from Athens of
1000 BC were to appear suddenly among us, he or she
would be among the brightest and most intellectually alive
of our colleagues and companions, with a good memory, a
broad range of ideas, and a clear-sighted view of important
issues. Furthermore, I would guess that he or she would be
among the most emotionally stable of our friends and
colleagues. I would also make this wager for the ancient
inhabitants of Africa, Asia, India, or the Americas, of
perhaps 2000–6000 years ago. The basis for my wager
comes from new developments in genetics, anthropology,
and neurobiology that make a clear prediction that our
intellectual and emotional abilities are genetically surprisingly
fragile.”
and
"Taken together, the large number of genes required for
intellectual and emotional function, and the unique susceptibility
of these genes to loss of heterozygosity, lead me
to conclude that we, as a species, are surprisingly intellectually
fragile and perhaps reached a peak 2000–6000 years
ago. But if we are losing our intellectual abilities, how did
we acquire them in the first place? This will be the topic of
the next section [15]. "