(HOLLA! @ HowlerKarma.)
Cognitive ability is different because it's not subject to such hard physical limits, and in addition because, in my opinion, we're not nearly as close to the limits of what people can learn as we are to what people can be trained naturally to do with their bodies. You can't double your height, so a very short person will be at a serious disadvantage for all time in the long jump. But people can greatly increase memory, for instance, through training. I don't think you can draw very many valid parallels between current limitations on intellectual abilities and athletics. Intellect is limited by teaching a lot of the time today; I guess that the closest I can come would be a child denied vitamins growing up, and turning out to have stunted growth. We've figured out nutrition to a greater degree than teaching and learning.
A long time ago, people weren't trained to do much intellectually compared to today; reading and writing was considered a big deal. Learned people of those days would likely have been astounded by the algebra skills of a fairly ordinary high school math student of today. And if you had tried to teach someone ordinary algebra in adulthood, perhaps a person with above-average IQ for that time, but dulled through lack of teaching that would be ordinary today, you would have had a much tougher time, perhaps an impossible one, much the same way a child raised by dogs can't learn much human language in the end.
I don't know what would prevent an ordinary person from learning much more than ordinary people learn today. I see no reason to think we've figured out optimal ways of teaching people. It just seems impossible to me. We all gripe here every day about ways in which schools are failing our kids, but they routinely fail most kids IMHO.
