Originally Posted by
Walter S
I agree that individuals adapt to the heat. I've experienced it personally and repeatedly. But you drastically over estimate the rate at which human evolution would pass on that trait. In order for natural selection to occur you first need for a random genetic change to make people function better when they're hot. Then you need for that specific thing to result in a survival advantage that influences one's opportunity to reproduce. Then that change needs to take hold and spread throughout the population.
Mutations can speed things along but even in common gene alleles and within families or tribes, there is always a range of function. Until just the past century or so, infant mortality was one of the biggest drivers of evolution and if a few kids in an adobe village got a fever during the hot season, the ones with the least genetic suitability to the desert climate would not pass on their genes. Also individuals who found the heat oppressive might pack up their DNA and move north ("I got winter in my blood").