You see? Once you start dissecting the question of what is natural, it dissolves.
Q: What about instinct? Didn’t humans act on pure instinct before we learned to think?
A: Instinct is also a very slippery concept. You can never say a behavior is instinctive, because all behavior is learned, at least to some extent. All behavior is a combination of input from the genes and input from the environment.
We always want to set ourselves aside, believing that humans are different from other animals because they operate on instinct and we have all of this culture. But in fact, culture affects our evolution.
Q: How does culture affect evolution?
A: The best evidence is the evolution of the gene that allows us to digest milk. After weaning, animals lose the enzyme required for breaking down milk sugar (lactose), so they have digestive difficulties (lactose intolerance). But some genetic variants allow us to consume milk.
We think people originally herded cattle for meat and hides, not for dairy. If a few people could drink milk after weaning, they’d have an advantage because they could use a food source other people couldn’t. They’d be more likely to survive and pass on the genes to their offspring.
Ultimately, more people could consume dairy products, thus encouraging the cultural practice of herding cattle. And the cultural practice of herding makes the genes more prevalent.
Q: What does evolution tell us about how we should exercise?
A: If you compare our skeletons with chimpanzees or early human ancestors, it looks like we’re adapted not just for walking but for running. For example, the way your skull sits on your neck and shoulders absorbs the shock during running.
Marathon running seems unnatural, but some suggest we may have evolved as long-distance runners. It is called the “persistence hunting hypothesis.” The idea is that early humans caught prey by running it down over a distance until it keeled over from heat exhaustion. That’s because people can offload heat through sweating, but animals can’t.
Still, this doesn’t lead to the conclusion that everybody should be running marathons.
Q: What do you think of the claim that sex crimes are caused by the evolutionary impulse to promote survival of the species?
A: Having sex is a big part of our makeup, but that’s a really far cry from using a “my genes made me do it” defense. That just doesn’t work biologically, legally or morally. We are not automatons.