To start with, here's a thought I had the other day: a great deal of the cholesterol denialism found in the paleo and low-carb communities seems informed by a fundamental misunderstanding about the nature of adaptations. Ditto for a great deal of anatomical denialism among vegans, many of whom embrace myths that humans are obligate frugivores.
Consider the following two statements.
1) Homo sapiens are more adapted to meat-eating than other primates.
2) Meat-eating is unhealthy, at least in large amounts.
There's no necessary contradiction in those two statements, but I think a lot of paleo and low-carb folks think there is.
And, to be fair, so do a lot of vegans.
I've encountered many statements on paleo blogs to the effect that natural selection wouldn't have made us better at meat-eating if meat-eating was so dangerous to our long-term health. Or, put another, slightly different way, meat-eating can't be unhealthy because we are adapted to it.
In the vegan community, sentiment tends to run in the opposite direction, taking the form of the tiresome "humans are natural herbivores" meme: because meat-eating is unhealthy, humans therefore are not really adapted to it, and we're just fooling ourselves if we think otherwise.
Both forms of denialism rest on an assumption that adaptation carries with it an obligation (in this case, on obligation to eat other animals): paleos & low-carbers embrace fossil evidence and the implied obligation but deny the health evidence about cholesterol and saturated fat; while vegans embrace the health evidence while rejecting the implied obligation, which leads them to deny the fossil evidence.
So, here's the mistake they both make: being adapted to something doesn't necessarily make it "good" from the standpoint of individual health. There are many adaptations that come with a high price, and many others that have outlived their usefulness and have thus become burdens to be overcome rather than traits to be indulged.
A few examples from modern humans illustrate the point:
- The unique human voice box, the anatomical source of our ability to use complex language, also makes it impossible for us to breathe and swallow at the same time... which means we're much more likely to choke to death on our food than other animals are. Language comes with the inherent risk of death by choking.
- Chronic lower back pain is a direct consequence of our upright posture; hominoid primates have a transverse lumbar process that is serially homologous with the spinous process (rather than with the styloid process, as it is in other mammals), allowing us to assume more upright postures. But this mutation puts a lot of mechanical stress on the lower vertebrae, often leading to herniated discs. This is a risk few other mammals run, because the weight of their upper bodies is distributed along a longer axis.
- Sickle cell anemia and the various forms of thalassemia are preserved in the human genome because they provide heterozygotes with greater resistance to malaria... despite the fact that heterozygotes suffer debilitating long-term health consequences from these adaptations, and recessive homozygotes die from them. For humans no longer living in malaria-prone areas of the world, these adaptations are now useless, and have become burdens rather than boons.
So, too, can it be for meat-eating. There's enough fossil evidence to plausibly argue that the genus Homo was given an edge in fitness by being better adapted to meat-eating than other primates. This allowed them to flourish on savannahs, under the selection pressures of the Pleistocene.
But this does not imply that meat-eating became healthier, from the point-of-view of degenerative disease. We still carry within us the mechanisms of atherosclerosis inherited from 22 million years of hominoid evolution, not to mention almost 200 million years of mammal evolution. We are, like nearly all extant organisms now living on this planet, a species of Rube Goldberg devices full of jury-rigged solutions that need only be good enough. If the traits of early Homo were good enough, there'd have been no need for the underlying biochemistry of cholesterol and atherosclerosis to be changed much, if at all.
Any advantage provided by adaptations to meat-eating would have been for the purpose of reproductive success, not "optimal health." That is, it would have provided early humans with a way of obtaining calories reliably enough, for long enough, to beat their rivals in the race to die with the most kids. Evolution, as I've pointed out before, is fundamentally about sex, not food or health. If being better at meat-eating allowed our ancestors to avoid starvation long enough to see their kids grow up, then it would have been preserved despite its atherosclerotic and other effects.
And thus, it's entirely possible that said adaptations are now burdens rather than boons, since we no longer face the selection pressures of life on the Pleistocene savannah. Something that helped us in the past can now be hurting us, and thus "eating like our ancestors" might be a terrible idea.
So, there's no need for denialism from either side of this paleo-vs.-vegan debate about diet. It turns out, they might both be right.