Paleo, and "ancestral-style" diets more broadly construed, seems to be based around a couple of principles.
The first is "Evolutionary Nutrition". This is the idea that humans evolved in particular circumstances, with particular foods available to them, and therefore are best adapted to consume those foods.
The second is the "thrifty gene hypothesis". In a nutshell, this attempts to explain why humans seem to be so prone to fattening and diabetes (as well as other "diseases of civilisation" or "diseases of abundance"). It posits that genes which allowed humans to store excess food away as fat would have been advantageous in hunter gatherer populations, facilitating survival through periods of famine. The thrifty gene hypothesis seems to be subsumed under the principle of evolutionary nutrition, in that it exemplifies the tenacity of adaptations over seemingly long periods of time.
I want to unpack the idea of evolutionary nutrition and give a few of my initial thoughts on the concept, before talking about my current position with respect to paleo. In the spirit of intellectual honesty, I'll admit that I was initially sceptical of the claims made by paleo when I began to research it. I have something of a lingering resentment for evolutionary psychological explanations for human social order, obtained through a number of years of studying the philosophy and sociology of science, and the ideas underpinning evolutionary nutrition seemed initially quite genetically deterministic in a way that set off alarm bells. Moreover, it seemed to explain too much, in that everything could be reduced down to "did our ancestors eat it?". I've learned since then that this is effectively a heuristic and that more well-versed paleo adherents have a nuanced approach to and understanding of their own doctrines.
Paleo's contention regarding evolutionary nutrition is effectively that the Neolithic Revolution was the original sin. The invention of agriculture and the migration of humans into permanent settlements which relied upon agrarian practices, and the shift towards a diet which contained a much greater proportion of grains and other farmed goods, is seen as having spawned many of the health problems we see today. The Neolithic Revolution is supposed to have happened ~25,000-10,000 years ago, meaning that there were many thousands of generations of hunter gatherers preceding the few hundred generations of farmers that followed. Evolution primarily works on extremely long timescales of tens of thousands to hundreds of thousands of years, so the argument goes, and therefore our constitutions have not yet adapted to the changes in diet. This means that our changes in patterns of consumption are directly responsible for obesity, diabetes. heart disease, IBS and myriad other illnesses that paleo practitioners contend did not exist in the Paleolithic Era, but instead only came about in Neolithic humans.
Paleo, then, tells its adherents that in order to optimise our bodies we ought to aim for a diet which is as close as possible to the kind of diet that our Paleolithic ancestors would have eaten. That means cutting out grains (which contain gluten and lectins, both of which are considered borderline Satanic), as well as cereal and (usually) dairy.
Dairy is where my first issue comes up. If it's the case that humans stopped evolving ten thousand years ago, and there has been little to nothing in the way of genetic change since then, why is it that ~90% of North Europeans still have the lactase enzyme when they age, compared to ~5% of Asians? That would seem to indicate some degree of evolution through the Neolithic period.
It wouldn't bother me so much if it weren't for the glorification of an idealised version of the scientific method which exists in alternative dieting and fitness communities (and elsewhere). There's this idea that Karl Popper's falsificationism is the be-all and end-all of science: if we want to know whether something is correct, we can't rely upon induction because of its intrinsic unreliability (see the Problem of Induction). Instead we have to look to deduction, which involves working within our observations, rather than attempting to infer from them. He argues that scientists should subject their hypotheses to rigorous attempts to prove them wrong, and that only after they survive numerous such attempts can we call them anything like "correct"*. If we find that our hypothesis has been falsified, we should give it up and then come up with another conjecture.
The lactose problem would seem to falsify the hypothesis that humans stopped evolving during the transition into agricultural societies. Now, I can perfectly well see why one might respond "no it doesn't, it just means that we haven't quite formulated the most nuanced and accurate version of that hypothesis". My problem isn't with that response, it's with the asymmetry of standards applied to evolutionary nutrition and other systems of thought. For example, it's generally taken as gospel within low-carb (and paleo) communities that the "calories in/calories out" theory of weight gain and loss has been falsified repeatedly over the years (often simply by failing to prove its existence in trials designed to do exactly that), and therefore ought to be dropped. Comparatively, the spread of particular kinds of genes through the human genome isn't taken as proof that the evolutionary nutrition hypothesis is flawed.
This refusal to drop a hypothesis after a supposed falsification is due to what is called "underdetermination" in philosophy of science. The idea behind this is that no matter how much evidence we have, there are always an infinite number of ways in which any given theory can be altered in order to fit with it. This means that when something seems to be falsified, rather than just dropping our theory it's possible to reinterpret or deny the evidence, or to alter our theory in some minor way. We can see this at its most extreme in some religions, where the apocalypse is predicted to come on a certain day, fails to come, and this is seen not as evidence of some fundamental flaw, but rather of a minor error they made in interpreting their text, or of some kind of test sent by their deity.
But underdetermination is also a big issue in nutrition science. If someone goes on a diet and fails to lose weight, there are innumerable reasons why this might be. They might not have counted their calories correctly, or consumed some food that is toxic, or not done as much exercise as they thought, or they might even be straight-up lying to themselves about their consumption and expenditure of energy. Alternatively, it might be that the diet itself is flawed, or it doesn't quite fit with their constitution. No matter what happens, we can always come up with a litany of explanations as to why it happened. This means that we can never definitively proof, to the satisfaction of everyone, that the "calories in/calories out" hypothesis or the "low-carb" hypothesis or the "evolutionary nutrition" hypothesis are correct or incorrect. Falsification just doesn't cut it.
What this means is that our decision to adhere to one diet or lifestyle over another is never going to be driven purely by evidence. The evidence can't point us squarely in one direction or the other, because all evidence requires interpretation, and we're often predisposed towards viewing evidence in a certain light. If we have a really strongly held view about the world, then evidence which seems to conflict with that view is less likely to be seen as undermining our worldview and more likely to be seen as flawed evidence.
With that in mind, I think it's reasonable to say that our reasons for believing in one diet over another are likely to be less grounded in the evidence that exists, and more to do with the kind of evidence we encounter or are given, how it is presented, and by whom. What I'm trying to do in my PhD research is to understand how and why people decide to believe one thing or one person over another. I've chosen the paleo community in part because it's so contested, and in part because so many of the people in the community are deeply scientifically literate and have a nuanced understanding of their subject matter.
I have some further thoughts about evolutionary nutrition, particularly with respect to the availability of particular kinds of foods in modernity and the way that paleo negotiates geographical and cultural differences in food consumption, but I'll leave those for a future post.
*Indeed, Popper thinks that we can never be proven correct, nor can we even know that we are getting closer to the truth. All we know is that something has stood up to more rigorous attempts at falsification than something else, and therefore can be considered to be less wrong or more reliable than other hypotheses which have not been subject to such testing, or have been and have failed.