Book review: The Driving Force, by Michael Crawford and David Marsh.
I read this book mainly to improve my understanding of what diets humans are adapted to. The book was mildly successful at that. Unfortunately, most of that improved understanding came from researching some suspicious parts of the book.
The book starts with some uninspiring discussions of evolution theory. E.g. chapter one is about the “great debate” between environmental stimulation versus natural selection. I remain a bit fuzzy about whether there have been lasting substantive disagreements in this debate, or whether it consists mostly of different framings.
The authors also express interest in adaptive mutation, as opposed to mutations being purely random.
These somehow get summarized on the back cover as “was Darwin wrong?”. Please don’t take that clickbait seriously.
The book eventually settles down into focusing on a clearer and more mundane “debate”: the complaint that discussions of evolutionary theory focus too much on survival, and pay inadequate attention to the effects of food.
I’ll overlook the hints that this is part of a debate about evolutionary theory, and agree that discussions of evolution paid too little attention to food back in 1989 when Driving Force was published, and maybe still do.
The Aquatic Ape
The most interesting part of the book deals with the aquatic ape hypothesis.
I’ve long suspected this hypothesis was partly correct, but I never got around to reading Elaine Morgan‘s book on the subject.
The Driving Force presents what look like strong arguments that our ancestors depended on swimming at some point, likely to get food. I’m unconvinced by other parts, such as the connection between an aquatic period and bipdelism.
The authors seem totally lost at sea when talking about what preceded the aquatic ape phase:
What is certain, is that nowhere in the world today is there any evidence of modern man having made any adaptations to the semi-arid savannahs.
Also, our ancestors never went up into the trees.
Their reasons for doubting that we had arboreal ancestors seem to be mainly that they don’t see advantages to living in trees. Yet I find it simple to look at chimpanzee lifestyles, and see that trees had more figs, and maybe fewer lions. Am I being unfair in using Wikipedia-based research to criticize what authors wrote back in the dark ages?
The authors do point out that it’s somewhat odd to have evolved, in a warm, dry climate such as a savanna, the ability to sweat away more water. Yet low-tech human societies survive in places such as the Kalahari Desert where they need to get most of their water from plants. And comparisons between primate species suggest that hot, dry climates tend to cause primates to evolve more sweat glands than other climates. So the savanna explanation for human sweat glands seems roughly as plausible as the aquatic explanation.
Why has the aquatic ape hypothesis been dismissed by mainstream experts?
Wikipedia led me to suspect that it was due to the early advocates of the hypothesis being obvious amateurs who made a few mistakes. The book seemed to confirm that impression.
It wasn’t until I searched for primate swim instinct that the hypothesis started looking really fishy. The diving reflex and the infant swimming reflex sounded like straightforward evidence – why else would humans have them, but other apes not?
The answer seems to be: like many claims about human uniqueness, the belief that other apes lack those adaptations seems to be based more on conceit than on research. Here’s a source claiming that all mammals have those reflexes. I haven’t tracked down any rigorous research on this topic, and I see no sign that the authors did either.
It seems notable that the book doesn’t discuss the benefits of cooking. Maybe there hadn’t been much research into the subject when Driving Force was published (1989). But now there’s a fair amount of research suggesting that our ancestors’ brain size was limited by the calories available to fuel it, and that cooking was an important technology that expanded the limit. This weakens the book’s claims that big brains are due to something connected to water.
I’ll conclude that there’s maybe a 20% chance that a watered-down version of the aquatic ape hypothesis is true. But all this book has done is muddy the water. We clearly have aquatic ancestors (all mammals do). I see only weak evidence of recent aquatic ancestors.
The Driving Force promotes an early version of a paleo diet, but the later versions of paleo diets that enjoy some success seem to have not been influenced by this version.
Crawford and Marsh fully bought in to the hype that saturated fats are the main cause of cardiovascular disease, and imply that the main benefit of a hunter-gatherer diet is that meat from wild animals has less saturated fat than modern commercial meat. It’s likely true that that is some sort of benefit, but it’s almost certainly a relatively small part of what distinguishes paleo diets from the standard American diet.
They occasionally mention the benefits of polyunsaturated fats, with a hint that it’s valuable to balance omega-3 and omega-6, and they briefly complain about excess sugar, refined carbs, and inadequate fiber. So a careful reader will see that they’re advocating a diet that’s pretty consistent with the evidence that has been published since 1989.
Their reasoning implies that seafood is healthier than farmed meat, due to the better ratio of polyunsaturated to saturated fat, and the aquatic ape thesis implies that we should expect to have adaptations for eating aquatic foods. In particular, they suggest we need more polyunsaturated fat than most animals, due to our brains using an unusual amount of it.
Theories about paleo diets typically don’t say much about seafood, but even weak versions of the aquatic ape hypothesis suggest that we may have become mildly dependent on seafood.
This combination of reasons seems stronger than the standard reasons for eating seafood, which depend more than I’d like on correlational evidence.
The Driving Force is one of the earliest books to point out that cardiovascular disease, diabetes, colon and breast cancer, etc, vary enough from culture to culture that they must be mostly or entirely due to lifestyle.
The book ends with a speculation that improved nutrition is likely to increase intelligence. The authors were likely unaware that the Flynn Effect had been discovered (but not yet publicized much). It still seems somewhat plausible that the Flynn Effect is driven by changes in chronic malnutrition.
The Driving Force bears some resemblance to a book that could have had a modest influence on our understanding of health and nutrition. Yet it apparently wasn’t persuasive or rigorous enough to have a noticeable impact.
On second thought, I’ll retract that “rigorous enough”. The more successful approaches to marketing paleo diets became popular mainly by taking a controversial stand on saturated fat. Some more rigorous pro-paleo arguments achieved a bit of popularity on the coattails of the controversial ones.
The diet implied by The Driving Force seems slightly healthier than the more controversial pro-beef versions of paleo diets. That indicates some combination of competence on the part of the authors, and the robustness of the basic insights behind paleo diets – it only takes a rudimentary knowledge of ancestral conditions to get to a better diet than the standard American diet.
I was amused by the authors’ hesitant hint that neanderthals were less intelligent than humans because they didn’t eat fish. I was initially suspicious because I have a high opinion of the view Henrich expresses in The Secret of Our Success: our best guess is that Neanderthals were individually a bit smarter than humans, but their smaller group sizes meant they accumulated less cultural wisdom.
A little investigation produced stronger reasons to doubt Crawford and Marsh: neanderthals probably did eat fish. Also, Crawford and Marsh imply that humans adapted to eating fish around when humans evolved bipedalism. But that appears to be a couple million years before neanderthals split off from humans. Are they disputing that timeline? Or saying that neanderthals lost the advantages of fish-eating? If so, have any human populations given up fish for long enough to experience anything similar? I don’t get the impression that Crawford and Marsh have a clear enough model to answer many of these questions.