National Geographic Daily News has just published an embarrassing article about early human evolution [Eating Crocodile Helped Boost Early Human Brains?]. An ancient kitchen dating back 1.95 million years was discovered in Kenya. Among the bones recovered at this were those of fish, turtles, and crocodiles. The paper has just been published in PNAS (Braun et al., 2010).
Here's what the National Geographic science writer (Christine Dell'Amore) reports ...
According to the study authors, the addition of water-based prey into early-human diets may have been what boosted brain size in certain hominins—humans plus human ancestral species and their close evolutionary relatives.This is one of those cases where the press report accurately describes what's in the paper. Here's the conclusion of the PNAS paper,
That's because reptiles and fish are particularly rich in long-chain polyunsaturated fatty acids. Some experts think this so-called good fat was "part of the package" of human brain evolution, said study leader David Braun, an archaeologist at the University of Cape Town in South Africa.
Discovering evidence for "brain food" in the late Pliocene (about 3 to 1.8 million years ago) may explain how bigger brains—for instance in our likely direct ancestor Homo erectus—arose in humans and their relatives about 1.8 million years ago, Braun said.
The evidence from FwJj 20 indicates that hominins were very effective at securing access to a wider variety of high-quality animal tissues than has been previously documented. Some of these resources would have provided necessary dietary resources without the added predation risks associated with interactions with large mammalian carnivores that are sometimes involved with the acquisition of elements of large mammal carcasses (28, 33). In addition, although animal tissues provide nutrient-rich fuel for a growing brain, aquatic resources (e.g., fish, crocodiles, turtles) are especially rich sources of the long-chain polyunsaturated fatty acids and docosahexaenoic acid that are so critical to human brain growth (2). Therefore, the incorporation of diverse animals, especially those in the lacustrine food chain, provided critical nutritional components to the diets of hominins before the appearance of H. ergaster/erectus that could have fueled the evolution of larger brains in late Pliocene hominins.There are so many problems here, I hardly know where to begin.
First, there's the implicit assumption that eating food rich in long chain polyunsaturated fatty acids contributes to brain growth. As far as I know, the total scientific evidence does not strongly support this assumption—although there are plenty of studies that make the claim. (Their reference 2 is to another anthropological study.) This is not an assumption that one should build a theory on, but, if you do, you'd better back it up with references to the primary biochemistry and physiology literature.
Second, it's easy to be confused about the importance of dietary lipids. For the record, you need to eat foods containing linolenate and linoleate because these are essential fatty acids. You can't synthesize them but you need them in order to make other important fatty acids. There's plenty of these essential fatty acids in plants, which is why our fellow primates (chimps and gorillas) survive quite nicely without eating crocodiles. The "magical brain food" kinds of fatty acids are the other omega-3 fatty acids that we can synthesize as long as we have an adequate supply of the essential fatty acids. Finally, let's think about the hypothesis being put forward. The idea is that our ancient ancestors weren't very smart. They had small brains. Some of them started to eat fish and crocodiles and that made their brain get bigger. Presumably this mostly affected the children since there's no evidence that diet can make an adult brain grow bigger.
As the years passed, the entire population acquired bigger brains as a result of their diet. Maybe this group out-competed their neighbors who didn't like fish so that eventually the entire hominid population of the region had big brains and ate fish.
What has this got to do with evolution? How do you get from a cultural preference for eating fish to changes in the genes controlling brain development? Are the authors implying some kind of Lamarckian inheritance? How, exactly, does eating fish translate into genetic changes (i.e. evolution)?
Am I missing something?
[Photo Credit: ProGolferDigest]
Braun, D.R., Harris, J.W.K., Levin, N.R., McCoy, J.T., Herries, A.I.R., Bamford, M.K., Bishop, L.C., Richmond, B.G. and Kibunjia, M. (2010) Early hominin diet included diverse terrestrial and aquatic animals 1.95 Ma in East Turkana, Kenya. Proc. Natl. Acad. Sci. (USA) 107: 10002-10007. [doi: 10.1073/pnas.1002181107]
Thank you thank you thank you!
ReplyDeleteYes, there are a lot of crypto-Lamarckian ideas about the relationship of diet and brain evolution in humans. The fish fat story is one, there are many others that involve other nutrients.
I get my students to think about a different example: What if you started dropping a lot of sugar beets out on your land for the deer to eat? Are you going to get deer with bigger brains? No, you're going to get more deer.
I suppose one might argue that potential brain-enlarging mutations couldn't have a phenotypic effect until hominins started getting more long-chain polyunsaturated fatty acids in their diet. But I agree that the reasoning is still bogus without better evidence that the fatty acids were limiting to begin with.
ReplyDeleteHow? I'm not a very good adaptationist story-teller, but this one's so easy that I can't pass it up.
ReplyDeleteSome folks eat more fish, and this shifts the distribution of smarts higher. Perhaps this beneficial fish-eating meme spreads by cultural evolution, but thats not the important part.
The important part is that, before this cultural innovation, most humans were just smart enough to maintain a 4-word vocabulary, and only a few brainiacs at the extreme end of the distribution had the smarts to turn out a sentence, which of course no one else understood, as it was rare to find 2 such brainiacs in the same small group.
But eating fish changed that. Before eating fish, sentences were rarely heard, but after the fish-eating meme spreads, whole sentences happen almost daily, and are frequently understood.
This cultural non-genetic evolution of sentence-making capacity shifts the evolutionary horizon such that all sorts of useful innovations in communication-- conjunctions, transitive verbs, and sarcasm-- became accessible, either via new mutations, or via combinations of available alleles. Genetic evolution of sentence-based communication takes off, as the result of eating fish.
Its called "genetic accommodation" or "genetic assimilation" . You can read about it on wikipedia (http://en.wikipedia.org/wiki/Genetic_assimilation)
Arlin
So, what happens when people stop eating fish (or crocodiles)? Do they become stupid? Do their brains revert to those of our ancient ancestors?
ReplyDeleteThat must mean vegetarians are .....
Larry--
ReplyDeleteNo, they don't become stupid again after they stop eating fish. That's the beauty of genetic assimilation.
Its been done experimentally. Take Drosophila and heat-shock them early in development. Some flies lose their posterior cross-vein, a little vein in the wing. Its a trivial little character, but that's not the point.
The point is that if you select for the capacity to lose the cross-vein upon heat-shock, then eventually, you get flies that develop without the cross-vein even if there is no heat-shock. The evolutionary changes obviate the need for the external stimulus.
Arlin
Hey Larry,
ReplyDeleteI am suspicious of the reasoning in this paper, but I think that the authors were trying to say that a fish (fatty-acid) rich diet sort of *enabled* the later evolution of larger brains, as it is (as you pointed out) assumed that these molecules are important to brain development.