Smart Crocodile Eaters?

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.

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.

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,

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?

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[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]