A just-so story, also called the ad hoc fallacy, is a term used in academic anthropology, biological sciences, social sciences, and philosophy. It describes an unverifiable and unfalsifiable narrative explanation for a cultural practice, a biological trait, or behavior of humans or other animals. The use of the term is an implicit criticism that reminds the hearer of the essentially fictional and unprovable nature of such an explanation. Such tales are common in folklore and mythology (where they are known as etiological myths โ see etiology).Wikipedia

“NPR has been doing a special series of reports during their “Morning Edition” program called “The Human Edge”, all about various aspects of human evolution. I think it’s just wonderful that they’re doing this, and the stories are available on the NPR website, which is also great.

I’ve been out of town and so haven’t been following closely. So I’m just noticing that some of these stories actually drive me up the wall. Every one of them is presented as what Stephen Jay Gould called a “just-so story”.

I’ll take one of the latest articles as an example: “Food For Thought: Meat-Based Diet Made Us Smarter”. The story begins with a short resume of the “expensive tissue hypothesis”, with quotes from one of expensive tissue’s main exponents, Leslie Aiello. This hypothesis is a serious one, which paleoanthropologists take seriously, and which has some empirical support in the comparative biology of primates. But here’s how the story poses the hypothesis:

“You can’t have a large brain and big guts at the same time,” explains Leslie Aiello, an anthropologist and director of the Wenner-Gren Foundation in New York City, which funds research on evolution. Digestion, she says, was the energy-hog of our primate ancestor’s body. The brain was the poor stepsister who got the leftovers.

Meat is packed with lots of calories and fat. Our brain โ which uses about 20 times as much energy as the equivalent amount of muscle โ piped up and said, “Please, sir, I want some more.”

As we got more, our guts shrank because we didn’t need a giant vegetable processor any more. Our bodies could spend more energy on other things like building a bigger brain. Sorry, vegetarians, but eating meat apparently made our ancestors smarter โ smart enough to make better tools, which in turn led to other changes, says Aiello.

That’s a “just-so story.” How did meat make us smarter? Is it a magical meat property? If I fed enough meat to the local deer, would they get smarter? The expensive tissue hypothesis proposes an energetic trade-off, but doesn’t provide any mechanism by which the evolution of smarter brains (or diet shift) would occur. A trade-off is simply “you can’t have your cake and eat it too.” It needn’t say anything at all about how you bake a cake, or what happens if you can’t eat it.”