"Just-so stories" driving me crazy03 Aug 2010
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 which uses about 20 times as much energy as the equivalent amount of muscle 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 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.
I’m not anti-expensive tissue, I just want to recognize the limits of these explanatory hypotheses. Energy cannot explain everything about human cognitive evolution. It’s an important constraint, but it cannot be the only one. Without some countervailing force, energy expenditure would always favor smaller brains. So we deserve some account of mechanism, not just energy budget.
The story about endurance running attempts to tackle the issue of mechanism: “For Humans, Slow and Steady Running Won the Race”. This story relies on interviews with Dan Lieberman, who favors the idea that Homo erectus adopted a form of long-distance running.
"Most animals are designed for speed, for power, not for endurance," Lieberman explains, as we make a turn onto the bridge. "And we are a special species in having been selected for endurance, not speed."
So we grew longer legs and lighter feet; the joints in the legs and pelvis got bigger to absorb a lot of impact; and we grew a bigger butt muscle.
Lieberman says these and other changes allowed us to run down and exhaust prey, like antelopes. He notes that "persistence hunters" in Africa have been known to do that. And the payoff would've been big for early humans: lots of high-calorie meat to feed a bigger brain.
Again, this is presented as a just-so story. It’s a plausible narrative, but the article doesn’t situate it in an evolutionary context. How exactly would you test this hypothesis? You could look at prey species profiles for early Homo (favoring low-endurance species), you could consider other cognitive and physiological requirements of persistence hunting (tracking ability, knowledge of water sources), you could look for evidence of gateway strategies (use of slow-acting poisons that require long-distance tracking). You could also try to refute alternative explanations for the anatomical features in question, such as their usefulness for long-distance walking, walking on irregular substrates, or simple allometry with body size or lifespan.
These are serious hypotheses with literature and evidence supporting them. I just wish that they could be reported in a way that made it sound like paleoanthropologists are skeptical scientists!