Michael Pollan's seven-word dictum for what we should eat is by now famous: "Eat food, not too much, mostly plants."
Yet in Pollan's four-part series Cooked, available on Netflix, Episode 1 ("Fire") is a veritable ode to meat, especially to barbecue. Adopting an evolutionary perspective, Pollan says that cooking made us human. He brings Harvard University biological anthropologist and author Richard Wrangham on screen to back up this statement.
I've taught Wrangham's 2009 book Catching Fire to my anthropology students for years and, for the most part, find it quite convincing. Wrangham looks at the significant anatomical changes in our ancestor Homo erectus right around 2 million years ago — bigger body and brain, smaller gut, smaller teeth compared to ancestors who came before — and explains them through a cascade of developments brought about by cooking vegetables like tubers.
Cooking made available to our ancestors unprecedented nutrients that fueled brain growth over time, and reduced the need for energy-expensive chewing of tough foods. (Our closest living relatives the chimpanzees, who eat only raw foods, may spend six hours a day chewing). One contentious issue with Wrangham's thesis is that there's only patchy evidence for control of fire and cooking in the time period where Wrangham's thesis most needs it to be. Is that just a gap in the archaeological record, or is cooking in fact not a behavior routinely mastered by Homo erectus individuals?
Wrangham's ideas, despite this outstanding question, have permeated popular culture as key explainers for how we became human.
Now, in the pages of Nature, evolutionary biologists Katherine D. Zink and Daniel E. Lieberman, also from Harvard, have published a notable challenge to the cooking hypothesis.
Zink and Lieberman see the same changes in Homo erectus noted by Wrangham, but say that cooking is unlikely the reason they emerged. Control of fire, they assert, became common only around 1 million years ago and cooking only at a half-million years ago, which is well after the development of the features of Homo erectus that cry out for explanation.
What our ancestors did have, before the routine control of fire and cooking, was stone-tool technology for the processing of foods, starting 3.3 million years ago.
The two researchers took an experimental approach to help tease apart the role of meat, mechanical processing and cooking in our past. As extensive media coverage last week explained (see here and here), Zink and Lieberman recruited volunteers to process either tough goat meat or yams, carrots and beets, and wired them up with electrodes to monitor the time and force required to do so under different conditions: The meat and vegetables were either unprocessed (raw) or processed (sliced, pounded or cooked).
One striking result: The unprocessed meat just couldn't be chewed into parts small enough for swallowing. (Truly, those raw-goat-chewing volunteers must have been very dedicated to science!)
Slicing and pounding made all the difference — and not only with the meat. These actions made vegetables, too, much more energetically efficient to process and eat, even in the absence of cooking.
Zink and Lieberman conclude this in their paper (USOs are vegetables, "underground storage organs" like tubers):
"The reductions in jaw muscle and dental size that evolved by H. erectus did not require cooking and would have been made possible by the combined effects of eating meat and mechanically processing both meat and USOs. Specifically, by eating a diet composed of one-third meat, and slicing the meat and pounding the USOs with stone tools before ingestion, early Homo would have needed to chew 17% less often and 26% less forcefully. We further surmise that meat eating was largely dependent on mechanical processing made possible by the invention of slicing technology."
Here's the upshot: Zink and Lieberman think they have found an energy-based explanation for why our bodies and brains increased in size, and our gut and teeth decreased in size, that fits better with human-evolution chronology as we know it than does Wrangham's set of ideas.
On Tuesday, I asked Wrangham what he makes of this new research. In an email message to me, he praised his Harvard colleagues' work: "Their research was beautifully designed and carried out, and provided some striking results including the large reduction in the amount of chewing of meat even from merely slicing it."
Wrangham then pointed out two problems he sees with Zink and Lieberman's rejection of the need for cooking as an explanatory factor for the described changes in Homo erectus.
While all three researchers agree that Homo erectus ate a lot of wild plants like tubers (USOs), Wrangham focuses on the difficulty in extracting enough energy from wild plants for our ancestors to thrive without cooking — even taking into account the beneficial results of pounding.
"So Homo erectus could have been an effective consumer of raw wild USOs only if their guts were very different from ours," Wrangham told me. "Current evidence, however, is that as with Homo sapiens, the guts of Homo erectus were too small to ferment long-chain carbohydrates as they would have needed."
Further, he noted, if cooking wasn't invented by Homo erectus, it's hard to see when it would have been: "Cooking has predictably much larger effects on evolutionary anatomy than non-thermal processing. Yet the changes in human anatomy after Homo erectus are relatively small."
What foods we ate millions of years ago and how we processed them were important factors in our species's evolution. It's fascinating to look side-by-side at the cooking hypotheses and the slicing-meat hypothesis: Catching Fire is highly enjoyable to read, and the Nature paper is open-access.
I invite you to do some comparing and, along the way, to enjoy some exciting evolutionary science.
Barbara J. King is an anthropology professor at the College of William and Mary. She often writes about human evolution, primate behavior and the cognition and emotion of animals. Barbara's most recent book on animals is titled How Animals Grieve. You can keep up with what she is thinking on Twitter: @bjkingape.