H. erectus cut, chewed way through evolution

Less energy needed to eat sliced raw meat led to smaller teeth, jaw

BITE CLUB  Early members of the Homo genus evolved relatively small jaws and faces because they sliced raw meat with stone tools, making it easier to chew and digest, a new study concludes (Homo erectus, represented by a 1.8-million-year-old African skull, shown).

Chip Clark, Smithsonian Human Origins Program 

Early members of the human genus had a flair for preparing sliced wild game tartare, a new study suggests. That meaty diet may have literally changed the face of Homo evolution, and enabled advances in talking and walking.

By 1.8 million years ago, Homo erectus sliced up raw meat with stone tools before eating it, say Harvard University paleoanthropologists Katherine Zink and Daniel Lieberman. It was a momentous move, making it possible to consume more calories while reducing chewing effort, the researchers report online March 9 in Nature. Faces and jaws got smaller while an energy-rich diet allowed brains and bodies to enlarge.

Evolutionary shrinkage of bones and muscles involved in chewing affected other parts of the body, the scientists say. An expanded vocal tract boosted the ability to make speech sounds. And a repositioned spinal cord, resulting from realignment of the base of the skull, increased the ability to walk and run long distances.

Cooking made it even easier to chew and digest meat and edible plants such as yams, Zink and Lieberman say. But they estimate that hominids began to regularly cook food only about 500,000 years ago. Cooking stimulated further jaw and facial shrinkage in Homo sapiens, but not in H. erectus as some researchers have suggested (SN Online: 8/22/11), the researchers say. Instead, a taste for sliced, raw meat got those facial changes off to a fast start in H. erectus, the scientists conclude.

Researchers have long noted that H. erectus skulls display relatively smaller jaws and faces than hominid species from more than 2 million years ago that preceded the Homo genus.

Using modern-day humans, Zink and Lieberman analyzed muscular effort and number of chews needed to ready different foods for swallowing. A total of 34 adults chewed standardized portions of goat meat or three starch-rich plants — jewel yams, carrots and red beetroots. Food came either unprepared, pounded with a stone to soften it, sliced or cooked.

After an average of 40 chews, participants still couldn’t break apart 3-gram chunks of goat meat. Meat slices, however, required an average of about 31 chews to break into pieces that could be easily swallowed and digested.

If one-third of total calories came from sliced meat, and the remaining calories came from stone-pounded plants such as jewel yams, H. erectus would have needed to chew its food 17 percent less often — more than 2.5 million fewer chews per year — and 26 percent less forcefully than if only unprepared plants were consumed, the scientists estimate.

The new study provides the first evidence for a decades-old assumption that tool-assisted meat eating prompted the evolution of smaller faces in early Homo, says paleoanthropologist Manuel Dominguez-Rodrigo of Complutense University of Madrid. “The key is the consumption of sliced meat, enabled by the use of stone tools.”

H. erectus must have regularly hunted prey to maintain a diet of about one-third meat (SN: 6/1/13, p. 13), Dominguez-Rodrigo says. 

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