Study shows how dietary changes and stone tools enabled reductions in size of teeth, jaws and gut
In comparison to earlier hominins, Homo erectus was bigger both in stature and brain size. As such, its energy requirements would have increased – but paradoxically the teeth and chewing muscles were smaller, maximum bite forces weaker and the gut size was reduced. It has long been assumed that this was made possible by increased meat consumption, slicing and pounding food with stone tools, and by cooking. However, the latter was uncommon until around 500,000 years ago. By these means, it is believed that Homo erectus and later humans reduced the both amount of chewing required for their food and workload of the gut in digesting it.
In a newly-published study, Zink and Lieberman report on a series of experiments intended to test these hypotheses. They measured chewing performance in adult human subjects fed size-standardized portions of meat and underground storage organs (roots, tubers, etc.) which are thought to have formed a major component of hominin diet. Goat meat, yams, carrots and beets were chosen for the test; goat is tougher than beef and therefore more similar to the wild game eaten by early hominins. The food was either unprocessed, processed by simple mechanical methods available in Lower Palaeolithic times (slicing and pounding), or roasted (the simplest form of cooking).
They found that the subjects were unable to chew the raw meat effectively, but slicing it resulted in substantial reductions in both the amount of chewing and bite forces required, and in smaller and more digestible meat particles were swallowed. Roasted meat required a greater chewing effort, but even smaller meat particles resulted. However, even unprocessed meat required considerably less masticatory effort than the raw USOs.
Although the advent cooking brought considerable benefits in terms of hygiene and increased energy yields, Zink and Lieberman believe that the reductions in dental size and jaw musculature observed in Homo erectus would have been made possible by the combined effects of eating more meat and mechanically processing both it and USOs. By eating a diet of one-third meat and two-thirds USOs, and slicing the meat and pounding the USOs with stone tools prior to eating, early humans would have reduced chewing by 17 percent and enabled a 26 percent reduction in bite forces.
Although it is possible that food processing and meat eating favoured evolutionary selection for smaller teeth and jaws, Zink and Lieberman believe that it is more likely that these relaxed the selective pressures maintaining robust masticatory anatomy, thus enabling selection to decrease facial and dental size for other functions such as speech production, locomotion, thermoregulation, and possibly even changes in the size and shape of the brain, so leading eventually to the modern condition of Homo. Regardless of what evolutionary factors favoured these changes, they would not have been possible without increased meat eating combined with food processing technology.
Zink, K. & Lieberman, D., Impact of meat and Lower Palaeolithic food processing techniques on chewing in humans. Nature (Published online) (2016).