Meat-eating and food processing were major drivers of human evolution

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.

Reference:
Zink, K. & Lieberman, D., Impact of meat and Lower Palaeolithic food processing techniques on chewing in humans. Nature (Published online) (2016).

Archaeological evidence for carcass processing at Kanjera, Kenya, 2 million years ago.

Earliest unambiguous evidence for meat-eating by early hominins.

Modern humans are the only existent primates anatomically adapted for the regular consumption of significant quantities of meat. The human gut is reduced compared with that of other primates, a configuration more suited to a meat-eating diet than the predominantly vegetarian diet of other primates. Although crucial to many models of hominin evolution, however, the timing of and circumstances in which early hominins began to include significant quantities of meat in their diet remain poorly understood.

The earliest-known stone tools, from Gona, Ethiopia, are 2.6 million years old and are often taken to be early evidence for meat eating (Semaw, et al., 1997; Semaw, 2000). No hominin remains were recovered in association with the tools, but in 1999, anthropologists working at the nearby Bouri Formation reported the discovery of large mammal bones bearing cut-marks apparently made by stone tools, possibly as a result of dismembering and filleting carcasses. Animals appeared to have been defleshed, and their long bones broken open, presumably to extract marrow. The bones were found in association with 2.5 million-year-old australopithecine remains, thought to be of Australopithecus garhi (de Heinzelin, et al., 1999).

It has also been claimed that 3.39 million-year-old animal bones from Dikika, Ethiopia, show stone tool cut-marks for flesh removal, and signs of having been struck with hammerstones to extract bone marrow (McPherron, et al., 2010). In the absence of any associated tools, there is no way to tell whether the cut-marks were produced with specially-made tools or naturally-sharp pieces of stone. Some are sceptical and argue that as the bones were buried in coarse-grained, sandy deposits, it is likely that trampling by animals produced the marks (Domınguez-Rodrigo, et al., 2011).

Even if the above is accepted as evidence of carcass-processing by early hominins, it is too insubstantial to show whether these were one-off forays into meat-eating or part of a more substantial shift in hominin dietary adaptations. To demonstrate ‘persistent carnivory’ requires a geologically-stratified series of relatively large assemblages of animal remains, each showing extensive signs of persistent hominin activity. The sum of the assemblages must demonstrate that this activity persisted over the course of at least a thousand years (Ferraro, et al., 2013).

Although rather more recent than the above dates, such evidence has now been reported from Kanjera South, a small site located on the shores of Lake Victoria, southwestern Kenya (Ferraro, et al., 2013). Three excavations along 50 metres have yielded several thousand well-preserved animal remains, approximately 2 million years old, and associated with stone tools. There is a consistent record of hominin activities throughout the stratified sequence, which spans hundreds or possibly thousands of years.

The animal remains included gazelle and other small bovids, together with a smaller number of medium-sized bovids. The remains showed clear evidence of butchery by hominins in the form of cut-marks and damage caused by hammerstones. Patterns of tooth-marks made by carnivores such as lions and hyenas suggest that these animals only had access to the carcasses after the hominins had removed the bulk of the meat and bone marrow. Carnivores typically chew on the mid-shafts of long bones, but the percentage of bones that were so marked was low.

Small bovids are invariably wholly consumed by carnivores within hours of death, implying that the hominins acquired and butchered them very soon after death. A possible implication is that these animals were hunted rather than scavenged, and that Kanjera represents the earliest archaeological record of hunting activities by hominins.

The skeletal remains of the small bovids suggest that they were transported to the site for butchery more or less intact. However, in the case of the medium-sized bovids, head and limb parts predominate. These animals were too large to transport intact, so the hominins removed the limb parts, leaving the rest of the body behind. Although head contents are nutritious, they are difficult to exploit and would thus be ignored by other scavengers. They therefore represent a niche that tool-using hominins could exploit. It is therefore likely that hominins scavenged leftover head parts from carnivore kills and transported them to the site for processing.

The Kanjera data not only provides the required evidence of hominin meat-eating over a period of many centuries: it also provides clues about specific activities. Thus, it seems, the hominins obtained much of their meat by hunting small bovids, but they also scavenged medium-sized bovid heads as a separate by complimentary activity. The date of 2 million years ago is somewhere between 200,000 and 500,000 years earlier than the previous earliest evidence for persistent hominin carnivory.

References:

1. Semaw, S. et al., 2.5-million-year-old stone tools from Gona, Ethiopia. Nature 385, 333-336 (1997).

2. Semaw, S., The World’s Oldest Stone Artefacts from Gona, Ethiopia: Their Implications for Understanding Stone Technology and Patterns of Human Evolution Between 2.6–1.5 Million Years Ago. Journal of Archaeological Science 27, 1197–1214 (2000).

3. de Heinzelin, J. et al., Environment and Behavior of 2.5-Million-Year-Old Bouri Hominids. Science 284, 625-629 (1999).

4. McPherron, S. et al., Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature 466, 857-860 (2010).

5. Domınguez-Rodrigo, M., Pickering, T. & Bunn, H., Reply to McPherron et al.: Doubting Dikika is about data, not paradigms. PNAS 108 (21), E117 (2011).

6. Ferraro, J. et al., Earliest Archaeological Evidence of Persistent Hominin Carnivory. PLoS One 8 (4) (2013).