Human ancestor Homo erectus probably wasn't the carnivore we thought

Suggestions that the evolution of ancient humans was linked to meat consumption have been found to be a little tough to swallow.

For decades, one of the leading theories for the development of apes into early humans has proposed that eating more meat contributed to the development of larger brains and bodies. This has been backed up by archaeological evidence of meat eating increasing after the appearance of Homo erectus in the fossil record.

However, a recent analysis of archaeological sites in Africa, where H. erectus is believed to have evolved, suggests that the increased amount of evidence is a result of more research into this time period at the expense of others.

Dr W A Barr, the study's lead author, says, 'Generations of paleoanthropologists have gone to famously well-preserved sites in places like Olduvai Gorge looking for, and finding, breath-taking direct evidence of early humans eating meat, furthering the viewpoint that there was an explosion of meat eating after two million years ago.

'However, when you quantitatively synthesise the data from numerous sites across eastern Africa to test this hypothesis, as we did here, the "meat made us human" evolutionary narrative starts to unravel.'

While this study, published in the Proceedings of the National Academy of Sciences, doesn't rule out meat's contribution to our evolution entirely, it calls for more research to find early evidence of it, as well as consideration of the other ways our ancestors could have developed.

Bringing home the bacon?

H. erectus is one of the first hominids to look recognisably human, with similar body sizes and proportions compared to modern humans.

With the earliest fossils of the species found around two million years ago in eastern Africa and western Asia, it is generally accepted that these early humans evolved in Africa before leaving to spread eastwards.

As a species H. erectus was very successful, living for over 1.5 million years before dying out around 200,000 years ago, although some studies suggest it may have lived on in Java for another 100,000 years.

The secret of its success is believed to rest in its suite of adaptations, including a larger brain and a smaller gut than its ancestors. Smaller guts are often associated with meat consumption as the protein in meat is easier to digest than plant material, while a larger brain would have needed significantly more energy which the easily digestible animal flesh could have provided.

In addition to providing these benefits, eating meat may also have saved energy, with less chewing required compared to eating plants. This may have been aided by tenderising it and perhaps even by cooking.

Changes in other parts of H. erectus's body have also been linked to meat consumption, with longer limbs better adapted to running than to climbing trees. It's been suggested that this might have been an adaptation to hunting.

This theory has been supported by substantial amounts of animal bones found in sites associated with H. erectus in Africa, such as at Turkana and Olduvai Gorge. Cut marks on the bones show evidence of butchery and being eaten by humans.

However, the group of American researchers wanted to assess whether the increasing amount of bones from these sites was a result of how well-studied they are, or because of an increased meat consumption in our early ancestors.

Their findings suggest that meat-eating may not have been as transformative in our evolution as is often suggested.

Putting meat on the bone

Studying finds reported from 59 sites spread across nine areas of eastern Africa dating between 2.6 and 1.2 million years ago, the scientists expected to find increasing evidence of meat consumption.

However, they found that sites that pre-dated the appearance of H. erectus were lacking, especially when compared with sampling after the species evolved. Instead, they found that the amount of effort put into the sampling was linked with the recovery of bones which showed evidence of consumption.

Excavations in well-studied areas, like Olduvai Gorge, have led to increased efforts to uncover more evidence of carnivory, causing an apparent spike in the number of bones uncovered from a particular time period. Meanwhile, other sites from similar periods, which fewer consumed bones have been uncovered from, have been less intensively investigated.

When the number of bones were adjusted by the amount of effort put into finding them, rather than the amount of meat being eaten increasing with time for H. erectus, it suggested that the level of meat-eating remained broadly the same.

The authors state that while they can't rule out the 'meat made us human' theory, more evidence from earlier sites is needed for it to hold up.

But it does leave open the possibility that other diets and ways of preparing food may have been a part of hominid evolution. Modern human ancestors may have eaten diets containing softer plant structures such as tubers, with suggestions these may have been roasted in fires.

There may also have been an increased reliance on scavenging rather than hunting animals, while studies on modern hunter-gatherers suggests that foraging could have provided enough calories to meet growing energy demands.

However, these theories also need more evidence to substantiate them, leaving H. erectus's dietary evolution an open question.

Dr Briana Pobiner, one of the study's co-authors, says, 'This study changes our understanding of what the zooarchaeological record tells us about the earliest prehistoric meat-eating. It also shows how important it is that we continue to ask big questions about our evolution, while we also continue to uncover and analyse new evidence about our past.'