The incorporation of meat into the diet was a milestone for the human evolutionary lineage, a potential catalyst for advances such as increased brain size. But scientists have struggled to determine when meat consumption began and who did it.
New research provides the first direct evidence that Australopithecus, an important early human ancestor that displayed a mix of ape-like and human-like traits, consumed very little or no meat, relying on a plant-based diet. The study determined the diet of seven Australopithecus individuals from South Africa dating to between 3.7 and 3.3 million years ago based on the chemistry of their tooth enamel.
"Meat likely played a significant role in the expansion of cranial capacity - larger brain development - during human evolution. Animal resources provide a highly concentrated source of calories and are rich in essential nutrients, minerals and vitamins that are critical for fueling a large brain," said geochemist Tina Lüdecke of the Max Planck Institute for Chemistry in Germany and the University of the Witwatersrand in South Africa, lead author of the study published on Thursday in the journal Science.
"Our data challenge the assumption that meat was a crucial dietary component for Australopithecus, despite some specimens being found in association with stone tools and cut-marked bones," Lüdecke said.
The findings suggest meat consumption was a later development, perhaps by subsequent populations of the various distinct Australopithecus species or by other species in the human evolutionary lineage, collectively called hominins. Australopithecus inhabited eastern and southern Africa from roughly 4.2 to 1.9 million years ago. Our species Homo sapiens appeared roughly 300,000 years ago.
The seven individuals studied were vegetarians.
"While occasional meat consumption is plausible, akin to modern non-human primates such as chimpanzees and baboons, our data suggest a diet primarily composed of plant resources," Lüdecke said.
This may have included browsing for fruits, tree leaves and certain flowering plants on the savannah landscape, Lüdecke said.
Australopithecus possessed ape-like face proportions and a brain about a third the size of our species, as well as relatively long arms with curved fingers, good for tree-climbing. Australopithecus stood on two legs and walked upright.
"Australopithecus provides critical insights into the evolution of bipedal locomotion and early tool use. While their brains were smaller than ours, their relative brain size was slightly larger than that of modern chimpanzees," Lüdecke said.
The most famous Australopithecus fossil is the one nicknamed Lucy, which was discovered in Ethiopia in 1974 and is about 3.2 million years old. Lucy, likely female, stood about one meter (3.5 feet) tall. Males would have been somewhat larger.
Lucy was a member of the species Australopithecus afarensis. The seven individuals in the study probably are members of the closely related species Australopithecus africanus.
The chemistry of the food consumed by an individual gets incorporated into tissue, including hard parts such as tooth enamel that are conducive to fossilization. The researchers analyzed seven fossilized molars found in the Sterkfontein cave near Johannesburg, part of South Africa's "Cradle of Humankind" area known for yielding early hominin fossils.
The ratio of two different forms - isotopes - of the element nitrogen in the Australopithecus teeth aligned closely with fossils of herbivorous animals in the same ecosystem such as antelopes rather than with carnivorous animals such as hyenas, leopards and saber-toothed cats.
The earliest evidence for possible meat consumption among hominins includes animal bones with cut marks dated to 3.4 million years ago in Ethiopia. Whether these represent butchering for meat has been a matter of debate.
The finding that Australopithecus, with a brain smaller than later hominins, "did not consume substantial amounts of mammalian meat is consistent with the hypothesis that a dietary change played a role in brain expansion," said study co-author Alfredo Martínez-García, head of the Max Planck Institute for Chemistry's organic isotope geochemistry laboratory.
"If we would have found that Australopithecus consumed substantial amounts of meat, we would have concluded that the subsequent expansion of cranial volume in other hominin species was not due to the onset of meat consumption," Martínez-García added.
Meat consumption also may have contributed to increased physical stature, reduced gut size, social complexity and tool use among hominins.
"The critical questions remain: who first started eating meat, when did it begin, and when did it become a resource significant enough to drive morphological adaptations?" Lüdecke said.
