Australopithecus afarensis (left), Homo rudolfensis (centre), Homo ergaster (right). Credit: Public domain; Don Hitchcock; Fernando Losada Rodríguez (rotated).
Chemical analysis of early human ancestors shows that hominins were eating carbohydrate-rich foods before they even had the ideal teeth to do so.
Such an adaptable diet may have been a driver of evolution in early humans.
Our ancient ape ancestors may have climbed out of the trees and started to walk on 2 legs as early as 7 million years ago. Scientists believe this was likely a response to the changing environment – the dense African forest where our ape ancestors evolved began to thin and be replaced with savannah and grassland as the global climate cooled and dried.
This change of scenery would have brought with it new challenges, including what early hominins would have eaten.
The need for ready sources of energy eventually led them to prefer grassy plants, grains and starchy plant tissue like underground tubers. Humans with long molars capable of chewing through these tough plant fibres didn’t evolve until about 700,000 years ago.
But a new study, published in Science, suggests early hominins were feasting on these carbs much earlier – even before they developed the teeth to properly deal with them.
The team analysed fossil teeth of different hominin species dating back to early human ancestor Australopithecus afarensis. They compared the chemical signatures of the hominins with those of extinct primates – giant terrestrial baboon-like monkeys from the genus Theropithecus and small leaf-eating monkeys from the genus Cercopithecoides.
Both the early hominins and the ancient monkeys moved away from a forest-based diet of fruits, flowers and insects toward a diet including grasses and sedges between 3.4 and 4.8 million years ago. This would have been a result of the changing habitats.
The different primate lineages had similar plant diets until about 2.3 million years ago when carbon and oxygen isotopes in the early hominin fossil teeth from Homo rudolfensis suddenly changed. The sudden drop in these isotopes is consistent with a lifestyle in which more oxygen-depleted water is consumed.
Such a change could indicate that Homo rudolfensis was drinking more oxygen-depleted water than other grassland animals, or even that this hominin species had developed a hippopotamus-like lifestyle of being submerged during the day and grazing on the savannah during the night.
Both scenarios, however, don’t match up with known physiology and behaviours or early hominins.
The explanation for the sudden isotope change which best fits known early human behaviour is that Homo rudolfensis – and likely other hominin species – were regularly eating underground grassland plant organs like tubers, bulbs and corms where oxygen-depleted water accumulates.
Such underground plant organs would have been a boon for early humans. They were plentiful, nutrient-rich, less risky than hunting and could have supported our ancestors’ growing brains and bodies. By this time, hominins had developed rudimentary tools for getting at the underground plant organs.
“We propose that this shift to underground foods was a signal moment in our evolution,” says lead author of the study Luke Fannin from Dartmouth College in the US. “It created a glut of carbs that were perennial – our ancestors could access them at any time of year to feed themselves and other people.”
The study challenges the assumption that smaller teeth in early hominins prevented them from exploiting these foods.
“Anthropologists often assume behaviours on the basis of morphological traits, but these traits can take a long time – a half-million years or more – to appear in the fossil record,” says senior author Nathaniel Dominy, a professor at Dartmouth.
“But these chemical signatures are an unmistakable remnant of grass-eating that is independent of morphology. They show a significant lag between this novel feeding behaviour and the need for longer molar teeth to meet the physical challenge of chewing and digesting tough plant tissues.”
“We can definitively say that hominins were quite flexible when it came to behaviour and this was their advantage,” Fannin adds. “As anthropologists, we talk about behavioural and morphological change as evolving in lockstep. But we found that behaviour could be a force of evolution in its own right, with major repercussions for the morphological and dietary trajectory of hominins.”
The researchers say the study could answer questions about why our ancient ancestors were so successful.
“One of the burning questions in anthropology is what did hominins do differently that other primates didn’t do? This work shows that the ability to exploit grass tissues may be our secret sauce,” Dominy says.