Early humans ate tough plants - and changed the world forever

Long before they had the teeth for it, early humans were already eating tough plants – grasses and underground starches.

That’s the big takeaway from a new study led by scientists at Dartmouth College. Their work shows that our ancestors didn’t wait for their bodies to evolve before making smart dietary choices.

Instead, they took risks, made use of what was available, and used their behavior to get ahead – literally eating their way into evolutionary change.

Behavior can shape human evolution

The researchers examined fossilized teeth from ancient hominins, tracking chemical traces left behind by graminoids – grasses and sedges. What they found was striking: hominins had already begun eating these tough, fibrous plants long before their teeth were optimized to chew them.

It took about 700,000 years for molars to evolve into the longer versions we see in modern humans – teeth that are better suited for grinding and digesting plant material. But humans didn’t wait. They just found a way to eat what they needed.

“We can definitively say that hominins were quite flexible when it came to behavior and this was their advantage,” said Luke Fannin, a postdoctoral researcher at Dartmouth and lead author of the study.

“As anthropologists, we talk about behavioral and morphological change as evolving in lockstep. But we found that behavior could be a force of evolution in its own right, with major repercussions for the morphological and dietary trajectory of hominins.”

Fossil clues hidden in chemistry

Studying ancient behavior is hard because it doesn’t fossilize. You can’t dig up a behavior the same way you dig up a bone. But chemical signatures on teeth can fill in the gaps.

“Anthropologists often assume behaviors 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,” said Professor Nathaniel Dominy, senior author of the study.

“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 behavior and the need for longer molar teeth to meet the physical challenge of chewing and digesting tough plant tissues.”

Shifting away from grass

The researchers studied fossil teeth from several early human species, starting with Australopithecus afarensis.

For comparison, they also examined fossils from two other extinct primates – theropiths (large, baboon-like monkeys) and colobines (small monkeys that ate mostly leaves).

Between 3.4 and 4.8 million years ago, all three species started eating more grasses and sedges and less fruit, flowers, and insects. None of them had the ideal teeth or digestive systems for it, but they made the switch anyway.

Then, around 2.3 million years ago, hominin tooth chemistry shifted again. Carbon and oxygen isotope levels dropped, suggesting a big dietary change. This likely reflects a move away from grass-eating by early humans like Homo rudolfensis.

The shift to underground foods

The team looked at three possible explanations: more water drinking, a hippo-like lifestyle of staying submerged during the day and feeding at night, or regular access to underground plant parts – like tubers and bulbs.

Their conclusion? Underground foods were the key. These plant organs, stored beneath the surface, are rich in carbohydrates and available year-round.

The underground foods were also harder to reach, which gave hominins with stone tools an edge. Other animals weren’t digging for these starchy powerhouses.

“We propose that this shift to underground foods was a signal moment in our evolution,” said Fannin. “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.”

Human teeth and evolution

To see how physical features caught up, the team tracked changes in tooth size and shape. Over time, hominin teeth shrank by about 5% every 1,000 years. The molars, though, grew longer.

It wasn’t until Homo habilis and Homo ergaster came along about 2 million years ago that teeth really adapted. Their molars had shapes better suited for chewing roasted or cooked starches – evidence of how cooking may have joined eating underground plants as a survival strategy.

Graminoids are everywhere. As humans moved through different landscapes, they were able to keep using these plants for nutrition. Over time, their teeth became more efficient at breaking them down.

“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,” said Dominy.

“Even now, our global economy turns on a few species of grass – rice, wheat, corn, and barley,” he said. “Our ancestors did something completely unexpected that changed the game for the history of species on Earth.”

The full study was published in the journal Science.

Image Credit: L to R: Public domain; Don Hitchcock; Fernando Losada Rodríguez (rotated)

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