A New Diet Didn't Make Us Human, The Change Came Later

A bonobo collecting aquatic herbs looks like it knows it is guaranteeing the growth of its offspring's brain. The observation has changed our thinking about the diets of human ancestors. Zana Clay/LuiKotale Bonobo Project

Two theories about how changes in diet enabled humanity's evolution have run into trouble in quick succession.

Around 2.1 million years ago, a species appeared in eastern Africa that was so different from its ancestors anthropologists class it as part of the genus Homo, to which we belong, not Australopithecus, from which it evolved. The question of what made these first humans so unlike those that went before is crucial to understanding our evolution, but the popular theory diet made the difference has bitten the dust.

Although chimpanzees and bonobos supplement their diet with meat, they eat a lot less of it than most humans. This observation has inspired some to jump to the conclusion – greatly encouraged by the livestock industry – that consuming more meat gave us extra nutrients, allowing our large brains to develop and our rise to world domination to begin.

Dr David Patterson of the University of North Georgia attempted to identify such a shift by looking at the carbon and oxygen isotopes in teeth and bones from human ancestors in the East Turkana region of Kenya.

The mechanism Patterson describes in Nature Ecology and Evolution does not directly identify the amount of meat in the diet. Instead, it measures the extent to which the ultimate energy source was from C3 or C4 plants. C4 plants have more carbon-13 than their counterparts, and this gets incorporated into the bones of those who eat them. The carbon-13 can be obtained directly, or through eating animals that feed on C4 plants.

Previous research has shown that ancestral hominins once relied primarily on C3 plants, but that more recently the C4 component was much larger. Patterson's work indicates the major shift occurred between 2 and 1.4 million years ago.

This means that the change in diet took place between early human species like H. habilis and the appearance of H. erectus, rather than around the time H. habilis appeared. The timing coincides with a rise in antelope butchery.

Today we consume a lot of C4 plants directly in the form of sugar cane, millet, and sweet corn, but our ancestors were more likely to eat animals that grazed on dry season grasses. So the shift suggests a meatier diet.

An increase in C4 consumption might also be explained through a climatic change that gave an advantage to C4 plants. However, a comparison with the bones of other types of animals living in the area at the time indicates their diets either didn't change or moved the other way.

People get passionate about diets, and the wild success of “eating paleo” indicates the enduring appeal of the theory that the healthiest diet is the one our ancestors ate.

Unfortunately for those who like to keep things simple, we've eaten a lot of different diets since we became human, only some of which had a substantial meat component.

A separate study has also challenged the idea humans needed to live near the ocean to get such big brains. Iodine is essential for brain development, which is why it is sometimes added to salt and bread. Ocean food sources are rich in iodine, but there have been questions about how human ancestors who didn't live close to the seashore could have got enough to allow big brains to grow.

Observations of bonobos harvesting aquatic herbs have changed that. In BMC Zoology the Max Planck Institute for Evolutionary Anthropology's Dr Gottfried Hohmann reports these herbs are surprisingly high in iodine. This not only explains how apes today avoid iodine deficiency, but shows a seafood diet was not the only option in humanity's past.

 

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