Evolution on the menu

EDINBURGH — While some diet books might suggest otherwise, nature has been very accommodating in its meal plan for Homo sapiens. Whereas other great apes, like gorillas and orangutans, are vegetarian, though some chimpanzees will eat monkey, evolution has made humans “omnivores” and left most culinary decisions up to us. So, when did evolution enable humans to eat meat? Two sources, our species’ family tree and the fossil record, offer clues.

The branches of the human family tree converge as they descend towards the root, revealing common ancestors where branches join. The shared ancestor of humans and chimps lived some 5 million years ago, and, while we do not know for certain what that ancestor ate, the best guess is that it was mainly, if not exclusively, vegetarian. If 5 million years ago is the earliest date that human ancestors could have opted for a diet that included meat, what is the latest date by which this might have happened?

One answer to that question can be gleaned from the skeletal remains of “Lucy”, a pre-human fossil ancestor of humans, discovered in 1974. Lucy belonged to a species called Australopithecus afarensis, which lived in East Africa between 3-4 million years ago and is thought to have been the immediate ancestor of our genus, Homo. 

The first evidence that Lucy’s species might have eaten meat came in 2009, when researchers excavating a site in Ethiopia unearthed two 3.39-million-year-old fossilised animal bones scored with markings made with a stone blade. The marks were interpreted to mean that the animals had been slaughtered, but sceptics questioned why no butcher’s “blades” were found in the vicinity.

Then, five years later, the missing evidence showed up at another site in East Africa where, 3.3 million years ago, some creature was making flaked stone tools for the apparent purpose of butchery. This date is too early for that creature to have belonged to a species of Homo, so it must have been an ancestor. The most likely candidate is Australopithecus afarensis, and if Lucy was no vegetarian, then her descendants — us — have very likely always been omnivores. Evolution may have equipped humans with the ability to eat meat, but it left us with plenty of latitude to decide how much, if any at all.

A more inflexible evolutionary legacy involves our relationship with milk. Mother’s milk may be the only food that humans were biologically programmed to consume. And yet two-thirds of adults worldwide cannot drink liquid milk, because they are intolerant to lactose, a sugar that is found almost exclusively in milk. The explanation for this intolerance is that the gene that codes for the production of an enzyme called lactase, which enables babies to digest the lactose, is normally switched off in late childhood.

There are two solutions to this problem. The first, invented by farmers in Anatolia about 10,000 years ago, is to allow bacteria to consume the lactose, turning milk into curd, cheese, and yoghurt. That is why a lactose-sensitive person can comfortably digest these dairy products.

The second solution was provided by evolution. About 7,000 years ago, a genetic mutation arose among farmers in Central Europe that prevented the lactase gene from being switched off in childhood. People carrying this mutation are able to drink liquid milk safely throughout their lives, without the unpleasant side effects that lactose-intolerant individuals suffer.

This mutation was a great advantage to those who inherited it, and carriers multiplied among farmers as they swept into Northern Europe, creating one of the fastest-spreading evolutionary event humans have ever experienced. Today, some 90 per cent of Northern Europeans and a similar percentage of North Americans who trace their ancestry to Europe, are lactose-tolerant. Other mutations with the same effect have evolved independently in parts of Africa, South Asia and the Middle East.

The human diet is unique among animals because we are ubiquitous and eat whatever is at hand. In fact, humans consume thousands of animal varieties and some 7,000 plant species. But while everything we eat has an evolutionary dimension, this rarely contains any prohibitions, as meat and milk illustrate in different ways. Simply put, choice may be the biggest factor shaping human diets. Biology provides the potential in our cuisine, but it is culture that writes the menu.

Jonathan Silvertown is professor of Evolutionary Ecology at the University of Edinburgh, and author of “Dinner with Darwin: Food, Drink, and Evolution”. Copyright: Project Syndincate, 2018. www.project-syndicate.org