Body Mass of Animals Shrank by 98% During Last 1.5 Million Years

Humans did cause mass extinction throughout the Pleistocene, Israeli archaeologists and zoologists prove in first meta-study on megafaunal extinction in the southern Levant

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A saber-toothed cat tries to drive a short-faced bear out of its territory.
A saber-toothed cat tries to drive a short-faced bear out of its territory.Credit: Daniel Eskridge / Shutterstock
Ruth Schuster
Ruth Schuster

The average body mass of animals hunted and consumed by early humans in the southern Levant shrank by more than 98 percent over the course of the Pleistocene – from 1.5 million years ago to 11,700 years ago, when the Holocene epoch of human civilization began – concludes an astonishing meta-study out of Tel Aviv University.

By 10,500 years ago, the mean body mass of animals in this region was only 1.7 percent of the mean body mass of animals 1.5 million years ago, report Jacob Dembitzer, Ran Barkai, Miki Ben-Dor and Shai Meiri of Tel Aviv University in Quaternary Science Reviews.

It has long been known that megafauna gradually vanished through the Pleistocene, especially following the last Ice Age, when modern humans spread everywhere. But only now are the dimensions and extent of this drastic phenomenon becoming clear, Barkai explains.

Credit: Jacob Dembitzer, Ran Barkai, Miki Ben-Dor and Shai Meiri

Forensic evidence

The relative contributions of climate changes and human predation pressure have been fiercely debated for decades regarding each animal in various locations. To quantify the phenomenon once and for all, Dembitzer and colleagues carried out a meta-study, reviewing dozens of articles, reports and books on hominin evolution and the fauna in the southern Levant since the Pleistocene. Their work has isolated the overriding cause for the first time.

The team found no correlation between the decline of animal body mass during the Pleistocene and changes in global temperature or rainfall. It wasn’t paleo-temperature, or paleo-precipitation: The culprit had to be paleo-us, littering our stamping grounds with the bones of our meals.

We entered the Pleistocene as early humans with rather small brains. We left it as modern humans with big brains embarking on the path to modern civilization. We had nothing but fossils and some cave art to remind us of the great towering animals that once lived among us, which we hunted. Various papers show evidence suggesting that hominins actively hunted, not just scavenged.

As we wiped out the megafauna we had to resort to eating smaller and smaller animals because that was all that was left. As the team puts it: “Throughout the Pleistocene, new human lineages hunted significantly smaller prey than the preceding ones. This suggests that humans extirpated megafauna throughout the Pleistocene and when the largest species were depleted the next largest were targeted.”

Rock art around the world from Asia to Europe and the Americas clearly shows animals long since extinct, including mammoths, and aurochs.Credit: Cave painter

The evolution of our brains from orange-sized to medium-cantaloupe-sized was actually driven by our need to develop better tools to hunt smaller and more evasive animals, Barkai and the team proposes. Simply put, you can brain an elephant with a wooden hunting spear if it’s not looking and you’re agile and strong enough. To catch a deer or a rabbit, you need a completely different toolkit.

It bears clarifying that the team is not suggesting that early or later humans bear the sole responsibility for all megafaunal extinctions everywhere. Also, some megafauna still survive, from the moose to the elephant and rhino to the Nile crocodile and the Columbian hippopotamus (thank you, Pablo Escobar). Giant snakes are also still a thing.

But European and American elephantids and rhinos are no longer a thing, the giant sloth is gone, the giant hyena of southeast Asia is no more, the lions of Europe and so, so many more.

The meta-study relates solely to the southern Levant. There is a wealth of data on early humans who crossed this region on their way out of Africa towards Eurasia (and back again) and the animals that existed in the different archaeological layers. How relevant are these findings to the rest of the world?

The evolution of our brains was actually driven by our need to develop better tools to hunt smaller and more evasive animals, the team proposes.Credit: Claude Valette

“Very relevant,” Barkai answers. It is true that no studies like this one have been done for other regions, which lack a comparable continuity of data going back 1.5 million years and more over so many sites. “But certainly there are indications that this was a global phenomenon. Everywhere, people likely focused on the big animals and when they were gone, they focused on small ones,” he explains.

Separate research has indeed shown that animal body size in Africa started declining more than a million years ago. That is well before modern humans evolved. The human line split from the chimp around 7 million years ago, and at some point, took a significantly carnivorous turn. In fact, Barkai and other researchers suspect that at least from the point of Homo erectus, our line became super-predators, subsisting largely on meat and fat, plus foraging for plant matter.

Thus, it is plausible that early humans were one driver of the megafaunal decline in Africa. The case in Australia and the Americas is much clearer-cut and not in our favor, according to various studies. In South America, 83 percent of megafauna species went extinct, including three whole orders of animals during the last million years. There was also a great extinction event in the Pampas 13,000 to 10,000 years ago, by which time humans had arrived.

As the megafauna grew scarce in the mid-Pleistocene, cave use increased, which the group thinks is no coincidence.Credit: Prof saxx

Settling down

In another global indication of our culpability, rock art around the world from Asia to Europe and the Americas clearly shows animals long since extinct, including mammoths, aurochs, lions in Europe and giant sloths. Clearly members of our species were intimately familiar with these giants.

And as the Ice Age waned, rendering great swaths of land suitable for vegetation to flourish, and as the animals vanished or became so small that they were hardly worth catching, even with the innovation of arrows – the Neolithic Revolution was born. Agriculture and domestic animals augmented and finally largely replaced hunting and gathering, according to the model Barkai and his team have been building.

“We claim that towards the Holocene there were basically no big animals to hunt so they started domesticating animals,” he explains. “Domestication and agriculture developed at around seven places around the word independently, probably for much the same reason.” Farming developed apparently independently at different times, in different places, but all during the Holocene: the Levant; in China; in Ethiopia, central America, north America, southern India and possibly other places too.

Previous separate work has suggested that agriculture might have arisen because mobile foragers were “forced” to settle down and grow stuff by overlords, because otherwise why would peacefully roaming hunter-gatherers have turned sedentary. Put otherwise, the rise of some form of states put pressure on people to stay put and farm so the states could collect the surplus grain and meat to feed their armies.

Credit: Griffith University

But for one thing, farming everywhere predates the earliest known states. Barkai and the team have a simpler explanation: people need to eat, they were running out of options, the climate had turned more conducive to plant proliferation and farming and animal husbandry solved the problem.

This meta-study demonstrating that animal body mass shrank by over 98% in the Pleistocene to the Holocene focused on mammals, leaving out reptiles. We know the early humans in Israel roasted tortoises, so why leave out our scaly meals? Because there isn’t enough data on reptile consumption as opposed to that of mammals – maybe we didn’t like eating reptiles or it’s a research bias, Barkai answers. Prehistoric sites are riddled with mammalian remains.

Another thing. You can’t lug a dead elephant back to your cave to share with the kids. But as the megafauna grew scarce in the mid-Pleistocene, cave use increased, which the group thinks is no coincidence.

Rock art in Madagascar depicting a giant sloth.Credit: David A. Burney / Julian P. Hume

The last megafauna in the Levant were the aurochs. The last one died about 3200 to 2500 years ago. It weighed on average about 900 kg.

A biological taboo

No, Barkai doesn’t think this story impacts human extinctions. Homo sapiens is the only survivor of the Homo group but he does not believe the patterns the group discovered in their meta-study were driven by sheer aggression – only need, and he does not believe cannibalism was rife, even if it may have happened here or there.

“We know there are evolutionary cons against cannibalism; it’s not only a cultural but biological taboo. Kuru,” he sums up in a word – the human version of mad-cow disease, caused by eating the nervous system and brain tissue from our species-mates. You eat a human brain with “bad” prions, you may get the condition too.

The study doesn't suggest that humans bear the sole responsibility for all megafaunal extinctions everywhere, but we certainly seem to have a had a hand in it.Credit: Claude Valette

Also, Barkai speculates, “Their respect for animals would have touched on other humans too.”

Despite the carnage implied by this story, Barkai retains his confidence that early humans of all types had respect for nature and for the very animals they hunted and ate. This theory is based partly on their evident attempts to utilize the carcass down to the last molecule, not letting any go to waste.

He also believes, since most modern humans clearly have no respect for animals whatsoever, and little for other humans, that somewhere along the line, we lost that quality. So what happened to the big animals if we respected them so much? “They had respect for the animals, but they had to make a living, and there seems to be a price for human prosperity," Barkai answers. "And this price is paid by the environment.”

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