Illustration of an animal in the genus Deinotherium, which went extinct after the evolution of humans
Heinrich Harder/Florilegius/Alamy
Human hunting was the key factor in the loss of dozens of elephant-like species in the past 2 million years, according to an AI-assisted analysis of thousands of fossils.
The extinction rate of these animals increased fivefold when early humans evolved around 1.8 million years ago, the study concludes, and rose even higher when modern humans appeared. Today, just three species of elephant remain from this group.
“If early humans had not appeared, the number of species would probably still be increasing,” says Torsten Hauffe at the University of Fribourg in Switzerland.
The number of species of elephant-like animals, known as proboscideans after the Latin for trunk, was rising in the millions of years before the arrival of humans, says Hauffe, probably thanks to their evolution of tougher teeth for eating grass.
There were around 30 species alive 1.8 million years ago when their territories started to overlap with those of early humans. For instance, there was a species found in Africa called Deinotherium bozasi, which had downwards and backwards-pointing tusks growing from its lower jaw. D. bozasi went extinct around a million years ago.
By the time modern humans began spreading around the world some 130,000 years ago, there were only 15 species of proboscideans still alive. Most of these species went extinct too, leaving only the Asian elephant, the African bush elephant and the African forest elephant.
To work out why, Hauffe and his colleagues developed a statistical model for estimating how the rate of extinctions and speciations has changed over time based on fossil finds, along with the likely reasons for those changes.
Previous models of this kind have been limited to looking at the effect of just one factor, such as climate, but by taking advantage of AI, the team’s model can estimate the relative contribution of numerous factors, says Hauffe. “We combined everything in a single analysis.”
The study’s conclusion is that overlap with humans is the single biggest factor linked with extinction, followed by geographic distribution and the shape of teeth and tusks. For instance, species limited to islands, such as the Sicilian dwarf elephant, Palaeoloxodon falconeri, were much more likely to go extinct.
Changes in the climate, which some think was the main cause of the extinctions, came in fourth behind these other factors. So the findings support the overkill hypothesis, says Hauffe – the idea that hunting by humans is mainly to blame.
A computer modelling study of woolly rhinos earlier this year backed up the idea that even a low level of hunting can drive slow-breeding animals to extinction, says Steven Zhang at the University of Helsinki, Finland, who wasn’t involved in the proboscidean study but did help assemble some of the fossil data that was analysed.
However, a 2021 analysis of this data by a team including Zhang concluded that while an early human impact is plausible, climate was the fundamental driving agent.
What is clear is that early people didn’t suddenly wipe out proboscideans, says Zhang. “In fact, it is within this timeframe that some of the most charismatic extinct elephant species emerged, including the gigantic Palaeoloxodon of Eurasia that stood 4 metres tall at the shoulder and weighed 25 tonnes, and the familiar woolly mammoth.”
Some sites where early humans butchered mammoths or Palaeoloxodon species date back more than a million years, says Zhang. “And both lineages survived into the last 25,000 years alongside prehistoric humans that only got more cognitively and technologically sophisticated across all this time.”
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