Early hominins may have been wiping out other species long before destruction-causing modern humans evolved, according to changes in the fossil record. About 5 million years ago, land-living turtle species, including tortoises, started going extinct at a much higher rate than they had been, suggests a new analysis.
鈥淥ur finding鈥� highlights how far back in time a negative hominin influence on biodiversity extends,鈥� write Anieli Pereira, formerly at the University of Gothenburg in Sweden, and her colleagues.
Modern humans have had a massive effect on ecosystems starting from . We wiped out big animals wherever we went. But many people suspect that the human impact on the planet began much earlier.
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For instance, team member S酶ren Faurby at the University of Gothenburg and his colleagues have shown that extinction rates in large carnivores in eastern Africa , while those in small carnivores remained stable, but it is hard to be sure that hominins were the cause.
Tortoises and other turtles may have been especially vulnerable to predation by early humans, because their defence mechanism of retracting into their shell isn鈥檛 effective against highly intelligent aggressors. Chimpanzees have been after smashing their shells against trees, for example.
鈥淵ou don鈥檛 need elaborate hunting techniques and weapons to kill a big tortoise 鈥� a hand axe or even a large rock would probably be enough to crack the shell open,鈥� says , UK.
Since the fossil record of turtles is unusually good, Pereira and her colleagues decided to analyse it to see how extinction rates changed over time. They excluded all sea turtles, all island turtles and also fossils whose dates are highly uncertain. That left about 3400 fossils representing 82 living species and 908 extinct ones.
The researchers found three major extinction events where turtle species went extinct faster than new ones evolved. Two events were ancient, with one coinciding with the extinction of the dinosaurs. The third began some 5 million years ago and continues to the present.
Looking at this period in more detail revealed that extinction rates rose only in the turtles that lived mostly on land, not those that spend most time in fresh water. What鈥檚 more, extinction rates in the Americas and Australia 鈥� which humans reached later 鈥� only went up around a million years ago, much later than in Africa and Asia.
鈥淥ur results point towards a hominin cause of extinctions rather than global climate change,鈥� the team concludes.
However, Longrich isn鈥檛 convinced. Humans reached Australia only around 70,000 years ago and the Americas around 35,000 years ago, he says.
This apparently huge discrepancy in the timing, however, could be an artefact caused by the fact that many recent fossils are dated only as being from the Pleistocene, an epoch that lasted from 2.6 million to 12,000 years ago, Pereira and her colleagues note. More accurate dating might show that the extinction rate really did coincide with the arrival of people.
Pereira declined to discuss the findings until publication in a peer-reviewed journal.
Reference: bioRxiv, DOI:
