Kenya 15 million years ago, with extinct elephant relatives feeding on different plant types Credit: Beth Zaiken

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Climate change drove elephant evolution

A new study tracking the changes in elephant tooth fossils over the past 26 million years found that as Africa became more arid, their teeth became more resistant to wear so that they could survive. 

However, when the environment got wetter, the teeth didn’t become less resistant. Instead, they stayed pretty similar until drier conditions drove the next burst of evolution.

Professor Adrian Lister, a specialist in fossil mammals at the Natural History Museum who co-authored the new study, says, ‘We found that there was an evolutionary trend in the shape of the teeth, even though the climate is fluctuating. While you might expect that the characters of the teeth would go back and forth as the climate does, they get higher step by step and don’t decline.’

‘We’ve described this phenomenon as an evolutionary ratchet, and it’s not something we’re aware has ever been demonstrated in fossils before.’

The findings of the study may also offer new evidence for why only three species of elephant survive today.

Elephants are a part of a group known as the proboscideans, which are thought to have evolved in Africa over 60 million years ago. Unlike modern species, the earliest forms weren’t very large, with species such as Eritherium estimated to have been only around five kilograms in size.

Modern elephants evolved out of a group known as the gomphotheres, along with extinct relatives such as the straight-tusked elephants and the woolly mammoth.

One way to track the evolution of these animals is with their teeth. They preserve very easily as fossils, allowing scientists to track changes in their diet over millions of years.

‘Modern elephant teeth are huge with high crowns, and their surface is formed of a series of ridges that look like the underside of a trainer,’ Adrian explains. ‘But back at the start of their evolution, their teeth are smaller, with lower crowns and more rounded cusps.’

‘Higher crowns protect against abrasion, which is important as elephants only have a set number of teeth. Once they’re all gone, the elephant will die, so it’s important to switch to higher crowns to maintain a similar life expectancy.’ 

While some theories suggest that the abrasion was caused by switching from soft foods like tree leaves to tougher grasses, others suggest that it was ancient climate change. As Africa became more arid, dust would be more common in the food the elephant was eating, which would increase tooth wear.

To look into what might be causing these changes, the team turned to research from two very different areas – human evolution and deep sea drilling.

Drilling in the Arabian Sea in previous decades has recovered a series of sediment cores, which record millions of years of geological history. If Africa was becoming more arid, the researchers would expect to see higher levels of dust at certain points in the core.

The dates of these dusty periods could then be compared to the age of the elephant teeth. These are well known because many of these fossils were uncovered alongside early hominin fossils in East Africa, which have been extensively dated to see where they fit into human evolution.

Analysis of the core drilled closest to where the teeth were found allowed the researchers to examine over eight million years of history. They found that the region was generally getting more dusty, even if levels of dust fluctuated over time.

Periods where levels of dust peaked were accompanied by bursts of tooth evolution. Statistical testing showed that dust was the best explanation for changes in crown height and ridge number in elephant teeth, while diet better explained the folding of the enamel.

However, it was the evolutionary ratchet that the team were most intrigued by. While a similar phenomenon has been found in studies of human and animal culture, it’s not something that’s been demonstrated before in evolution through the fossil record.

The researchers think that this mechanism is probably relatively common in nature, but a lack of fossils in other animals has kept it hidden from scientists.

Over time, the ratchet would have been one factor driving the evolution of extreme grazers such as Palaeoloxodon jolensis, a straight-tusked elephant which was highly dependent on grasslands. Climate fluctuations over the past million years would have affected these habitats, and the elephants may have been unable to adapt. 

‘While the ratchet allowed elephants to survive arid climate and the spread of grasslands, becoming overspecialised could have contributed to their extinction,’ Adrian says. ‘Modern African elephants are quite generalist in their teeth and diet, which may have helped them to survive as their relatives died out.’

Today, the savanna and forest elephants are the only proboscideans left on the continent, together with the Asian elephant the last representatives of a group containing almost 200 species. With all three species heavily endangered, urgent action is required to keep their once mighty lineage from being lost forever.

The paper Fluctuating climate and dietary innovation drove ratcheted evolution of proboscidean dental traits is published in Nature Ecology & Evolution,.

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