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How deep-sea drilling is helping to unravel elephant evolution
Fossil teeth and deep-sea drilling are guiding scientists through the evolution of the largest living land animals.
As Africa became increasingly arid over the past 10 million years, elephant teeth became increasingly adapted to resist wear, allowing them to adapt to the spread of tropical savannas on the continent.
Elephants and their relatives evolved in a previously unappreciated way.
A new study has used sediment cores from deep-sea drilling and ancient human fossils to help track the changes in elephant tooth fossils over the past 26 million years. It 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 is a specialist in fossil mammals at the Museum who co-authored the new study alongside University of Helsinki researcher Dr Juha Saarinen.
Adrian 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, published in the journal Nature Ecology & Evolution, may also offer new evidence for why only three species of elephant survive today.
By studying the teeth of ancient elephants like Palaeoloxodon (left) and Protanancus (right), the researchers are able to tell what they were eating and thus what the environment was like at the time they were alive. Image ©Juha Saarinen / The Trustees of the Natural History Museum
How did elephants evolve?
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 first proboscideans weren’t very large, with early species such as Eritherium estimated to have been only around five kilograms in weight.
After a slow start, the group rapidly began to diversify around 30 million years ago as they spread around the world. Over time, this led to a whole range of different species, from tiny elephants on Mediterranean islands to mastodons in North America.
Back in Africa, the changing environment also drove the evolution of new species. 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 through their teeth. This is because teeth preserve very easily as fossils, allowing scientists to track changes in 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 develop 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 elephants were 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.
Modern elephants are the last members of a group stretching back more than 60 million years. Image © Martin Finnerty/Shutterstock.
Revealing the evolutionary ratchet
Drilling in the Arabian Sea in previous decades has recovered many 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 the tree of 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 dustier, with the periods where levels of dust peaked 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. The researchers think that this could be an important and relatively common type of evolutionary change, but that a lack of fossils and climate data over such a long period 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 are known to 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 African savanna and forest elephants, together with their cousins the Asian elephant, are the only proboscideans left in the world. With all three species heavily endangered, urgent action is required to keep their once mighty lineage from being lost forever.
