Animals first evolved to live exclusively on land about 370 million years ago 鈥� but on dozens of occasions since then land animals have gone back to exploiting the seas. That might be because the shallow seas around continents are so full of food that they proved an irresistible lure.
Some of the most spectacular species now living in the sea have land-living ancestors. Whales are descended from animals similar to deer, while walruses and seals evolved from animals a bit like modern otters.
What鈥檚 more, many land-dwelling species go to sea for food: seabirds are one example, and Galapagos iguanas are another. Even our species exploits the oceans on a massive scale. We began doing so 125,000 years ago or perhaps even earlier, and there is evidence that we have been fishing for deep-sea tuna for 42,000 years.
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鈥淲e are very definitely marine organisms,鈥� says at the University of California-Davis. 鈥淲e鈥檙e also terrestrial too, obviously, but ecologically speaking we鈥檙e enormously important to the seas.鈥�
What encourages land animals to go marine? Vermeij and his colleague decided to test two competing ideas.
A whale of a time
One is that land animals turn marine right after mass extinctions, taking advantage of the ensuing chaos in ocean ecosystems. They use this opportunity to adapt to marine life, enabling them to compete with marine natives when the oceans recover. It鈥檚 an idea that found support in a 2014 study, which uncovered 66 million years ago.
But there鈥檚 an alternative. Maybe land animals can muscle their way into the sea at any time. They might .
Vermeij and Motani identified 69 occasions when land animals began living in or exploiting the seas over the last 250 million years. There was little evidence of land animals turning to the seas after mass extinctions.
In fact, for two of the biggest mass extinctions in Earth鈥檚 history 鈥� one at the end of the Triassic 201 million years ago, and the end Cretaceous extinction 鈥� there was no spike in land animals turning marine. 鈥淭hat surprised us,鈥� says Vermeij.
This finding contradicts the results of the 2014 study. at the University of KwaZulu-Natal in Durban, South Africa, who led that study, thinks this might be because his team only looked at living species. They didn鈥檛 consider land animals that turned marine and later went extinct, whereas Vermeij and Motani did. 鈥淭his of course makes their study more inclusive,鈥� says Proche艧.
Going underwater
Instead, Vermeij and Motani found support for the idea that land animals can move into the sea at any time.
Their study suggests that the rate land animals turned marine has increased steadily over the last 60 or so million years. This interval doesn鈥檛 include any significant mass extinctions, but it does coincide with an increase in marine productivity. For instance, , so more food was available in shallow seas. Vermeij thinks this increase in food availability might have made oceans a tempting target for land animals.
But Proche艧 thinks it鈥檚 unwise to reduce this to an either/or debate. There is probably merit to both ideas that attempt to explain why land animals return to the seas, he says. 鈥淎ll such events and variables are inevitably interlinked.鈥�
Paleobiology
