Among the most famous of all events in evolutionary history (if something that took 30 million years could be called an event) is when a lineage of fish-like animals emerged from the water onto the land, setting the scene for the radiation of amphibians, reptiles, birds and mammals.
But over the following 370-odd million years, some of these terrestrial vertebrates performed spectacular U-turns and returned to the sea, including ichthyosaurs, plesiosaurs, sea turtles, penguins, whales, dolphins, manatees, dugongs, seals, sea lions and walruses.
Who could tell from looking at a whale or a dolphin that they are descended from a group of terrestrial mammals that includes hippos and cattle? Or that manatees and dugongs are the closest living relatives of the elephants?
What's the difference between sea lions and seals?
The ancestry of seals and sea lions – which, together with fur seals, elephant seals and walruses, comprise the pinnipeds – is perhaps a little more obvious.
Seals are not called ‘dogs of the sea’ for nothing. Their faces especially have a canine quality about them.
Their terrestrial ancestors were actually closer to weasels, otters, skunks and racoons than to dogs, but pinnipeds do sit squarely amongst the Carnivora, the order of mammals that contains cats, dogs, bears and other predators. Unlike whales, dolphins, manatees and dugong, which spend all their time in the water, pinnipeds still have one foot – or flipper – on terra firma, hauling themselves out onto land to breed.
There are 33 living species of pinniped (the name derives from the Latin words for ‘fin’ and ‘foot’) distributed across the world’s oceans, although most occur in temperate and polar regions. They comprise three distinct families, the largest two of which contain the ‘true seals’ (seals and elephant seals) and the ‘eared seals’ (seal lions and smaller, hairier fur seals). The third is occupied solely by the unmistakable walrus.
Though they appear similar, true seals and eared seals can be easily distinguished. First, true seals lack the external earflaps of the eared seals. Second, their styles of locomotion are different.
True seals use their rear flippers to power them through the water and their front ones mainly for steering. In eared seals, it’s the other way round.
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On land, true seals wriggle around on their bellies, while eared seals are able to fold their rear flippers and use them almost like legs.
The biggest and smallest pinnipeds are both true seals. Male southern elephant seals can exceed four tonnes in weight, making them the biggest of all the carnivores.
The smallest is the Baikal seal, just over a metre long and about the weight of a human. It’s also the only freshwater pinniped, living in Russia’s Lake Baikal, thousands of kilometres from the nearest coastline – and no one is quite sure how it got there.