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Whales are some of the most extreme mammals alive on the planet. But how they evolved from furry, four-legged critters to giants of the ocean has not been fully understood.
Now, with the help of the most comprehensive set of whale skulls scans ever produced which spans their entire 50 million years of evolution, researchers have been able to pinpoint the key moments of their life on Earth.
From the largest animal that has ever lived to those which dive to three kilometres beneath the surface in search of their prey, whales are some of the most extraordinary mammals that have ever evolved.
But while the general evolution of whales over the past 50 million years is well recorded in the fossil record, the intricacies of how and why the group of marine mammals diverged and spread to dominate the oceans has been harder to elucidate.
A new study has been looking at the skulls of extinct and living whales to try and piece this together. By 3D scanning the skulls of some 201 species of whale spanning their entire evolutionary history, the researchers have been able to pinpoint the key moments in the evolution of whales and get a glimpse at what ecological factors were influencing these marine behemoths.
Dr Ellen Coombs, a Postdoctoral Fellow at the Smithsonian's National Museum of Natural History, completed this research whilst at the Museum.
'We've gathered the most expansive cranial data set for whales which exists on the planet,' explains Ellen. 'The main thing we were looking at was what was happening to the rates of evolution, so where are you seeing peaks and troughs in the speed of evolution, throughout their history.
'But not only did we look at what was happening to the rates of evolution and the change in skull shape, but we were also looking at what might have affected that.'
The team found that there were three main bursts of evolution for whales. The first occurred when the mammals initially made the transition from land to water, the second when the two main groups of whales diverged from each other, while a third is seen when toothed whales refined their echolocation and diversified.
The results have been published in Current Biology.
Wherever they live in the world, whales and dolphins are the dominant predators. They can be found swimming in the depths of the oceans, along rocky coastlines, and even up estuaries and into the planet's major river systems.
Known more formally as cetaceans, the group contains whales, dolphins and porpoises, but cetaceans as a whole can be split into two main groups. One of these is the mysticetes, or big baleen whales such as the blue whale, while the other is the odontocetes, or toothed whales such as dolphins and sperm whales.
All of these are descended from a land-living ancestor around 50-55 million years ago, but the spark for this transition to water is thought to have been the asteroid which killed the dinosaurs 66 million years ago.
While the impact devasted the land, it also had a significant effect on the marine environment, as it finished off the large marine reptiles, such as plesiosaurs, which were dominating the oceans.
'This means there was plenty of empty ocean, which starts to get populated by mammals about 50 million years ago,' explains Ellen. 'Within eight million years, the ancestors of whales go from being fully terrestrial, such as the four-legged Pakicetus which lived around the edge of the Tethys Sea, to fully aquatic.
'This is super quick in evolutionary terms.'
It was this rapid transition which caused the first - and largest - boost in evolution for the whales, as the move to water caused some significant changes to their skull shape.
'What you are seeing are changes in the position of the nasals, or nose, from the tip of the snout up to the top of the head as the animals are becoming more aquatic,' says Ellen. This was also associated with other major changes in the body of the early whales, namely the near-complete loss of their hind limbs.
For about 10 million years the early whales rapidly spread right around the world and into all the vacant marine environments, before another big wave of evolution rippled through the group.
This is when the two main groups of modern whales (the neocetes) split, as the baleen and the toothed whales start to do their own very separate, very different things.
'The second really interesting rise in the rates of evolution occurs about 39 million years ago,' explains Ellen. 'The big baleen whales don't have baleen at this point as they are still toothed, but they start to go down their own functional pathway, very different to the echolocating toothed whales.
'It is really interesting because when baleen then evolves about 27 million years ago and the mysticetes start to mass filter feed, within a few million years their rates of evolution and the changes in their skull come to standstill.
'We think that is just because they got really good at filter feeding and the shape of their skulls hasn’t had to change much since.'
When it comes to the toothed whales, however, Ellen and her team think that it was the evolution of echolocation which really drove the high levels of change in their skulls.
Echolocation meant that the toothed whales suddenly had a huge advantage, as they no longer needed to actually see their food to find it. This meant they could feed on pretty much any prey, and so the whales were able to enter all available niches in the aquatic world.
The toothed whales were suddenly able to dive down to inky depths on the hunt for squid, or swim up murky rivers in the pursuit of fish.
'What we really need to find in the fossil record is an Eocene odontocete whale that can echolocate,' says Ellen. 'Because we think that the earliest odontocetes could echolocate, or at least had very basic echolocation, and they took this innovation and ran with it.'
Finally, it was this ability to echolocate which led to the third spurt of evolution.
While the baleen whales evolved to become the largest animals ever to live, employing a strategy so successful their skulls have changed little since, the toothed whales were still innovating and refining their echolocation.
'About 10-18 million years ago the toothed whales, particularly the sperm whales, had some really strange changes going on in their skulls,' explains Ellen. 'We start to see high levels of shape change throughout toothed whale diversification as their echolocation is becoming more sophisticated, and alongside that we start to get a lot of weird changes in their skull.
'The whales really started to diversify in what they were eating and their body size changed as well.'
Most animals have symmetrical skulls, but one of the main changes in the skulls of echolocating whales is a highly asymmetrical or wonky skull. It is thought that this is to make space for the echolocating organ known as the 'melon'. This asymmetry is taken to the extreme in sperm whales.
The study is helping to tell the full story of whale evolution, covering not only the entire history of the animals but also their geographic spread around the world as the researchers included a lot of new material in the analysis from the southern hemisphere.
While the diet and ecology of the animals was found to be a major influence on their evolution, the next step for Ellen is to look at the abiotic factors such as climate. It is hoped that by looking at how this has impacted their development in the past it might give some clue as to how the changing environment will influence them in the future.