More sociable mammals evolve quicker than solitary ones

When the asteroid smashed into the planet 66 million years ago, it put the diversity of life onto a completely new trajectory. While it snuffed out the dinosaurs, it gave mammals the opportunity they had been waiting over a hundred million years for.

In the years following the cataclysmic event the diversity of mammals exploded, with many of the major groups we now see today appearing within a few million years after the extinction event.  

But questions still lingered about the importance of the asteroid to mammal evolution. For one, the molecular data teased from the DNA of living mammals didn't quite match up with what the fossil record was saying. Did this diversity of mammalian life exist while the dinosaurs were still strutting their stuff and the extinction only unleash it, or did this diversity only come into being when the dinosaurs were dead and buried?

To try and help answer some of these questions, Professor Anjali Goswami has been looking in extraordinary detail at the evolution of the mammalian skull. By scanning the skulls of hundreds of species of placental mammals, both living and extinct, she has then been able to build up a picture of how this group has evolved over time.

The researchers found that the largest peak in the evolution of placental mammals occured right around the extinction of the dinosaurs, with a number of increasingly smaller peaks in the following tens of millions of years to present day. 

'Everything just speeds up for mammals once the dinosaurs are out of the way,' explains Anjali. 'But because of the uncertainty of when the origin of mammals actually occurred, we can't clearly tie the peaks of evolution to certain environmental events.'

'But my guess is that that first peak is certainly related to the extinction of the dinosaurs, while the later peaks seem to correlate with key periods of climate change during the Cenozoic.'

The results of this new study have been published in the journal Science. 

Tracing the evolution of mammals

By travelling to over 20 museums and collections around the world, the researchers were able to build up a vast dataset of 322 mammalian skulls which covered the whole range of diversity both living and extinct.

Each skull was 3D scanned, before then being analysed digitally. This involved marking specific points on the digital skulls, and thus allowing the team to see precisely how the different aspects of the bones have changed over time across the entire group.

This information was then combined with a family tree for mammals and a timeline, so they were then able to see how the rate of these changes - and therefore mammalian evolution - varied across the branches of the mammal family tree through time.

'There is an early burst, so the biggest peak is right around  the dinosaur extinction 66 million years ago and then it drops really fast,' says Anjali. 'But then there is another smaller peak, and then then another smaller one and another.'

'This is what in physics is a called an attenuated wave, one which loses power after time, and this is exactly what we see with the placental mammal model of evolution.  Essentially mammal evolution seems to be slowing down, in terms of their general forms.'

While it is difficult to tie each peak to certain events, it seems likely that they coincide with different environmental factors occurring on Earth at those specific times, such as when the Earth was cooling or warming up. This would perhaps have caused different groups of mammals to adapt to different environments, and thus created a spurt of evolution, although the researchers cannot say this for certain. 

While it could be argued that the peak of evolution following the end of the dinosaurs is quite intuitive, the extraordinary dataset also allowed the researchers to tease apart different factors that might have an impact on the speed of a group's evolution.

One of the clearest outliers is the whales. These large-bodied animals have modified their skulls in some of the most extreme ways possible, rearranging countless elements of their skull as they adapted to an entirely new environment, food source and lifestyle.

But even within the other groups, there were a number of surprising characteristics that caused mammals to evolve more quickly or slowly.

'One characteristic that was really unexpected was sociality,' explains Anjali. 'Social structure hugely influences how fast mammals evolve.'

'In general, social mammals evolve much faster than solitary mammals.'

This is likely because mammals that live in groups, such as deer, rely on social signalling and competition. This can therefore lead to the evolution of flashy ornamentations such as horns and antlers. Another aspect of living in a group is communication, which can help explain the rapid evolution of, for example, echolocation and sound production in social whales.

Another surprising characteristic that was linked to fast-evolving groups is the young being independent at birth, known as being precocial. This includes animals such as antelopes and horses, which can run around soon after being born, while those mammals that are reliant on their parents for a long time, like primates or carnivores, evolve much slower.

What did the first mammal look like?

The detail gathered from these scans also allows the researchers to time travel. By piecing the data together, it means that they can in effect recreate what the skull of the earliest ever placental mammal may have looked like.

It comes out a small, likely insectivorous animal, not unlike a modern shrew or mouse. But this ability to predict the skull of early ancestors was also applied to some of the other major mammalian groups.

'We reconstructed the hypothetical ancestors for the main super orders of mammals, so the sloths and armadillos, the elephants, the primates and rodents, and the carnivores and ungulates,' explains Anjali.

'And if you look at the estimated ancestors for all of those groups, they all look the same.'

This might at first appear fairly insignificant, but it may help to answer a problem that has long foxed palaeontologists.

All these ancient groups are thought to have appeared before the extinction of the dinosaurs, but it has been exceedingly difficult assign different early mammal fossils to each group, making some question whether or not some of the groups had actually evolved at this time. It now seems likely that they did, but that they simply all looked incredibly similar, which makes it hard for scientists to identify their fossils with certainty.

But in addition to looking into the past, the work could also help other scientists look into the future. By understanding which groups are able to more quickly evolve, it could allow researchers to predict what mammals may do better, and which ones may do worse, as the planet continues to warm at an increasingly rapid rate.