When the hot summer weather kicks in you may find yourself feeling concerned about furry pets and other animals we live alongside. But fur and hair are ancient adaptations that allowed human ancestors to develop more active lifestyles and varied diets. All mammals have some kind of fur. It’s part of what sets us apart from other animal groups. Fur comes in all shapes and types, including as human hair. Its thermoregulation properties can tell us about how our ancestors’ lifestyles changed as they diverged from reptile-like animals, becoming more active hunters.
Mammals are highly social and fur colouration also helps in distinguishing allies and enemies. It can help animals hide too. Think of a sand-coloured fox in the desert or a polar bear in the snow. Birds are known to have evolved from the reptilian dinosaurs and feathers are highly modified dinosaur scales. The origin of mammalian hair in deep time is less clear because it preserves less well than hard scales, bones and feathers.
Fossil imprints of hair are found in Middle Jurassic species, such as the beaver-like Castorocauda, about 220 million years ago. And a mammalian ancestor with fossilized fur, the rat-like Spinolestes, which had a mohawk of bristles, is known from the Cretaceous period, 125 million years ago.
Such fossils are rare. But scientists have found food residues that contain hair-like structures inside fossilized faeces from the Permian period, around 260 million years ago. We can take a look, however, into mammal ancestor evolution to get a better understanding of fur and hair evolution.
We know the mammalian and the reptilian lineages separated from each other more than 300 million years ago. The first mammalian ancestors still looked very much like reptiles. But they had a different diet. They hunted on large prey with their fangs, whereas reptiles originally fed on small spiders, millipedes or insects. Constant temperature is essential for mammals to have the stamina to hunt. This can be accomplished, among other things, by developing fur. Hunting also implies the need for spontaneous movement like sprinting off and pouncing on a prey. Daily sunbathing is, in contrast, essential for agility in reptiles, who sit on warm rocks and wait for insects to scuttle past.
The first animals to experiment with body temperature were mammalian forerunners, the early synapsids. Some of them had large lizard-like bodies and developed sails on their back, supported by outgrowths of the vertebrae. Thanks to a dense blood vessel system in the surface of the skin, these sails could quickly absorb heat from the sun and release excessive body heat (like elephants and fennec foxes do with their large ears today). But this strategy did not work out for them: the sailed mammalian ancestors became extinct.
