What makes a mammal?

Author Liam Drew explains what sets us apart from the other classes of the animal kingdom

Find out what separates us from birds and reptiles

It was while my daughter was being breastfed — which happened right after I’d spent months occasionally getting an ultrasound peep at her growing inside a womb — that I became interested in my being a mammal.

I’d known that I was a mammal for as long as I could remember, and I had some basic grasp that it meant I’d once subsisted solely on milk and that I had hair and was warm-blooded. I also knew that humans were placental mammals, not pouched marsupials or egg-laying mammals like the platypus and its closest cousins, and so we develop in wombs, nourished there by placentas. But beyond these basic facts I wasn’t at all sure what defined a mammal or how this type of animal had evolved. Suddenly, consumed with an interest in the elemental aspects of my biology, I decided to find out more about what makes me like a fox, hedgehog and giraffe, and unlike an anaconda, turtle or pigeon.

My approach to this question was initially pretty 18th century. Back then people defined groups of plants and animals not by their being descended from shared ancestors but according to which unique features they shared. Shared reproductive biology and a need to breathe air made Carl Linnaeus declare in 1758 that whales and dolphins weren’t fish but members of a natural group with furry, warm-blooded land animals.

To name this new group he’d created, Linnaeus chose — with no explanation — the mammary gland as the defining shared feature they’d be named after, thereby coining the term ‘mammalia’, which we translate as mammals. But a century after mammals were first grouped together, Charles Darwin published his great book On the Origin of Species by Means of Natural Selection and biologists came to widely accept that animals evolved.

According to Darwin, mammals were a group of animals more closely related to one another than to any other type of creature, and the features they share are owed to them having been inherited from a common ancestor. To explain where these traits came from one had to first explain what the advantages were of having them conferred on their possessors and also how they had come to exist from an ancestor that had not had them. How, for instance, had a mammary gland evolved from a milk-less ancestral state?

Mammals’ closest living relatives are reptiles, who are members of a much larger class of animals called Sauropsida, which also includes birds. And we know now that the mammalian and reptilian lineages diverged about 310 million years ago. Their last shared ancestor looked decidedly more reptilian than mammalian. And from that cold-blooded, milk-less and hairless (not to mention small-brained, sprawlinglimbed and primitively toothed) starting point, the succession of pre-mammalian ancestors who morphed into true mammals evolved all the traits that we associate with mammals today.

The first recognisably mammalian creatures lived about 210 million years ago. Much of what we know about the evolution of mammals therefore comes from fossil records. But this information is also today combined with conclusions drawn from comparing the bodies — and DNA — of all the various 5,500 living mammal species.

“mammals’ closest living relatives are reptiles”