Fur & Nipples: What Makes A Mammal?
You may have noticed that we don’t live among the dinosaurs these days. You may have noticed that most of the animals you see in the wild are mammals. Well, a lot can change in 65 million years, and one of the most remarkable success stories of the post-dinosaur world is the evolution of mammals. I’m going to throw out a couple of things that really impress me about mammalian development, and hopefully you’ll find them pretty fascinating too.
After the dinosaurs kicked the asses of our pseudo-mammalian ancestors and their cousins into the bookends of prehistorical success, the ancestors of mammals remained pretty small and stayed mostly in the background during the Jurassic and Cretaceous periods of earth’s history, although, of course, there was quite a bit of evolution going on in the meantime, we are talking about millions of years between our decline and rise to the top after all. In fact, there was already some pretty noticeable divergence going on, as you may notice on this diagram from Nature. The left hand column is representative of the traditional view of mammalian development, with little change happening until the dinosaurs were all but gone, but newer evidence suggests an earlier split into a number of groups, shown in, well, all the other column.
So by the time we got a chance to stretch our legs we’d already done enough of a warm up to start running, if you’ll pardon the stretched and rare sports metaphor. Anyway, these early mammals already had impressively large brains in comparison to their body size, and they’d already developed fur. In fact, fur might have evolved as far back as two hundred millions years ago, and it certainly helped when the devastating climate change caused by an extraterrestrial impact rocked earth. That impact.
The marsupial/placental split had already happened by this point, and thus the last common ancestor we share with the kangaroo will have seen a couple of dinosaurs in its time. Don’t be jealous, they’d probably want to eat it. The mammals generally remained pretty small while crocodiles and large, flightless birds dominated the post-mesozoic ecosystems, but once mammals began to grow larger, they started to outcompete their more classically reptilian contemporaries. Big cats and the ancestors of wolves would prove to be more efficient predators than the previous kings of the food chain, and between 56 to 34 million years ago – the Eocene – hoofed mammals diversified rapidly and many groups of mammals got pretty big. You can see a few of these in the image below.
One of the reasons that the mammals were so successful in reclaiming a planet the dinosaurs had lost, was their methods of reproduction and rearing their young. Whereas reptiles and monotreme mammals (like the platypus) give birth to very small young, the methods that marsupial and placental mammals employ mean that by the time the young mammal is walking around it’s already pretty big, and even then it stays with its mother, getting nourishment from nipples that quickly let it grow large enough to fend for itself. It’s an efficient system and I’d like to finish this baby off by explaining how we managed to evolve the lactation that makes it possible.
It starts with sweating, which goes along with fur to protect against the sun’s intense heat. Although we might think of fur as an insulator, it also doubles as a sunblock, although it doesn’t fare anywhere near as well without sweat. Some of these sweat glands produced chemicals to kill bacteria, as if the skin wasn’t already a harsh enough environment for them, eh? These protective substances happened to split larger sugars into lactose and help to dissolve fat. Eventually, these glands, initially used for cooling down and protecting, became specialized for providing nutrition in the area where we see mammary glands in mammals, making use of properties that were already there and exploiting them for the benefit of the next generation. And isn’t that what evolution is all about?
[image credits: national geographic, Nature]