Chris has a master's degree in history and teaches at the University of Northern Colorado.
We love our dogs, cats, gerbils, and hamsters. We love cows and pigs for an entirely different reason. And, we're also pretty fond of humans, at least most of the time. So, what do all of these have in common? Yeah - they're mammals, a class of animals characterized by the presence of hair, warm-blooded bodies, and live birth. Mammals include the largest animals on the planet, the whales, as well as the most intelligent - many people assume that's us, but for a growing number of scientists, that's up for debate. In any case, if we're all related, why are mammals so different from each other - so diverse? I mean, I'm not about to mistake my neighbor for an armadillo. A sloth, maybe, but that's beside the point. Let's take a look back through evolution and see how far back we can trace our family tree.
Ancestors of Mammals
Okay, here we go, the ancestor of mammals. This one-celled critter is the first living organism on the planet. Hmm. Maybe we don't need to go back quite that far. Let's fast-forward a bit. Okay…vertebrates, fish, amphibians, ooh - Ichthyostega, that's a fun one, just a bit more, and here we are. We are in the Carboniferous period, at roughly 312 million years ago. See this little guy, this is Archaeothyris, one of the first synapsids. A synapsid is an animal with an opening in the skull, a small hole called a temporal fenestra, behind each eye. This fenestra, this opening, allows for a stronger jaw, since more muscles can develop. The most dominant of the early synapsids were called the pelycosaurs, an early reptile-like group of creatures without scales; instead, they had something closer to skin. The pelycosaurs were, believe it or not, the bridge between reptiles and mammals, called mammal-like reptiles due to their skin and temporal fenestra.
Okay, let's jump forward a bit more. We've made it to the Permian period. It's roughly 275 million years ago, and this is a therapsid. Therapsids evolved from pelycosaurs to have legs that are directly under their bodies, the way that modern mammals do today, rather than jutting from the sides like reptiles. However, one group of therapsids was more successful than all the others, the cynodonts, who evolved roughly 260 million years ago. Cynodonts were pretty close to mammals. They had defined jaws and teeth, a larger brain cavity in the back of the skull, and likely some form of warm-blooded heating. But they also laid eggs and don't seem to have had true fur. Cynodonts were generally small, somewhat shrew-like creatures who burrowed and seem to have lived in small groups.
Appearance of Mammals
Around 250 million years ago, there was a mass extinction that killed off about 70% of land-based life. The cynodonts, safe in their burrows, survived, and emerged to find that many of their many predators were dead. The main danger were crocodile-like reptiles that hunted during the day, so the cynodonts evolved to be nocturnal, have great senses of smell and hearing for safety, and developed better whiskers and fur for getting around in the dark. By around 210 million years ago in the early Triassic period, roughly the same time as the appearance of the first dinosaurs, these cynodonts had evolved into the first mammals. This is Eozostrodon, one of the oldest mammals we've discovered. Cute, isn't it? Throughout the period of the dinosaurs, which was roughly 250-65 million years ago, early mammals thrived, but never grew much larger than small rodents. They diversified, some becoming aquatic, some adapting to trees or shrubs or rocks or plains or burrows. Around 160 million years ago, these mammals divided into two groups, which would eventually become the marsupials and placentals, which is what we are.
Now, as any 8-year old could tell you, around 65 million years ago the world's most successful group of animals, the dinosaurs, died. Soon after that, the world fell into an ice age that lasted millions of years. Now, what's pretty nice to have in an ice age? Fur. It really comes in handy. So, the dinosaurs die off, the world turns into a giant snow globe, and mammals are in absolute heaven. For the next 60 million years, mammals evolved at an incredible rate, occupying every ecological niche left open by dinosaurs. We had saber tooth cats and gigantic pigs as top predators, 10 foot-tall ground sloths, miniature horses on the plains, early whales and porpoises in the oceans, bats in the skies, and mammoths! Mammoths! Come on, how cool is that? Most orders of mammals had evolved by roughly 30 million years ago, and all of them were developed by 3 million years ago. Of course, it wasn't until about 200,000 years ago that humans - as we are today, anyway - appeared on the scene. So, we're relatively newcomers to the world. But, we're alive at a good time. It's a mammal's world.
This world is full of mammals, a class of animals characterized by the presence of hair, warm-blooded bodies and live birth. Mammals can trace their ancestry back to roughly 312 million years ago with the appearance of the synapsids, animals with temporal fenestrae behind the eyes. The most dominant of these early synapsids, the pelycosaurs, eventually evolved into therapsids roughly 275 million years ago. Therapsids, such as the group called the cynodonts had legs under their bodies, something similar to warm-blooded heating, and were developing early versions of fur. After a mass extinction 250 million years ago, the cynodonts evolved into true mammals by roughly 210 million years ago. These mammals expanded during the age of the dinosaurs, but were generally the size of a small dog at largest. It wasn't until after the extinction of the dinosaurs 65 million years ago and the emergence of the ice age that mammals really exploded into thousands of diverse species occupying every ecological niche on the planet.
To unlock this lesson you must be a Study.com Member.
Create your account
Register to view this lesson
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.Become a Member
Already a member? Log InBack