The Origin & Early Evolution of Mammals

Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

In this lesson, we'll explore the ancestry and evolution of mammals and discover how reptiles hundreds of millions of years ago ended up evolving into one of the most successful orders of animals in history. Then, you can test your understanding with a brief quiz.


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.

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  • 0:02 Mammals
  • 0:52 Ancestors of Mammals
  • 2:59 Appearance of Mammals
  • 5:36 Lesson Summary
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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.

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