Mammals are familiar beasts. From a squirrel on a power line to a blue whale swimming through the sea, we share the world with more than 6,000 mammal species of all shapes and sizes. While we can easily distinguish a creature like a jaguar from a reptile or a bird in the modern world, however, mammals as we know them are the result of hundreds of millions of years of evolutionary changes. In fact, many of the key features that make us mammals evolved even before the dinosaurs.
Paleontologists have known for decades that mammals emerged from a broader, diverse group of creatures called synapsids. The very first synapsids of about 306 million years ago were small and lizard-like, but distinguishable by a single opening in their skull behind their eye socket. (We have a modified version of this hole, the space between your cheekbone and your cranium where a jaw-closing muscle runs through.) Nevertheless, a big evolutionary gap exists between a very early, lizard-like synapsid and modern mammals like ourselves.
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