Mammal Beginnings

The first mammals were small, shrewlike animals that were about 5 inches (12 cm) long. Related to today's monotremes, they first appeared during the Triassic Period, about 220 million years ago. They were descended from reptiles called synapsids, which appeared about 300 million years ago. These primitive mammals evolved into different groups during the Jurassic and Cretaceous periods (208-65 million years ago). Most of these early mammals were carnivores (meat eaters), but some, such as the tree-living multituberculates, which ranged from animals the size of mice to some as big as beavers, ate plants. The ancestors of today's marsupials, insectivores and primates first appeared in the Cretaceous Period (145-65 million years ago). When the dinosaurs died out at the end of the Cretaceous Period, these modern mammals spread to every continent and evolved into thousands of new species.

The First Mammal

- Megazostrodon -

Megazostrodon, which lived in Africa about 220 million years ago, is the oldest known mammal. This insect eater was only 5 inches (12 cm) long and probably laid eggs like today's monotremes. 

The First  Person

- Australopithecus afarensis -

The earliest known human was Australopithecus afarensis, who lived in northern Africa about 3 million years ago. About 4 ft (1.2 m) tall, Australopithecus was first identified from a series of footprints found in hardened volcanic ash. In 1974, the skelton of a female Australopithecus, named "Lucy" by its discoverers, was found in Ethiopia. 

Good Dog

- Cynognathus - 

Cynognathus, whose name means "dog jaw" was a mammal-like reptile that lived 240 million years ago. It grew to about 3 ft (1 m) in length, but its head, with massive jaws, was more than 1 ft (30 cm) long. 

Spiky Ancestor 

- Dimetrodon - 

Sail-backed Dimetrodon was a mammal-like reptile, It belonged to a group of animals that had large openings in their skulls behind the eye sockets. Mammals gradually evolved from members of this group. 

Circling their Prey 

- a Arsinoitherium fighting with Hyaenodons - 

On the plains of northern Africa 40 million years ago, a female Arsinoitherium defends her young against a pack of 4 ft (1.2 m) long predators called Hyaenodon. Although Arsinotherium grew to nearly 13 ft (4 m) long, they were actually relatives of today's rabbit-sized hyraxes. 

Did You Know? 

Camels and their near relatives now live in South America, Asia and Africa. They evolved in North America but died out there during the Pleistocene Period, about 12,000 years ago.