Living Things Change
The process of natural selection works very slowly. It usually takes thousands, if not millions, of years for new species to evolve. Today, mammals are an incredibly diverse group of animals, but they all developed from a single group.
Monotreme Mammals
Scientists believe that the first mammals laid eggs just like reptiles and amphibians. Only two egg-laying mammals are still around today: the platypus and the echidna. These both belong to a group called monotremes and live in Australia.
Placental Mammals
Most modern mammals, including humans, are part of a group called placentals, in which the baby fully develops inside the mother. After birth, she feeds the baby and cares for it.
Marsupial Mammals
Marsupials, such as kangaroos, are a small group of mammals from an older…