Advance of the Mammals

Once the mammals had improved on the reptiles' method of reproduction, by keeping the eggs in the body of the female, where they were fertilized and developed, two separate forms evolved. These were on the one hand placental mammals, and on the other the marsupial or pouched mammals.

Placental mammals

In the placental mammals, the embryo is attached to the wall of the mother's uterus or womb at the placenta. The young animal is born at quite an advanced stage of development. This is what happens in the vast majority of mammals, including humans.

Marsupials

Young marsupials, by contrast, are born at a very early stage of development. The tiny creatures then immediately crawl into the mother's pouch and live there, feeding on milk from one of the teats inside the pouch. They remain in the pouch until they are quite well grown.

Marsupials or pouched mammals now live mainly in Australia and South America. The kangaroo, koala, opossum and wombat are all examples.

The ancestors of marsupials separated from the other mammals about 100 million years ago. During the late Cretaceous they lived in every continent, but they could not compete with the more efficient placental mammals. They survived only in the comparative isolation of the Americas and Australia. In the latter continent, where the only placentals are bats, a few rodents and some mammals introduced by people, dozens of different kinds of marsupials can be found.

Newly-born kangaroo suckling inside pouch

An adult kangaroo

Insulating fur

The mammals' insulating covering of fur, and warm blood, were particularly useful during the Palaeocene and Eocene, when the world grew cooler. As the world moved towards the Ice Age, the warm-blooded birds and mammals had a big advantage.

Palaeocene mammals

The Palaeocene mammals included the remote ancestors of today's elephants, rhinoceroses, pigs and cattle. In the sea, the whales were replacing the ichthyosaurs, plesiosaurs and mosasaurs. By the Eocene, 54 million to 38 million years ago, we find ancestors of the primates, a group of mammals to which the apes, monkeys and we humans belong. Some of these creatures were not unlike today's monkeys and gibbons. In this period we also find a group of animals no bigger than fox terrier dogs, the ancestors of today's horses.