How have Mammals Evolved?

Do you know how have mammals evolved? As you know the group of animals known as mammals are successful, widespread, and very diverse. Modern mammals range in size from animals such as the lesser shrew of Britain at a total length of a little over six centimeters and weighing little  more than 4.5 grams to the giants of the oceans such as the blue whale weighing in at up to 150 tonnes.

Modern mammals occupy a range of different habitats: they can live in deserts, as does the polar bear, in jungles where many species abound, in temperate woodlands like the European badger and so on. These are the mammals that live on land. As we have seen, there are those that roam the oceans such as the whales.

How have mammals evolved?

Ocean dwelling mammals have more problems than their counterparts on land because they still have to maintain their body temperature. Obviously hair would be useless to help them retain heat. Consequently, they have evolved thick layers of fat usually known as blubber.

On the other hand, because the salt water helps them to support their body weight, they have been able to grow to these enormous sizes. In fact, the largest whales are the largest animals that have ever lived, larger even than the mighty, vegetarian dinosaur, Brontosaurus.


The whale is a mammal that has returned to a marine way of life.

Mammals have also conquered the air, perhaps not as efficiently as birds, but nevertheless bats could still be considered to be successful animals in the struggle for survival.

How, then, has this great diversity been achieved? In  Triassic  times, some 200 million years  or so ago, mammal-like  reptiles had developed at least partial control of their body temperature  and may have  been covered in fur or hair  rather than scales.

It is difficult to say with certainty from fossil evidence, exactly when the first true mammals appeared, but it was certainly not for another thirty or forty million years, when at the end of Triassic times animals, not unlike modern shrews, were alive.


The duck-billed platypus (above) and the spiny anteater are the only surviving mammals that lay eggs and they are very primitive indeed.

The most primitive mammals probably still laid eggs rather than giving birth to living young, and today there are still mammals surviving which are also egg-laying. The duck-billed platypus of Australia, for example, lays its eggs at the bottom of a burrow.

It has been able to survive (it could be described as a living fossil) because the Australian continent has been cut off by the drifting apart of the continents for at least 100 million years. Marsupials, also found mainly in Australia, represent a kind of intermediate stage of evolution with the young being born alive at a very early stage and remaining in the mother’s pouch.


The Tasmanian wolf and the kangaroo are examples of marsupials that have survived in Australasia because this area become separated from the rest of the world by continental drifting, as a result, these animals had little competition from more advanced mammals that were developing elsewhere.

From Tertiary  times until today, in geologic terms, the  evolution of mammals  has been very  rapid  with the development of animals  has been  very rapid  with the development  of animals  such as the horse  and the  elephant  being the easiest  to trace.

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