哺乳類の起源は2億年前に遡り、祖先に近い形を保ったものも、環境に適応すべく進化を遂げたものもあります。地下で暮らすものもいれば、飛行するもの、水陸両棲のもの、完全に水中で生活するものなど多種多様な種です。そうした哺乳類の高い活動水準、比較的大きな脳や高い適応性は、体温と代謝率に追うところが大きいと著者は指摘します。化石をもとに哺乳類の進化の過程を辿り、淘汰された種や人類につながる樹林霊長類にも言及します。
 

• Introduces the nature, evolutionary history, and modern diversity of mammals
• Considers how one of the defining characteristics of mammals, their warm bloodedness, has affected their physiology and enabled them to adapt to a wide variety of environments.
• Discusses the evidence in the fossil record for the origin and spread of mammals, showing how fossils illustrate patterns of evolution.

From a modest beginning in the form of a little shrew-like, nocturnal, insect eating ancestor that lived 200 million years ago, mammals evolved into the huge variety of different kinds of animals we see today. Many species are still small, and follow the lifestyle of the ancestor, but others have adapted to become large grazers and browsers, like the antelopes, cattle, rhinos, and elephants, or the lions, hyaenas, and wolves that prey upon them. Yet others evolved to be specialist termite eaters able to dig into the hardest mounds, or tunnel creating burrowers, and a few took to the skies as gliders and the bats. Many live partly in the water, such as otters, beavers, and hippos, while whales and dugongs remain permanently in the seas, incapable of ever emerging onto land.

In this Very Short Introduction T. S. Kemp explains how it is a tenfold increase in metabolic rate - endothermy or "warm-bloodedness" - that lies behind the high levels of activity, and the relatively huge brain associated with complex, adaptable behaviour that epitomizes mammals. He describes the remarkable fossil record, revealing how and when the mammals gained their characteristics, and the tortuous course of their subsequent evolution, during which many bizarre forms such as sabre-toothed cats, and 30-tonne, 6-m high browsers arose and disappeared. Describing the wonderful adaptations that mammals evolved to suit their varied modes of life, he also looks at those of the mainly arboreal primates that culminated ultimately in Homo sapiens.