A good and informative book about the evolution of mammals, from their beginnings to the present day. The author shows that mammals never evolved from reptiles, but instead evolved alongside them. While mammals were small and easily overlooked during the age of the dinosaurs, that did not mean they were restricted: in fact, she shows that mammals kept evolving and filling ecological niches at that time, before they began to take on bigger challenges after the extinction of the non-avian dinosaurs.

Her descriptions of the fieldwork done in the past and today to form the modern picture of the evolution of mammals is fascinating. She also points out the colonialism, racism and sexism that was rife in palaeontology, that she and others are now changing, getting more women and local people involved in palaeontological research.

In short, a book that should be read to get the modern picture of the origins and evolution of mammals.

What follows is a chapter by chapter summary of the book:

Chapter one introduces the author, her field work on fossils on the Scottish Isle of Skye, and on the beginnings of the group that would give rise to mammals.

Chapter two gives a summary of the history of mammal fossils. When first found in rocks dating back to the age is the dinosaurs, mammal fossils were considered primitive and identified as belonging to marsupials. This is a misconception due to the idea of animals changing to become more sophisticated (and human) over time. It is only in modern times that modern marsupials are shown to be just as sophisticated as placental mammals and evolved alongside, not from, the reptiles and dinosaurs.

Chapter three looks back in time to the Carboniferous period, where most groups of life were starting to emerge on land. After the plants, the insects were the most abundant landform. But following behind were the animal groups that would give rise to the mammals and reptiles. While early mammals forms are often described as 'reptile-like', this is wrong. As the author shows, mammals and reptiles developed from different groups of animals (as shown through examination of their fossil skull bones in this period) and only resemble each other at that stage of life, leading to the misunderstanding.

Chapter four looks at the Permian period. It is during this time that the Synapsids began to get big, with animals like the well known Dimetrodon and others. Various explanations are provided for that animal's sail on its back. But it was also during this period that herbivores (plant eating animals) first developed.

Chapter five looks at other groups of animals that were also present during the Permian that descended from the Synapsids. One group, the Therapsids, would give the world predators and herbivores that are distinguished from other groups of animals by possessing specialized teeth for cutting and chewing. Another group, the Cynodonts, who would become the ancestors of the mammals, also developed a more upright gait, that would let them move more efficiently. It was around this time that evidence for hot-blooded animals can be found in the fossils.

Chapter six looks at the Triassic period, which came after a major extinction event that marked the end of the Permian. It took millions of years for the major groups of animals to recover, but when it did, the world would look very different. Gone were the Therapsids that dominated the Permian. In their place were the reptile groups that would give rise to various forms of crocodiles and that most well known of ancient animals, the dinosaurs. Why this happened is still subject to research.

Chapter seven continues to look at the development of animals during the Triassic that would eventually lead to animals that can be recognized as mammals. During this period, these animals would become smaller and become nocturnal, a successful way of living. Their hearing and sense of touch and smell would also become well-developed. Their teeth would also develop into specialized forms for eating.

Chapter eight looks at some is the technology, like CT scans and Synchrotron machines, that are able to penetrate stones and fossils to reveal their inner structure. This technology would be critical for analysing mammal fossils, as there are often small and fragile. The scanning technology world help to illuminate how some of the basic features of mammals, like whiskers, hair and the ability to provide milk, would begin to appear in ancient mammals.

Chapter nine looks at the mammal-like forms developing in the Jurassic period. Many were small, which initially caused many people to overlook them, concentrating on the much larger dinosaurs in that period. But closer examination shows that the mammal-like forms were developing new ways of living, like burrowing or gliding. Their small size may also have helped them to develop more sensitive hearing by helping to free up jaw bones to be used as inner ear bones.

Chapter ten looks at the ancestors of mammals and mammal-like animals in the Cretaceous. With dinosaurs now dominating the landscape, it can be easy to overlook the much smaller mammals during this time, but fossils continue to show mammals diversifying and finding new ways of living in a landscape that was also undergoing change, with the development of flowering plants.

Chapter eleven looks at what happens after the asteroid impact that lead to the extinction of the non-avian dinosaurs. Various forms of mammals (and other life forms) that survived flourished, leading to the world we now know. But the usual story that the dinosaurs were preventing mammals from developing, before the impact, may not be correct, if the author's research is correct, indicating that other kinds of mammals may also have had a hand.