All modern mammals evolved from a group of reptiles that lived more than 200 million years ago. Over time these reptiles developed the various adaptations that define mammals today: the skull and jaw bones were rearranged, they developed different types of teeth, they brought the limbs underneath the body rather than sprawling out to the sides, and they developed a diaphragm and palate to help them breathe efficiently and eat at the same time. At some point during their development, the mammals also started to regulate their own body temperature through internal heat production and started to feed their young with milk. Since these are features which don't fossilise it is impossible to say when these changes occurred.
The world of these early mammals was very different from the world today. There was only one continent on Earth, and the climate was much warmer. The other animals around at the time were the first dinosaurs, and many other reptilian groups. Invertebrates had been around for a very long time and were very diverse, as were the fish. There were no flowers, but plenty of non-flowering plants.
The first mammals were small, and probably nocturnal since their internal heat production allowed them to stay active even when the temperatures dropped at night. They used their sense of smell to find prey and each other.
By about 100 million years ago there were several types of mammal. We are familiar with the monotremes, marsupials and placentals of today, but there were also other kinds, such as multituberculates, which are now extinct. The world was changing, though. The one mass of land began to break up, and the separating continents took with them living cargoes of animals.
The first split was a north-south divide. North America and Eurasia broke away from South America, Africa, Australia and Antarctica. Then Africa broke away from the Southern continent, and India too went adrift. Placental mammals were divided and started to develop in four different ways.
In Africa, a group developed known as the Afrotheres. Today these are represented by the elephants, the sea cows, the elephant shrews, the golden moles, the tenrecs and the aardvark. In South America, a group known as the Xenarthrans developed, today represented by the anteaters, sloths and armadillos. Across North America and Eurasia lived the Laurasiatherians, a large group containing the carnivores, hoofed animals, whales, bats and other animals. Also in the north lived the fourth group known as the Euarchontaglires: the rodents and primates.
The marsupials were not confined to the Southern hemisphere as they are today. Opossums lived in Europe and North America and even invaded Africa when it finally came into contact with Europe around 30 million years ago. The monotremes have a much poorer fossil record, and so it is not known how diverse and widespread they might once have been.
Having split apart, the continents eventually started to collide with each other, and their different groups of mammals started to mix. First Africa collided with Europe. Primates, hoofed animals and carnivores flooded in, while elephants and other Afrotheres spread north. Then South America and North America touched, allowing Xenarthrans like the giant ground sloths and armadillos to move north, whilst hoofed animals and carnivores moved into South America.
Today there is such a mix of animals throughout the globe that it is only recently, with the help of molecular analysis, that we have begun to piece together their complicated past. The formation of this haphazard jigsaw has helped us realise just how distantly related some very similar-looking animals are.
Follow the movement of the different kinds of mammals in the last 65 million years on the Walking with Beasts site, or see how different animals are related to each other with the mammals' family tree.
Fruit bat babies weigh a quarter the weight of their mother, who can still fly whilst carrying them.