By classical Linnaean classification, Mammals belong to class 'Mammalia'. Modern, phylogenetic classification too seems to put mammals in their own place in the tree of life, with the earliest mammals appearing some 250 million years ago. Mammal diversity increased much after the extinction of dinosaurs, some 66 million years ago, allowing mammals to become larger in size, and take a variety of forms. Mammals share several characteristics, in that they have a kind of layer of hair, are warm-blooded, and produce milk. Most mammals, placentals, give live birth, though a smaller number, the marsupials, grow their young in a pouch until they grow larger, and more defined. For where egg-laying mammals, monotremes, were once common, they have greatly diminished, in that the single species of platypus and four species of echidna of Australia remain the only living Monotremes.

The act of nourishing the young with milk is an instrumental characteristic throughout the entirety of mammal diversity, and in fact the word 'mammal' derives from the Latin 'mamma', meaning 'breast'. Some mammals, such as the platypus, do not have teats, and instead sweat out the milk.

The first mammals are thought to have appeared some 180 million years ago, in a time when reptiles were dominant of the land, and the dinosaurs were growing in size. Such mammals vaguely resembled today’s rodents, though they were still not far diverged from their synapsid ancestors. A long-standing notion is that primitive mammals would have been supressed by the dinosaurs of their time, with the reptiles dominant of the land. Whilst this is reasonable to some extent, it’s possible that larger mammals would have easily thrived amongst the dinosaurs – even feeding upon smaller dinosaurs and mammals. Alongside the non-avian dinosaurs, a great number of mammals too were destroyed by the extinction at the end of the Cretaceous, allowing a lineage of smaller mammal, the Therians, to succeed them. Today, mammals are the most successful of any land-based vertebrate, with over 6,000 different species, themselves belonging to over 100 different families. Over time, several ‘fathers’ of these families would diversify themselves across the continents, and some mammals would also specialise in an aquatic life-style – all whales and dolphins today are derived from land-based ancestors, which would have taken to the water to get to a then-underlooked food-source. The quasi-supercontinent comprising Europe, Asia, and Africa surely was a breeding ground for advanced mammals. The Northern Hemisphere encompassing much of Europe and Asia is mostly temperate, with warm summers, and harsh winters. Much of it was once forested and adorned with trees, making for opportunity in the mammal world. For herbivores such as deer and cattle, a forested landscape provides carnivores with a challenge of finding food – though for the carnivores, such as wolves and cats, this also obscured them from the evading herbivores. Some of such animals would also end up in Africa, which too was a continental factory of evolution in itself – laid-back antelope are easily adapted to open plains, as opposed to forested deer in Eurasia. Cats would also reach Africa, and would diverge into various sizes – ranging from the black-footed cat, about the size of a domestic cat, to the African Lion, skilled at bringing down large prey. Opportunity would also wait for megafauna in Asia, with great resources for large animals of the likes of elephants and rhinoceros. The first cat, perhaps native to Asia, may have resembled the Clouded Leopard, the most ancient-known species of cat. Larger cats too would prosper in Asia, namely the likes of Snow Leopards and Tigers, which may in fact be closely related. Cattle would also spread to Asia, and diverge into buffalo, yak, and on some islands, buffalo would shrink in size, producing anoa. As well as in Africa, Asia would be a land of prosperity of a number of primates, such as the Orangutans, Gibbons, and Langurs. The smaller primates too are abundant here – tarsiers, tiny, nocturnal tree-hopping primates, are perhaps one of the most ancient, as are perhaps the loris, similarly nocturnal, but less skittish. In some cases, mammals from either Europe or Asia can cross into each other, though doing so takes a rather long journey. Indeed, such animals as elephants or rhinoceros were once found native in Europe, but this was not to last. On arrival to North America, it can be noted that much of the megafauna resembles what is found in Europe – that is, America too has bears, wolves, rabbits, and deer. This could be for a land-bridge between Europe and the Americas, which some animals may have crossed. Either this, or the inhabitants have been native to either side of America for many years - in the case of the Virginia Opossum, its ancestors were once found further south in South America, until the two Americas joined, when opossums started moving upwards. The same is true for the armadillo, which appeared in South America 50 million years ago, and continues today to move upwards, as the planet warms. Colossal armadillos too once roamed the Americas, which may have reached a metre and a half at the top of their domed shell. Being slow-moving and large, however, is a disadvantage if humans live on the continent, as it is believed that humans may have hunted the Glyptodon to extinction. Much of the brilliant fauna of South America has existed there for millions of years, with the rainforest landscape providing opportunity for monkeys, jaguars, sloths and a range of other mammals to prosper. On Antarctica, the Southernmost continent, no terrestrial mammals, aside from human scientists, live – instead, whatever mammals come to land on Antarctica tend also to live much of their lifespan in the surrounding ocean. Although most Marsupials are native to Australia, and the islands that surround it, it is believed that an opossum-like ancestor gave rise to the marsupials, which eventually got sent to Australia by migration. Until recent times, no grand number of placental mammals lived around Australia, until the arrival of the Dingo about 3500 years ago. From there, animals of any one part of the globe would be brought anywhere else – all of which were all brought by a single species – humans, the most abundant of any mammal, numbering about 8 billion in all. Humans are arguably more dexterous and intelligent than any other primate, and it seems that with such power, humans may hold the future of biodiversity in their own hands. Humans by far have exploited natural resources than any other mammal, and are the only species which has developed agricultural farming, which today is practiced on a massive scale.