The first mammals evolved about 190 million years ago. These early mammals were small, insect-eating creatures that lived at the same time as the dinosaurs. When the dinosaurs disappeared about 65 million years ago, mammals began to diversify into many forms. There are now about 4500 different species of mammals living in almost every environment on earth including the oceans, fresh water, on and below the ground, in the treetops and even in the sky.
All mammals (including humans) have the same distinctive features. These include:
- fur or hair growing from the skin
- mammary glands that, in females, produce milk for feeding the young
- three bones (the malleus, incus and stapes) in the middle ear for transmitting sound to the inner ear
- a single bone (the dentary) on each side of the lower jaw
Our place in the animal kingdom
Humans possess many unique characteristics but we also share a number of similarities with other animals.These similarities and differences are revealed through our genetic make-up, the ways our bodies are constructed and our behaviour. They help us to understand our place in the animal kingdom by allowing us to work out the evolutionary relationships between ourselves and other animals. Studies of our closest living relatives, the apes, also provide valuable clues about our early ancestors’ bodies and lifestyles.
Humans are classified as mammals because humans have the same distinctive features (listed above) found in all members of this large group.
Humans are also classified within:
- the subgroup of mammals called primates;
- and the subgroup of primates called apes and in particular the 'Great Apes'
Humans, however, also possess many unique characteristics and are therefore classified within a unique subgroup of the Great Apes called the hominins.