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The monotremes are more closely related in evolution to the reptiles than are any other recent mammals. They are not the ancestors of the marsupials or the placental mammals, but rather they represent a distinct line of mammalian evolution. They possess certain mammalian features, such as hair and mammary glands, and they are warm-blooded. On the abdomens of the females, milk oozes from paired areas of tubular glands (they possess no teats for suckling) and is lapped up by the young. They have a four-chambered heart and some skeletal features associated with mammals. Other features, such as the presence of a cloaca, along with features of the vertebrae and ribs, are very reptilian.

The echidna (ih-KID-nuh) is a type of primitive mammal found in Australia and New Guinea. Echidnas belong to the taxonomic family called Tachyglossidae. These animals are also monotremes. Monotremes, which are limited to platypuses and echidnas, are a group of mammals that lay eggs rather than give birth to live young as most other mammals do. Echidnas are considered the most primitive living mammals and may be the oldest surviving mammal species. They are also sometimes called spiny anteaters, though they are not part of the anteater family.