Mammals can be identified by the presence in females of mammary glands that produce milk for offspring. Mammals are warm-blooded vertebrates with sweat glands, hair, three middle ear bones and a neocortex region in the brain. Their gradual evolution from mammal-like “reptiles” called “synapsids” spanned about 70 million years. The first clear evidence of fully mammalian jaw joints and middle ears was found about 200 million years ago; mid-Jurassic fossils show early evidence of hair or fur; and lactation occurred in monotremes, egg-layers that urinated, defecated, and reproduced through a single hole, though not at the same time. They are believed to have secreted milk not from nipples but through a hairy patch on their bellies. (The platypus and four echidna species are the sole surviving mammalian egg-layers.) Mammals now encompass approximately 5,400 species, including humans.


Photo © Robin Silver