Mammals, the predominant vertebrates in the animal kingdom, evolved 220 mya (million years ago). Their ability to survive environmental stresses exceeds that of reptiles, amphibians, fishes, and even birds, the only other warm-blooded animals. Mammals have "amnions" (water-tight membranes enclosing their embryos), are born alive and suckled with milk, and have insulating fur or hair to prevent excessive body-heat loss or gain. "Warm-blooded" actually means maintaining a constant body temperature which takes about 75 percent of our metabolic activity.

A mammal also has differentiated teeth in its jaws (canines for stabbing, incisors for slicing, and molars for chewing) to allow a broader diet. It also has a palate across the roof of the mouth to permit nasal breathing while biting or chewing, and three ear bones to enable better hearing. Mammals have a four-chambered heart to enable more efficient circulation (particularly to the brain), and leg bones extending straight down from the pelvis to permit fast, efficient locomotion (a characteristic shared with dinosaurs who are "cousins" to mammals; having evolved along different lines from a common reptile ancestor), and synapsid openings behind the eye sockets, through which jaw muscles extend to their attachment places atop the outside of the skull. These features, which give mammals evolutionary advantages, did not all arise as beneficial change mutations simultaneously.

Amnion membranes, which prevent desiccation of embryos, first appeared along with shelled eggs 310 mya. These enabled newly-evolved reptiles to inhabit land areas far from water. "Synapsids," an extinct intermediate group that descended from reptiles 300 mya (from which mammals later evolved), had synapsid openings, but otherwise retained most reptilian characteristics, including one ear/bone, jaws filled with spike-like teeth that were ineffective for chewing, and upper leg bones extending outward to give them a sprawling slower gait. The best-known synapsid is "Dimetrodon," which could raise or lower a large retractable fin on its back that could help adjust its body temperature to external conditions. Through chance mutations which improved their environmental adaptation, later synapsids evolved vertical upper leg bones, differentiated teeth, palates, three ear bones, and the other features that distinguish mammals from their ancestral synapsids and reptiles.

The first true mammals were tiny ones that had to contend with large predators which nearly wiped them out during the reptile and dinosaur heyday, the Mesozoic Era. They included small "triconodonts" and (later) "multituberculates" who had sharp multi-cusped primitive molars. The latter might be considered the most successful of all mammals because they survived the longest 100 mya (million years) before becoming extinct. Both probably were primitive egg-laying mammals, similar to the platypus and echidna, monotremes that now barely survive in Australia.

Marsupials, who are born as blind, legless, and hairless partially developed embryos, use their hands to climb up to the mother's pouch. There they drink breast milk for many weeks as they complete their embryonic development. Marsupials and monotremes were the only existing mammals in Australia until 1790, when England's Captain James Cook (during scientific expeditions for which Benjamin Franklin obtained safe passage for his ships during the Revolutionary War) discovered Australia.

Captain Cook, who was en route to the Indian Ocean to observe a rare Venus "transit" (a passing of that planet directly between the sun and the earth), released the first placental wildlife on that continent - rats, mice and cats and dogs which quickly reverted to a feral condition which soon threatened to wipe out the more primitive, less competitive monotremes and marsupials. Placentals (including humans) share a common "placenta" between the mother and the enclosed developing embryo, through which nutrients and oxygen enter and waste materials exit.

Placentals evolved on one of the other continents after the Indonesian land-bridge connecting Australia to Asia had submerged enough to prevent them from reaching that continent and killing off all marsupials and monotremes; as they did on all the other continents that were interconnected. The only extant marsupial outside Australia is the possum, which even survives on Long Island. The fossil history of placental mammals in the most recent Cenozoic Era (65 mya to the present) will follow.