Mammals Endure and Prevail

Before the total extinction of dinosaurs, mammals lay suspended. They were primitive things, hardly larger than the eggs of their supreme neighbors, and scavenging for insects and other small game.

It was not until the Cenozoic Period that the mammals had a chance to come into their own as they became more advanced than any creatures before them.

A major difference between mammals and the creatures before them was their solution to the problems of giving birth. Dinosaurs laid their eggs and left them without protection and gave them little attention after birth. The young were therefore often in danger. Scientists have evidence that birds from this same time period were more attentive, at least incubating the eggs in a nest to keep them safe. Fossil evidence supports the theory that mammals developed evermore sophisticated ways of dealing with these problems surrounding birth and survival of the newly born.

The first mammals, the monotremes, laid eggs like their reptile cousins. Monotremes were the transition between the reptile-ruled world of the Mesozoiic to the mammal-ruled world of the Cenozoic. The platypus is representative of monotremes, and has survived to this day, though not discovered until 1798. Females lay their two eggs in their burrow located at the end of a river. The relatively underdeveloped babies hatch after about 10 days after the eggs are laid. They develop inside the burrow for approximately 4 months, and then they move out to make their home in the river. Their curious looking bills are super-sensitive to seek out food on the river bottom. Being  nocturnal, their eyesight is not very good, though adapted to the darkness. The males of the species are one of only a few poisonous mammals. They have hooks behind their ankles from which they can inject poison into an unsuspecting victim. The venom is so powerful it has been known to kill dogs. Platypuses and their cousins, the echidnas, the only remaining monotremes, can be  found in Australia and New Guinea.

Most likely, marsupials stem from one of many of the extinct monotremes, and represent a more advanced solution to giving birth and protecting offspring. Like the monotremes, marsupials give birth to underdeveloped  young; however, rather than keeping the offspring in a burrow, the marsupials carried their offspring in specially evolved pouches until the young were ready to live more independently. Many marsupials have survived to the present time, such as the kangaroo in Australia and the opossum in North America.

Though there are no fossil records of such a creature, there almost certainly were creatures that internalized the birth pouch and represent a transition to placenta birth. Internal gestation of the young through a placenta and birthing to a more fully developed offspring became the norm for mammals. This method of birth and protection of the offspring was more advanced and became favored through natural selection, as mammals supplanted reptiles in many niches and placenta-birthed species won dominance over monotremes and marsupials.

Along with favored birthing methods, mammals also possessed another key advantage. Unlike the reptiles, mammals were warm-blooded. Mammals did not have to wait for the sun's warmth to bring on vitality and activity. They could hunt at night or in the day time. But mammals also needed more energy, and therefore needed more food and oxygen. The mammalian diaphragm muscle was developed to satisfy this need. Mammals also developed the ability to breathe while they eat. With noses, the air can pass through their lungs while engaged in devouring its quarry. Mammals' teeth are also much more useful for grinding food.

The Mesozoic mammals had been little more than scavengers and fruit eaters, taking advantage of the outburst of flowering plants. These feeding habits allowed the mammal digestive systems to evolve. After the extinction of the dinosaurs, mammals went from a tenuous status of enduring as survivors in a reptile ecosystem to prevailing in the much-changed competitive environment of the Cenozoic Period.

The stage was set for mammal evolution to race forward, as the dinosaurs faded, leaving a niche for just such new animals employing radical new strategies for survival and dominion, for just such emerging mammals...

Into this void came the distant descendants of man, the early primates. They thrived in the trees where there were fewer predators and their nimbleness and ingenuity were leveraged. Man's descendants were probably among the first primates to leave the trees and to make the ground their home, gradually further and further away from groves of trees as they gained mobility and comfort in their new surroundings. In the trees, these forerunners of man had regularly stood erect with their fingers holding on to limbs. On the ground, this support against gravity was not present, and these  ape-like creatures walked, almost hopping, primarily using the muscles of their back legs supported for balance and for extra speed pushed by their front arms.