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In the past 300 million years, an innumerable variety of vertebrate species have evolved from the bony fishes of the early Paleozoic. At several points in evolutionary history, there have been periods of relatively rapid adaptation and expansion of organisms known as adaptive radiations. These are limited only by a speciesO adaptive potential and the adaptive opportunities of the environment. There have been three major adaptive radiations of terrestrial vertibrates.

The first of these was the rapid diversification of reptilians leading up to the age of reptiles during the Mesozoic. The predecessors of reptiles were amphibians, capable of producing a stronger and self-sufficient egg. When they adapted to a fully terrestrial life cycle, reptiles had virtually no competition for ecological niches. Over a relatively short period of time, reptiles became the dominant life forms on the planet.

Approximately 65 million years ago at the beginning of the Cenozoic, another major adaptive radiation took place. With the decline of the dinosaurs, a wide variety of ecological niches were opened to mammals. The first mammals are believed to have arisen from small mammal-like reptiles. They were small and resembled modern rodents. Possibly the most important characteristic of the mammals is their advanced learning abilities and behavioral flexibility. A larger brain size developed in the evolving mammals with a larger percentage of brain mass being made up by the cerebrum and neocortex.

In addition to their neurological advantages mammals have many unique physical characteristics that have facilitated their evolutionary success. Mammals are heterodontal, they possess specialized and differentiated teeth. This, in conjunction with their jaw strength increased their dietary diversity. Homoiothermy, a self-regulated body temperature, although not unique to mammals helped them settle in colder ecological niches and allowed for nocturnal activity in areas with colder nights. Finally, mammals have an increased investment in their offspring. Mammalian young develop in utero, or in an external pouch. After birth, mammals are still extremely dependent upon their mothers for milk (also unique to mammals) and protection for a considerable percentage of their life span. This is necessary to allow for brain development and the learning of survival behaviours.

The primate radiation is only a major evolutionary innovation when seen in an anthropocentric view. Primates only became widely distributed with the arrival and migration of H. Erectus. Primates are believed to have evolved from small shrew-like mammals. Cartmill suggests that the major adaptations of the primates arose from hunting insects in the underbrush. Whatever environment selected for primate innovations, they were specialized in the trees. Primates have developed elongated digits, opposable thumbs, and nails rather than claws to facilitate grasping of small food items and tree limbs. Primates rely heavily on vision; their close set eyes enable better depth perception. Primate dentation is generalized to allow for a highly diverse diet. The semi-erect stance of primates lead eventually to the bipedal stance of hominids.

These adaptive radiations demonstrate how organisms, through the forces of evolution can expand over a wide variety of habitats. During the Ages of Reptiles, Mammals, and Humans, three very different types of organisms have occupied and dominated nearly every ecological niche worldwide

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