CHAPTER 12. CENOZOIC ERA (65 MYA - present)

The earlier part of the Cenozoic era, known as the Tertiary period, is divided into five distinct epochs, all of relatively brief duration. These are the Paleocene (66-55 mya), Eocene (55-37.5 mya), Oligocene (37.5-24 mya), Miocene (24-25 mya), and Pliocene (5-2 mya). The last period, known as the Quaternary, takes us up to the present day.

Tertiary Period (65-2 mya)

In the Cenozoic era, all the present-day orders of mammals became established (including some that later became extinct), followed by present-day families and subfamilies. Then appeared the genera, and in some cases, the species that still survive today.

The first part of the Cenozoic era is the Paleocene epoch (66-55 mya). In this epoch, mammals rapidly diversified, but were still unlike those alive today. The fossil record shows that the higher insects were undergoing diversification. The rise of the higher orders of animals occurred with the radiation of flowering plants, the two being mutually interdependent. A general trend was that the flowering plants underwent a transition from promiscuous pollination by unspecialized beetles to restricted pollination by specialized insects and other animals.

Arising in the Tertiary period, or perhaps earlier, were the primates. True primates arose from 70 to 65 mya. The order primates contains humans, as well as lemurs, monkeys, and apes. Especially significant about this order is that its members have hands and feet that each have five distinct digits with the innermost toes and thumb usually opposable so that grasping is readily available. The first primates arose in the Paleocene and were very rodent like, resembling the present day tree shrews of southeast Asia. The next ones arose in the Eocene epoch (55-37.5 mya) and were comparable to lemuroids and tarsiroids. These later developed into the prosimians. These primates contain three families of lemurs, along with indris, the aye-aye, the loris, and bushbabies. Also considered prosimian is the family of tarsiers. Prosimians have small, procumbent incisors and are usually nocturnal. They also are very reliant on the sense of smell. Unlike the higher primates, they do not have color vision or a high degree of stereoscopic vision (promoting depth perception), and are less social.

The new world monkeys arose from 34 to 12 mya, beginning in the Oligocene epoch (37.5-24 mya). Except for the owl monkey (Aotus trivirgatus), they are diurnal, sleeping mostly in tree holes at night. New world

monkeys have a flat nose with two nostrils set widely apart and usually have a prehensile tail. They live in trees, never descending to the ground. They are widespread through South America and into central America. They are mainly vegetarian, but can feed on insects, worms, and small vertebrates. Examples are the capuchins, marmosets, howlers, and squirrel monkeys.

The old world monkeys developed in the Miocene epoch (24-5 mya). They have nostrils set close together and a rather pronounced nose that hangs down. The first one, Colobinae, contains the colubus monkeys, langurs, and proboscis monkeys. The second subfamily, Cercopithecinae, contains the baboons, mandrills, gelada baboons, and macaques.

The superfamily Hominoidea contains the ape families and human beings. (There is some evidence that ape ancestors developed before the old world monkeys.) The apes are in two families, family Hylobatidae, which contains the brachiating gibbons, and family Pongidae, which contains the less agile orangutans, gorillas, and chimpanzees.

Occurring in the Pliocene epoch (5-2 mya) was the development of the genus of human beings (Homo). Humans will be discussed in following chapters.

Quaternary Period (2 mya to Present)

This is the period when the primate genus Homo became strongly established. The evolution of man will be discussed in detail in a later chapter. The Quaternary era is divided into two epochs. The first epoch, the Pleistocene, began about 1.7 mya. This is the epoch when Ice Age mammals were abundant, as the ice caps advanced and retreated. The next epoch, the Holocene, which began only .01 mya, is the epoch of modern mammals. The number of human beings increased on all continents.

The Primate Brain

The most recent improvement to the brain in its evolution over millions of years involves its largest, uppermost portion. This is the surface gray matter, or cerebrum (also known as the cortex and neocortex). So that the neocortex would not outgrow the skull, the cerebral cortex wrinkled. The surface literally folded into convolutions so it could fit within the braincase. Some mammals increased in size and enlarged the area of the neopallium disproportionately, so they increased in intelligence as well.

The neocortex became the command post for the deep, "old" parts of the brain. This made it possible for primates to get more, but certainly not complete, control over their more primitive emotions. It also permitted the necessary greater intelligence to solve various situational problems.


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