Anthropoids include monkeys, apes, and humans. In general, prosimians tend to be nocturnal in contrast to diurnal anthropoids and exhibit a smaller size and smaller brain than anthropoids. The first primate-like mammals are referred to as proto-primates. They were roughly similar to squirrels and tree shrews in size and appearance.
The existing fossil evidence mostly from North Africa is very fragmented. These proto-primates remain largely mysterious creatures until more fossil evidence becomes available. The oldest known primate-like mammals with a relatively robust fossil record is Plesiadapis although some researchers do not agree that Plesiadapis was a proto-primate.
Fossils of this primate have been dated to approximately 55 million years ago. Plesiadapiforms were proto-primates that had some features of the teeth and skeleton in common with true primates. They were found in North America and Europe in the Cenozoic and went extinct by the end of the Eocene. These early primates resembled present-day prosimians such as lemurs.
Evolutionary changes continued in these early primates, with larger brains and eyes, and smaller muzzles being the trend. By the end of the Eocene Epoch, many of the early prosimian species went extinct due either to cooler temperatures or competition from the first monkeys.
Figure 1. The howler monkey is native to Central and South America. It makes a call that sounds like a lion roaring. Anthropoid monkeys evolved from prosimians during the Oligocene Epoch. New World monkeys are also called Platyrrhini —a reference to their broad noses Figure 1.
Old World monkeys are called Catarrhini —a reference to their narrow noses. There is still quite a bit of uncertainty about the origins of the New World monkeys. At the time the platyrrhines arose, the continents of South American and Africa had drifted apart. Therefore, it is thought that monkeys arose in the Old World and reached the New World either by drifting on log rafts or by crossing land bridges. Due to this reproductive isolation, New World monkeys and Old World monkeys underwent separate adaptive radiations over millions of years.
The New World monkeys are all arboreal, whereas Old World monkeys include arboreal and ground-dwelling species.
Apes evolved from the catarrhines in Africa midway through the Cenozoic, approximately 25 million years ago. Apes are generally larger than monkeys and they do not possess a tail. All apes are capable of moving through trees, although many species spend most their time on the ground. Apes are more intelligent than monkeys, and they have relatively larger brains proportionate to body size. The apes are divided into two groups. The lesser apes comprise the family Hylobatidae , including gibbons and siamangs.
The great apes include the genera Pan chimpanzees and bonobos Figure 2a , Gorilla gorillas , Pongo orangutans , and Homo humans Figure 2b. The very arboreal gibbons are smaller than the great apes; they have low sexual dimorphism that is, the genders are not markedly different in size ; and they have relatively longer arms used for swinging through trees. Figure 2. The a chimpanzee is one of the great apes.
It possesses a relatively large brain and has no tail. The family Hominidae of order Primates includes the hominoids : the great apes Figure 3. Evidence from the fossil record and from a comparison of human and chimpanzee DNA suggests that humans and chimpanzees diverged from a common hominoid ancestor approximately 6 million years ago.
Several species evolved from the evolutionary branch that includes humans, although our species is the only surviving member. The term hominin is used to refer to those species that evolved after this split of the primate line, thereby designating species that are more closely related to humans than to chimpanzees. Determining the true lines of descent in hominins is difficult. In years past, when relatively few hominin fossils had been recovered, some scientists believed that considering them in order, from oldest to youngest, would demonstrate the course of evolution from early hominins to modern humans.
In the past several years, however, many new fossils have been found, and it is clear that there was often more than one species alive at any one time and that many of the fossils found and species named represent hominin species that died out and are not ancestral to modern humans. Three species of very early hominids have made news in the past few years. The oldest of these, Sahelanthropus tchadensis , has been dated to nearly 7 million years ago.
There is a single specimen of this genus, a skull that was a surface find in Chad. It is not thought at this time that this species was an ancestor of modern humans. With the extinction of the dinosaurs at the end of the Cretaceous, many terrestrial niches became available and predation pressures were somewhat relaxed. In addition, temperatures were higher than in the recent past see Figure 3.
The spread of flowering plants resulted in an adaptive radiation of insect pollinators and herbivores plant-eaters , as well as insectivorous and herbivorous arboreal vertebrates.
The earliest primates likely descended from a small, nocturnal, insectivorous mammal. The tree shrews and colugos also known as flying lemurs are the closest living relatives to primates. The tree shrew is used as a living model for what the earliest primates, or primate predecessors, might have been like.
At some point, primates or their ancestors moved into the trees and adapted to an arboreal environment. Two theories regarding the evolution of some primate characteristics, such as grasping or prehensile hands, forward-oriented eyes, and depth perception, are the Arboreal and Visual Predation Theories. The Arboreal Theory posits that primate characteristics, such as grasping hands and feet and the presence of nails instead of claws, are the result of moving into and adapting to an arboreal environment.
Imagine the casualties! The Visual Predation Theory asserts that characteristics that were well-suited to scurrying around in trees and visual features in particular, such as convergent orbits, are adaptations to insect predation. Short of a butterfly net, grasping hands, visual acuity, and depth perception are essential for catching insects, but I guess they would be kind of handy for using a butterfly net as well!
The map in Figure 3. They are divided into two superfamilies, Adapoidea and Omomyoidea. In general, the adapoids were diurnal, lemur-like animals that are thought to be the ancestors of the strepsirrhine primates, i. The smaller, nocturnal omomyoids are good candidates for the ancestors of modern-day tarsiers. However, due to the early dates for ancestral tarsiers, it is possible that the omomyoids and tarsiers were sister lineages.
During the Eocene Epoch, the early strepsirrhine-like primates experienced an adaptive radiation and expanded into numerous niches over a broad geographic area. The northern expansion of early primates into Europe and North America was possible because Eurasia and North America were joined as the large landmass known as Laurasia and, as mentioned, it was warm enough for tropical animals to move into northern latitudes. Due to subsequent global cooling, the early primates in North America and Europe eventually went extinct.
Strepsirrhine primates spread into Africa after it docked with Laurasia. By at least the late Eocene, the first anthropoid primates had evolved. There is debate over the origin of the anthropoids, i. There are four different theories of our ancestry, each with its share of supporters: 1 adapoid, 2 omomyoid, 3 tarsier, or 4 independent origin as yet undiscovered.
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