The Evolution of Primates

Characteristics and Evolution of Primates

All primates exhibit adaptations for climbing trees and have evolved into two main groups: Prosimians and Anthropoids.

Characteristics of Primates

All primate species possess adaptations for climbing trees, as they all descended from tree-dwellers. This arboreal heritage of primates has resulted in adaptations that include, but are not limited to: 1) a rotating shoulder joint; 2) a big toe that is widely separated from the other toes and thumbs, that are widely separated from fingers (except humans), which allow for gripping branches; and 3) stereoscopic vision, two overlapping fields of vision from the eyes, which allows for the perception of depth and gauging distance. Other characteristics of primates are brains that are larger than those of most other mammals (larger brain/body ratio than similar-sized non-primates), claws that have been modified into flattened nails, typically only one offspring per pregnancy, and a trend toward holding the body upright.

The Order Primates is divided into two groups: prosimians and anthropoids. Prosimians include the bush babies and pottos of Africa, the lemurs of Madagascar, and the lorises of Southeast Asia. Tarsier, also from Southeast Asia, show some prosimian-like and some anthropoid-like features. Anthropoids include monkeys, apes, and humans. In general, prosimians tend to be nocturnal (in contrast to diurnal anthropoids, excluding the nocturnal Aotus, owl monkey) and have a smaller brain/body ratio than anthropoids.

Evolution of Primates

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 fragmentary. These proto-primates will remain largely mysterious creatures until more fossil evidence becomes available. The oldest known primate-like mammal 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 had some features of the teeth and skeleton in common with true primates. They were found in North America and Europe in the Cenozoic, going extinct by the end of the Eocene.

The first true primates were found in North America, Europe, Asia, and Africa in the Eocene Epoch. 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.

Evidence shows that the anthropoid monkeys evolved from prosimians during the Oligocene Epoch. By 35 million years ago, evidence indicates that monkeys were present the Old World (Africa and Asia) and in the New World (South America) by 30 million years ago. New World monkeys are also called Platyrrhini: a reference to their broad noses. Old World monkeys (and apes) 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 by drifting on log rafts or mangrove floating 'islands'. 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 during the Miocene epoch, approximately 25 million years ago. Apes are generally larger than monkeys and 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 as 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), Gorilla (gorillas), Pongo (orangutans), and Homo (humans). 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/brachiating through trees.

Early Human Evolution

Modern humans and chimpanzees evolved from a common hominoid ancestor that diverged approximately 6 million years ago.

Human Evolution

The family Hominidae of order Primates includes chimpanzees and humans. 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 (or hominid) 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. Hominins, who were bipedal in comparison to the other hominoids who were primarily quadrupedal, includes those groups that probably gave rise to our species: Australopithecus africanus, Homo habilis, and Homo erectus, along with non- ancestral groups such as Australopithecus boisei. 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. It is possible that there were 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. However, it is also possible that too many new species have been named.

Very Early Hominins

There have been three species of very early hominoids which have made news in the past few years. The oldest of these, Sahelanthropus tchadensis, has been dated to nearly seven million years ago. There is a single specimen of this genus, a skull that was a surface find in Chad. The fossil, informally called "Toumai," is a mosaic of primitive and evolved characteristics. To date, it is unclear how this fossil fits with the picture given by molecular data. The line leading to modern humans and modern chimpanzees apparently bifurcated (divided into branches) about six million years ago. It is not thought at this time that this species was an ancestor of modern humans. It may not have been a hominin.

A second, younger species (around 5.7 million years ago), Orrorin tugenensis, is also a relatively-recent discovery, found in 2000. There are several specimens of Orrorin. It is not known whether Orrorin was a human ancestor, but this possibility has not been ruled out. Some features of Orrorin are more similar to those of modern humans than are the australopiths, although Orrorin is much older.

A third genus, Ardipithecus ramidus (4.4 million years ago), was discovered in the 1990s. The scientists who discovered the first fossil found that some other scientists did not believe the organism to be a biped (thus, it would not be considered a hominid). In the intervening years, several more specimens of Ardipithecus, including a new species, Ardipithecus kadabba (5.6 million years ago), demonstrated that they were bipedal. Again, the status of this genus as a human ancestor is uncertain, but, given that it was bipedal, it was a hominin.

Early Hominins

The hominin Australopithecus evolved 4 million years ago and is believed to be in the ancestral line of the genus Homo.

Early Hominins: Genus Australopithecus

Australopithecus ("southern ape") is a genus of hominin that evolved in eastern Africa approximately 4 million years ago and became extinct about 2 million years ago. This genus is of particular interest to us as it is thought that our genus, genus Homo, evolved from Australopithecus about 2 million years ago. Australopithecus had a number of characteristics that were more similar to the great apes than to modern humans. For example, sexual dimorphism was more exaggerated than in modern humans. Males were up to 50 percent larger than females, a ratio that is similar to that seen in modern gorillas and orangutans. In contrast, modern human males are approximately 15 to 20 percent larger than females. The brain size of Australopithecus relative to its body mass was also smaller than modern humans and more similar (although larger) to that seen in the great apes. A key feature that Australopithecus had in common with modern humans was bipedalism, although it is likely that Australopithecus also spent time in trees. Hominin footprints, similar to those of modern humans, found in Laetoli, Tanzania, are dated to 3.6 million years ago. They show that hominins at the time of Australopithecus were walking upright.

There were a number of Australopithecus species, often referred to as australopiths. Australopithecus anamensis lived about 4.2 million years ago. More is known about another early species, Australopithecus afarensis, which lived between 3.9 and 2.9 million years ago. This species demonstrates a trend in human evolution: the reduction of the dentition and jaw in size. A. afarensis had smaller canines and molars compared to apes, but these were larger than those of modern humans. Its brain size was 380–450 cubic centimeters, approximately the size of a modern chimpanzee brain. It also had prognathic jaws, which is a relatively longer jaw than that of modern humans. In the mid-1970s, the fossil of an adult female A. afarensis was found in the Afar region of Ethiopia, dated to 3.24 million years ago. The fossil, which is informally called "Lucy," is significant because it was the most complete australopith fossil found, with 40 percent of the skeleton recovered.

Australopithecus africanus lived between 2 and 3 million years ago. It had a slender build and was bipedal, but had robust arm bones and, as with other early hominids, may have spent significant time in trees. Its brain was larger than that of A. afarensis at 500 cubic centimeters, which is slightly less than one-third the size of modern human brains. Two other species, Australopithecus bahrelghazali and Australopithecus garhi, have been added to the roster of australopiths in recent years.

A Dead End

While most australopiths had a relatively slender, gracile build and teeth suited for soft food, there were also australopiths of a more robust build, dating to approximately 2.5 million years ago. These hominids were larger and had large grinding teeth. Their molars show heavy wear, suggesting that they had a coarse and fibrous vegetarian diet as opposed to the partially carnivorous diet of the more gracile australopiths. They include Australopithecus robustus of South Africa, and Australopithecus aethiopicus and Australopithecus boisei of East Africa. These hominids became extinct more than 1 million years ago and are not thought to be ancestral to modern humans, but rather members of an evolutionary branch on the hominin tree that left no descendants.

Genus Homo

The human genus Homo, which includes modern humans as well as extinct human relatives, appeared around 2.3 million years ago.

Early Hominins: Genus Homo

The human genus, Homo, first appeared around 2.3 million years ago. For many years, fossils of a species called Homo habilis were the oldest examples in the genus Homo, but in 2010, a new species called Homo gautengensis was proposed that may be older, although it is not well accepted. In comparison to Australopithecus africanus, H. habilis had a number of features more similar to modern humans. H. habilis had a jaw that was less prognathic (forward projection of the jaw) than the australopiths and a larger brain, at 600–750 cubic centimeters. However, H. habilis retained some features of older hominin species, such as long arms. The name H. habilis means "handy man," which is a reference to the stone tools that have been found with its remains.

H. erectus appeared approximately 1.8 million years ago. It is believed to have originated in East Africa and was the first hominin species to migrate out of Africa. Fossils of H. erectus have been found in India, China, Java, and Europe, and were known in the past as "Java Man" or "Peking Man." H. erectus had a number of features that were more similar to modern humans than those of H. habilis. H. erectus was larger in size than earlier hominins, reaching heights up to 1.85 meters and weighing up to 65 kilograms, sizes similar to those of modern humans. Its degree of sexual dimorphism was less than earlier species, with males being 20 to 30 percent larger than females, which is close to the size difference seen in our species. H. erectus had a larger brain than earlier species at 775–1,100 cubic centimeters, which compares to the 1,130–1,260 cubic centimeters seen in modern human brains. H. erectus also had a nose with downward-facing nostrils similar to modern humans, rather than the forward facing nostrils found in other primates. Longer, downward-facing nostrils allow for the warming of cold air before it enters the lungs and may have been an adaptation to colder climates. Artifacts found with fossils of H. erectus suggest that it was the first hominin to use fire, hunt, and have a home base. H. erectus is generally thought to have lived until about 50,000 years ago.

Humans: Homo sapiens

A number of species, sometimes called archaic Homo sapiens, apparently evolved from H. erectus starting about 500,000 years ago. These archaic H. sapiens had a brain size similar to that of modern humans, averaging 1,200–1,400 cubic centimeters. They differed from modern humans by having a thick skull, a prominent brow ridge, and a receding chin. Some of these populations survived until 30,000–10,000 years ago, overlapping with anatomically-modern humans.

There is considerable debate about the origins of anatomically-modern humans or Homo sapiens sapiens. As discussed earlier, H. erectus migrated out of Africa and into Asia and Europe in the first major wave of migration about 1.5 million years ago. The multiregional hypothesis holds that humans first arose near the beginning of the Pleistocene two million years ago and subsequent human evolution has been within a single, continuous human species. This species encompasses archaic human forms such as Homo erectus and Neanderthals as well as modern forms, which evolved worldwide to the diverse populations of modern Homo sapiens sapiens. The hypothesis contends that humans evolve through a combination of adaptation within various regions of the world and gene flow between those regions. Proponents of multiregional origin point to fossil and genomic data and continuity of archaeological cultures as support for their hypothesis.

The primary alternative hypothesis is the recent African origin of modern humans, which holds that modern humans arose in Africa around 100,000–200,000 years ago, moving out of Africa around 50,000–60,000 years ago to replace archaic human forms with limited interbreeding: at least once with Neanderthals and once with Denisovans.