The existing, very fragmentary fossil evidence from Asia, Europe, North Africa, and especially Western North America suggests that they were adapted to an arboreal way of life in warm, moist climates. They probably were equipped with relatively good eyesight as well as hands and feet adapted for climbing in trees. These primate-like mammals Plesiadapiformes will remain rather shadowy creatures for us until more fossil data become available.
The primate-like mammals do not seem to have played an important role in the general transformation of terrestrial animal life immediately following the massive global extinctions of plants and animals that occurred about 65,, years ago.
The most dramatic changes were brought about by the emergence of grazing and browsing mammals with tough hoofs, grinding teeth, and digestive tracts specialized for the processing of grass, leaves, and other fibrous plant materials. The evolution of these herbivorous mammals provided the opportunity for the evolution of the carnivorous mammals specialized to eat them.
These new hunters and scavengers included the evolutionary lines that would later produce the dogs, cats, and bears of our time. Adaptive radiation was resulting in the rapid evolution of new species to fill expanding ecological niches, or food getting opportunities. Most of these new animals were placental mammals. With the exception of bats, none of them reached Australia and New Guinea. This explains why they did not exist there until people brought them in recent times.
South America had also drifted away from Africa and was not connected to North America after 80,, years ago. However, around 20,, years ago, South America reconnected with North America and placental mammals streamed in for the first time, resulting in the extinction of most of the existing marsupials there.
Early Prosimians. The beginning of the Eocene Epoch In addition, placental mammals with larger bodies and bigger brains began to appear in the fossil record at this time. Paul Falkowski has suggested that this is due to the fact that the amount of oxygen in the earth's atmosphere more or less doubled around 50 million years ago.
Larger mammals have relatively fewer capillaries for the distribution of oxygen to the cells of their bodies. Subsequently, they must breathe air that is more oxygenated. Brains have especially high oxygen requirements. In addition, pregnant placental mammals must transmit a substantial portion of the oxygen in their blood to their fetuses. This also would have been a major factor in the rapid evolution of animals and plants at the time. Overall, climates were significantly warmer during the Eocene than now.
There were crocodiles in the Arctic, pine forests in the Antarctic, and palm trees in Wyoming. There was no polar ice. As a consequence, sea levels were close to feet m. The first true primates evolved by 55 million years ago or a bit earlier, near the beginning of the Eocene Epoch. Their fossils have been found in North America, Europe, and Asia. They looked different from the primates today. They were still somewhat squirrel-like in size and appearance, but apparently they had grasping hands and feet that were increasingly more efficient in manipulating objects and climbing trees.
The position of their eyes indicates that they were developing more effective stereoscopic vision as well. Smilodectes lemur-like family Adapidae from the Eocene Epoch Among the new primate species were many that somewhat resemble modern prosimians such as lemurs, lorises, and possibly tarsiers. The Eocene was the epoch of maximum prosimian adaptive radiation. There were at least 60 genera of them that were mostly in two families--the Adapidae similar to lemurs and lorises and the Omomyidae possibly like galagos and tarsiers.
This is nearly four times greater prosimian diversity than today. Eocene prosimians also were much more widely distributed around the world than now. It was during this epoch that they reached the island of Madagascar, where they flourished up to modern times. The great diversity of Eocene prosimians was probably a consequence of the fact that they did not have competition from monkeys and apes since these latter more advanced primates had not yet evolved.
Their brains and eyes were becoming larger, while their snouts were getting smaller. At the base of a skull, there is a hole through which the spinal cord passes. This opening is the foramen magnum literally the "large hole or opening" in Latin.
The position of the foramen magnum is a strong indicator of the angle of the spinal column to the head and subsequently whether the body is habitually horizontal like a horse or vertical like a monkey. During the Eocene, the foramen magnum in some primate species was beginning to move from the back of the skull towards the center. This suggests that they were beginning to hold their bodies erect while hopping and sitting, like modern lemurs, galagos, and tarsiers.
By the end of the Eocene Epoch, many of the prosimian species had become extinct. This may be connected with cooler temperatures and the appearance of the first monkeys during the transition to the next geologic epoch, the Oligocene about 34 million years ago. Early Monkeys and Apes.
The body size of mammals in many species lines progressively increased after the end of the age of dinosaurs as they took advantage of the vast expanses of land and plant food made available by the extinction of the giant reptiles.
The biggest land mammals ever to live evolved by around million years ago near the end of the Eocene Epoch and flourished during the subsequent Oligocene Epoch The largest of them was a hornless rhinoceros Indricotherium transouralicum living in Eurasia that weighed However, H.
The name H. It is believed to have originated in East Africa and was the first hominin species to migrate out of Africa. Fossils of H. 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. 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. A number of species, sometimes called archaic Homo sapiens , apparently evolved from H. These archaic H. 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,—10, years ago, overlapping with anatomically-modern humans.
Homo sapiens neanderthalensis Tools : The Homo sapiens neanderthalensis used tools and may have worn clothing. There is considerable debate about the origins of anatomically-modern humans or Homo sapiens sapiens. As discussed earlier, H. 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 ,—, years ago, moving out of Africa around 50,—60, years ago to replace archaic human forms with limited interbreeding: at least once with Neanderthals and once with Denisovans.
Privacy Policy. Skip to main content. Search for:. 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. Learning Objectives Identify key characteristics of primates.
Key Takeaways Key Points All primates are descended from tree-dwellers, exhibiting adaptations which allow for tree climbing that include: a rotating shoulder joint, separated big toes and thumb for grasping, and stereoscopic vision.
True primates, ancestral to prosimians, first appear in the fossil record in the Eocene epoch around 55 million years ago; they were similar in form to lemurs.
Anthropoids ancestral to both Old World and New World monkeys appear in the fossil record in the Oligocene epoch around 35 million years ago. Anthropoids ancestral to apes appear in the Miocene epoch around 25 million years ago. Apes are divided into two main groups of hominoids: lesser apes or hylobatids gibbons and siamangs and great apes Pongo : orangutans, Gorilla : gorillas, Pan :chimpanzees, and Homo : humans.
Key Terms dimorphism : the occurrence in an animal species of two distinct types of individual adaptive radiation : the diversification of species into separate forms that each adapt to occupy a specific environmental niche. Early Human Evolution Modern humans and chimpanzees evolved from a common hominoid ancestor that diverged approximately 6 million years ago. Learning Objectives List the evolved physical traits used to differentiate hominins from other hominoids.
Key Takeaways Key Points Modern humans are classified as hominins, which also includes extinct bipedal human relatives, such as Australopithecus africanus , Homo habilis , and Homo erectus. Few very early prior to 4 million years ago hominin fossils have been found so determining the lines of hominin descent is extremely difficult. Within the last 20 years, three new genera of hominoids were discovered: Sahelanthropus tchadensis , Orrorin tugenensis, and Ardipithecus ramidus and kadabba , but their status in regards to human ancestry is somewhat uncertain.
Key Terms hominin : the evolutionary group that includes modern humans and now-extinct bipedal relatives hominoid : any great ape such as humans belonging to the superfamily Hominoidea.
Early Hominins The hominin Australopithecus evolved 4 million years ago and is believed to be in the ancestral line of the genus Homo. Learning Objectives Describe the physical characteristics of the Australopiths and compare them to those of modern humans. Key Takeaways Key Points The early hominin Australopithecus displayed various characteristics which show more similarity to the great apes than to modern humans: great sexual dimorphism, small brain size in comparison to body mass, larger canines and molars, and a prognathic jaw.
Australopithecus africanus lived between 2 and 3 million years ago and had a larger brain than A. The gracile australopiths had a relatively slender build and teeth that were suited for soft food and may have had a partially carnivorous diet, while the robust australopiths probably ate tough vegetation. Key Terms dentition : the type, number and arrangement of the normal teeth of an organism or of the actual teeth of an individual sexual dimorphism : a physical difference between male and female individuals of the same species bipedalism : the habit of standing and walking on two feet.
Genus Homo The human genus Homo , which includes modern humans as well as extinct human relatives, appeared around 2. Learning Objectives Compare and contrast the evolution and characteristics associated with the various Homo species: Homo habilis, erectus, and sapiens.
Key Takeaways Key Points Homo erectus , appearing 1. Compared to Homo habilis , Homo erectus was more similar to modern humans due to its height and weight, brain size, limited sexual dimorphism, and downward-facing nostrils. Archaic Homo sapiens had a similar brain size to modern humans Homo sapiens sapiens , but, unlike modern humans, they had a thick skull, prominent brow ridge, and a receding chin.
The multiregional hypothesis of modern human origins states that there is an unbroken line of evolution involving regional adaptations and gene flow from H. The recent out of Africa hypothesis of modern human origins states that H. BY LUCK, I had chosen to study two species that fell at opposite ends of the continuum between slow and rapid passage of food. It is now clear that most primate species can be ranked somewhere along this continuum, depending on whether they tend to maximize the efficiency with which they digest a given meal or maximize the volume of food processed in a day.
This research further shows that even without major changes in the design of the digestive tract, subtle adjustments in the size of different segments of the gut can help compensate for nutritional problems posed by an animals dietary choices.
Morphological compensations in the digestive tract can have their drawbacks, however, because they may make it difficult for a species to alter its dietary habits should environmental conditions change suddenly. These digestive findings fascinated me, but a comparison of brain size in the two species yielded one of those eurekas of which every scientist dreams. I examined information on the brain sizes of howler and spider monkeys because the spider monkeys in Panama seemed smarter than the howlers--almost human.
Actually, some of them reminded me of my friends. I began to wonder whether spider monkeys behaved differently because their brains were more like our own. My investigations showed that, indeed, the brains of howler and spider monkeys do differ, even though the animals are about the same size. Same-sized animals generally have like-sized brains. The spider monkey brain weighs about twice that of howlers. Now, the brain is an expensive organ to maintain; it usurps a disproportionate amount of the energy glucose extracted from food.
So I knew natural selection would not have favored development of a large brain in spider monkeys unless the animals gained a rather pronounced benefit from the enlargement. Considering that the most striking difference between howler and spider monkeys is their diets, I proposed that the bigger brain of spider monkeys may have been favored because it facilitated the development of mental skills that enhanced success in maintaining a diet centered on ripe fruit.
A large brain would certainly have helped spider monkeys to learn and, most important, to remember where certain patchily distributed fruit-bearing trees were located and when the fruit would be ready to eat. Also, spider monkeys comb the forest for fruit by dividing into small, changeable groups.
Expanded mental capacity would have helped them to recognize members of their particular social unit and to learn the meaning of the different food-related calls through which troop members convey over large distances news of palatable items.
Howler monkeys, in contrast, would not need such an extensive memory, nor would they need so complex a recognition and communication system. They generally forage for food as a cohesive social unit, following well-known arboreal pathways over a much smaller home range. Also, howlers tend to use only one or perhaps two large fruiting trees a day, whereas spider monkeys often visit five, 10 or more. If I was correct that the pressure to obtain relatively difficult-to-find, high-quality plant foods encourages the development of mental complexity which is paid for by greater foraging efficiency , I would expect to find similar differences in brain size in other primates.
That is, monkeys and apes who concentrated on ripe fruits would have larger brains than those of their leaf-eating counterparts of equal body size. To pursue this idea, I turned to estimates of comparative brain sizes published by Harry J. Jerison of the University of California, Los Angeles. To my excitement, I found that those primate species that eat higher-quality, more widely dispersed foods generally have a larger brain than do their similar-sized counterparts that feed on lower-quality, more uniformly distributed resources.
As I noted earlier, primates typically have larger brains than do other mammals of their size. I believe the difference arose because all primates feed very selectively, favoring the highest-quality plant parts--for instance, even primates that eat leaves tend to choose very immature leaves or only the low-fiber tips of older leaves. A review of the fossil record for the hominid family--humans and their precursors--provided some intriguing clues. Australopithecus , one important genus in our family, emerged in Africa more than 4.
As is true of later hominids, they were bipedal, but their brains were not appreciably larger than those of todays apes. Hence, selection had not yet begun to favor a greatly enlarged brain in our family. The fossil record also indicates Australopithecus had molar teeth that would have been well suited to a diet consisting largely of tough plant material.
Toward the end of the Pliocene, climate conditions began to change. The next epoch, the Pleistocene lasting from about two million to 10, years ago , was marked by repeated glaciations of the Northern Hemisphere. Over both epochs, tropical forests shrank and were replaced in many areas by savanna woodlands. As the diversity of tree species decreased and the climate became more seasonal, primates in the expanding savanna areas must have faced many new dietary challenges. In the Pleistocene the last species of Australopithecus --which by then had truly massive jaws and molars--went extinct.
Perhaps those species did so, as my colleague Montague W. Demment of the University of California, Davis, speculates, because they were outcompeted by the digestively specialized ungulates hoofed animals. The human, or Homo , genus emerged during the Pliocene.
Early species of Homo were similar in body size to Australopithecus but had notably larger brains. These species were replaced by the even larger-brained H. In parallel with the increases in brain size in the Homo genus, other anatomic changes were also occurring. The molar and premolar teeth became smaller, and stature increased. To me, the striking expansion of brain size in our genus indicates that we became so successful because selection amplified a tendency inherent in the primate order since its inception: that of using brainpower, or behavior, to solve dietary problems.
Coupled with the anatomic changes--and with the associations in living primates between larger brains and a high-quality diet--this increase also points to the conclusion that the behavioral solution was to concentrate on high-quality foods. In fact, I suspect early humans not only maintained dietary quality in the face of changing environmental conditions but even improved it.
Expansion of the brain in combination with growth in body size and a reduction in the dentition supports the notion of a high-quality diet for a couple of reasons. When one examines present-day orangutans and gorillas, it becomes clear that in our superfamily, Hominoidea apes and humans , an increase in body size combined with decreased dietary quality leads to a slow-moving, fairly sedentary and unsociable ape.
Yet our Homo ancestors apparently were mobile and sociable--more resembling the lively, social and communicative chimpanzee. Unlike orangutans and gorillas, which eat quantities of leaves and other fibrous materials as well as ripe and unripe fruits, chimpanzees feed preferentially on large quantities of high-quality, energy-rich ripe fruits. Likewise, the reduction in the molars and premolars shows that the texture of foods we ate had somehow been altered such that the dentition no longer had so much work to do.
In other words, either these early humans were eating different less fibrous, easier-to-chew foods than was Australopithecus , or they were somehow processing foods to remove material that would be hard to chew and digest. Indeed, stone tools found with fossil remains of H. The probability that hominids persisted in seeking energy-rich foods throughout their evolution suggests an interesting scenario.
As obtaining certain types of plant foods presumably became more problematic, early humans are thought to have turned increasingly to meat to satisfy their protein demands. One can readily envision their using sharp stone flakes to cut through tough hides and to break bones for marrow.
To incorporate meat into the diet on a steady basis and also to amass energy-rich plant foods, our ancestors eventually developed a truly novel dietary approach. They adopted a division of labor, in which some individuals specialized in the acquisition of meat by hunting or scavenging and other individuals specialized in gathering plants.
Many of the foods thus acquired were saved instead of being eaten on the spot; they were later shared among the entire social unit to assure all members of a balanced diet. Survival of the individual thus came to depend on a number of technological and social skills. It demanded not only having a brain able to form and retain a mental map of plant food supplies but also having knowledge of how to procure or transform such supplies.
In addition, survival now required an ability to recognize that a stone tool could be fashioned from a piece of a rock and a sense of how to implement that vision. And it required the capacity to cooperate with others for instance, to communicate about who should run ahead of a hunted zebra and who behind , to defer gratification to save food until it could be brought to an agreed site for all to share and both to determine ones fair portion and to ensure that it was received.
Such demands undoubtedly served as selective pressures favoring the evolution of even larger, more complex brains. IN OTHER WORDS, I see the emergence and evolution of the human line as stemming initially from pressures to acquire a steady and dependable supply of very high quality foods under environmental conditions in which new dietary challenges made former foraging behaviors somehow inadequate.
Specialized carnivores and herbivores that abound in the African savannas were evolving at the same time as early humans, perhaps forcing them to become a new type of omnivore, one ultimately dependent on social and technological innovation and thus, to a great extent, on brainpower.
Edward O. Wilson of Harvard University has estimated that for more than two million years until about , years ago , the human brain grew by about a tablespoon every , years. Apparently each tablespoonful of brain matter added in the genus Homo brought rewards that favored intensification of the trend toward social and technological advancement.
Although the practice of adding some amount of meat to the regular daily intake became a pivotal force in the emergence of modern humans, this behavior does not mean that people today are biologically suited to the virtually fiber-free diet many of us now consume.
In fact, in its general form, our digestive tract does not seem to be greatly modified from that of the common ancestor of apes and humans, which was undoubtedly a strongly herbivorous animal. Yet as of the mids no studies had been done to find out whether the gut functions of modern humans were in fact similar to those of apes. It was possible that some functional differences existed, because anatomic evidence had shown that despite similarity in the overall form of the digestive tract, modern humans have a rather small tract for an animal of their size.
They also differ from apes in that the small intestine accounts for the greatest fraction of the volume of the human digestive tract; in apes the colon accounts for the greatest volume.
To better understand the kind of diet for which the human gut was adapted, Demment and I decided to compare human digestive processes with those of chimpanzees, our closest living relatives. We hoped to determine whether, over the course of their respective evolutionary histories, humans and chimpanzees had diverged notably in their abilities to deal with fiber.
We were greatly encouraged in this effort by Glynn Isaac, who was then at the University of California, Berkeley. The feeding habits of chimpanzees are well known.
Despite their skill in capturing live prey particularly monkeys , these apes actually obtain an estimated 94 percent of their annual diet from plants, primarily ripe fruits. Even though the fruits chimpanzees eat tend to be rich in sugar, they contain less pulp and more fiber and seeds than do the cultivated fruits sold in our supermarkets.
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