Primate Evolution Chapter 16 Chapter Reinforcement and Study GuideReinforcement and Study Guide In your textbook, read about the characteristics of a primate. Complete the chart by checking those structures or functions that are characteristic of primates. In your textbook, read about primate origins. For each statement below, write true or false.

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Darwin's great insight, and the unifying principle of biology today, is that all species are related to one another like sisters, cousins, and distant kin in a vast family tree of life. The implications are breathtaking; if we could travel back far enough in time, we would find common ancestors between ourselves and every other living organism, from porcupines to flamingoes to cactuses. Our immediate evolutionary family is comprised of the hominoids, the group of primates that includes the 'lesser apes' (siamangs and gibbons) as well as the 'great apes' (chimpanzees, bonobos, gorillas, and orangutans). Among the great apes, our closest relatives are the chimpanzees and bonobos (Figure 1). The fossil record, along with studies of human and ape DNA, indicate that humans shared a common ancestor with chimpanzees and bonobos sometime around 6 million years ago (mya). We begin this discussion of our species' evolution in Africa, near the end of the geological time period known as the Miocene, just before our lineage diverged from that of chimpanzees and bonobos.

In order to understand the evolution of any species, we must first establish its ancestral state: what sort of animal did it evolve from? For our lineage, this requires that we try and reconstruct the Last Common Ancestor of humans and chimpanzees (marked 'A' in Figure 1). The Human-Chimpanzee Last Common Ancestor (HC-LCA) is the species from which the hominin lineage and the chimpanzee & bonobo lineage diverged. Hominins are species on our branch of the hominoid tree after the split with the chimpanzee & bonobo line, including all of the extinct species and evolutionary side branches (Figure 1).

There was a great diversity of ape species in the Miocene, with dozens of species known from the fossil record across Africa, Europe, and Asia. These species varied in their anatomy and ecology, and it is not clear which, if any, of the fossil species discovered thus far represent the HC-LCA (Kunimatsu et al. 2007; Young and MacLatchy, 2004). Nonetheless, we know from fossil and comparative evidence that it was much more similar to living apes than to living humans. The HC-LCA would have had an ape-sized brain and body, with relatively long arms and fingers and a grasping foot that allowed it to forage in the trees. The canine teeth were probably large and sharp, as seen in several Miocene hominoids. Moreover, the canines were probably sexually dimorphic, with males having much larger canines than females, as seen among the living great apes and Miocene fossils. Like living apes it would have walked quadrupedally (on all fours) when on the ground, and its diet would have consisted almost entirely of plant foods, primarily fruit and leaves.

Changes from an ape-like anatomy are discernible in hominoid fossils from the late Miocene in Africa. Some hominoid species from this period exhibit traits that are typical of humans but are not seen in the other living apes, leading paleoanthropologists to infer that these fossils represent early members of the hominin lineage. The first human-like traits to appear in the hominin fossil record are bipedal walking and smaller, blunt canines.

The oldest hominins currently known are Sahelanthropus tchadensis from Chad (Brunet et al. 2005) and Orrorin tugenensis from Kenya (Senut et al. 2001). Sahelanthropus, dated to between 6 and 7 mya, is known from a largely complete skull and some other fragmentary remains. Its brain size, 360cc, is within the range seen in chimpanzees, and the skull has a massive brow ridge, similar in thickness to male gorillas (Brunet et al. 2005). However, the position and orientation of the foramen magnum, the hole in the base of the skull through which the spinal cord passes, suggests that Sahelanthropus stood and walked bipedally, with its spinal column held vertically as in modern humans rather than horizontally as in apes and other quadrupeds (Zollikofer et al. 2005). Orrorin is known primarily from postcranial fossils, including a partial femur. The proximal portion of the femur shows similarities to those of modern humans, suggesting the species was bipedal (Pickford et al. 2002). No skulls of Orrorin have been recovered, and so its cranial morphology and brain size are uncertain. In both Orrorin and Sahelanthropus the canine teeth of males are larger and more pointed than in modern humans, but are small and blunt compared to the canines of male apes. This suggests that canine sexual dimorphism — and by extension, competition among males for mating access to females — was diminished in these early hominins compared to the great apes.

By far the best known early hominin is Ardipithecus ramidus, a 4.4 million year old species from Ethiopia, which is known from a nearly complete skeleton as well as numerous other dental and skeletal remains (White et al. 2009). Ar. ramidus and an older, related species known from fragmentary remains, Ar. kadabba (5.8–5.2 mya), have reduced canines similar to those of Orrorin and Sahelanthropus. The skull of Ar. ramidus is rather ape-like and broadly similar to that of Sahelanthropus, with a small chimpanzee-sized brain of 300–350cc (Figure 2). The Ardipithecus postcranial skeleton is intriguing. Although badly fragmented, the pelvis recovered reveals a morphology quite different from that of living apes, with a shorter, more bowl-like shape that strongly suggests Ardipithecus walked bipedally; this is consistent with the foramen magnum position, which suggests an upright posture. However, its long forelimbs and fingers and its divergent, grasping first toe (hallux) suggest Ardipithecus spent much of its time in the trees. The overall impression is of a largely arboreal species that walked bipedally whenever it ventured to the ground.

Australopithecus, Homo erectus, and humans.' />

Around 4mya we find the earliest members of the genus Australopithecus, hominins which were adept terrestrial bipeds but continued to use the trees for food and protection. The first specimens of Australopithecus were discovered in South Africa in 1924 (Dart, 1925), and research efforts over the subsequent eight decades have produced hundreds of fossils from several species at sites all across East and Southern Africa. We now know that Australopithecus was a highly successful genus that persisted for nearly three million years (Figure 1).

The best-known Australopithecus species are A. afarensis (3.6–2.9 mya) from East Africa and A. africanus (3.2–2.0mya) from South Africa. The pelvis and lower limb of these species clearly indicates that they were fully bipedal: the pelvis is short and bowl-shaped, bringing the gluteal muscles around to the side of the body, as in modern humans, for trunk stabilization during bipedalism, and the first toe is in line with the other toes (Ward, 2002; Harcourt-Smith and Aiello, 2004). The Australopithecus foot may even have had a human-like arch, based on analysis of the metatarsals and the fossilized Laetoli footprints (Ward et al. 2011). Nonetheless, compared to modern humans, the forearms were long and the fingers and toes were long and somewhat curved, suggesting that Australopithecus regularly used the trees to forage and perhaps as a refuge from predators at night. This mixed terrestrial & arboreal strategy would have served these species well in the mixed woodland and savannah environments they inhabited.

Brain size in Australopithecus ranged between 390 and 515cc, similar to chimpanzees and gorillas (Falk et al. 2000), suggesting cognitive abilities were broadly similar to living apes (Figure 2). Body size in Australopithecus was rather small and sexually dimorphic, about 30kg for females and 40kg for males (McHenry, 1992). This level of dimorphism is not reflected in the canines, which were small, blunt, and monomorphic as in earlier hominins.

Unlike the canines, molar teeth in Australopithecus were much larger than those of earlier hominins, and had thicker enamel. This suggests their diet included hard, low quality plant foods that required powerful chewing to process. A subgroup of Australopithecus, known as the 'robust' australopiths (often labeled by a separate genus Paranthropus) because of their enormous teeth and chewing muscles, took this adaptation to the extreme. Most Australopithecus species were extinct by 2 mya, but some robust forms persisted until about 1.2 mya in East and South Africa.

The earliest fossils of our own genus, Homo, are found in East Africa and dated to 2.3 mya (Kimbel et al. 1997). These early specimens are similar in brain and body size to Australopithecus, but show differences in their molar teeth, suggesting a change in diet. Indeed, by at least 1.8 mya, early members of our genus were using primitive stone tools to butcher animal carcasses, adding energy-rich meat and bone marrow to their plant-based diet.

The oldest member of the genus Homo, H. habilis (2.3–1.4 mya) is found in East Africa and is associated with butchered animal bones and simple stone tools (Blumenschine et al. 2003). Its more formidable and widespread descendant, H. erectus, is found throughout Africa and Eurasia and persisted from 1.9 mya to 100 kya, and perhaps even later (Anton, 2003). Like modern humans, H. erectus lacked the forelimb adaptations for climbing seen in Australopithecus (Figure 2). Its global expansion suggests H. erectus was ecologically flexible, with the cognitive capacity to adapt and thrive in vastly different environments. Not surprisingly, it is with H. erectus that we begin to see a major increase in brain size, up to 1,250cc for later Asian specimens (Anton, 2003). Molar size is reduced in H. erectus relative to Australopithecus, reflecting its softer, richer diet.

Around 700 kya, and perhaps earlier, H. erectus in Africa gave rise to H. heidelbergensis, a species very much like us in terms of body proportions, dental adaptations, and cognitive ability (Rightmire, 2009). H. heidelbergensis, often referred to as an 'archaic' Homo sapiens, was an active big-game hunter, produced sophisticated Levallois style tools, and by at least 400 kya had learned to control fire (Roebroeks and Villa, 2011). Neanderthals (H. neanderthalensis), cold-adapted hominins with stout physiques, complex behaviors, and brains similar in size to ours, are thought to have evolved from H. heidelbergensis populations in Europe by at least 250 kya (Rightmire, 2008; Hublin, 2009).

Fossil and DNA evidence suggest our own species, H. sapiens, evolved in Africa 200 kya (Relethford, 2008; Rightmire, 2009), probably from H. heidelbergensis. The increased behavioral sophistication of H. sapiens, as indicated by our large brains (1,400cc) and archeological evidence of a broader tool set and clever hunting techniques, allowed our species to flourish and grow on the African continent. By 100kya, our species spilled into Eurasia, eventually expanding across the entire globe into Australia and the Americas (DiGiorgio et al. 2009). Along the way our species displaced other hominins they encountered, including Neanderthals in Europe and similar forms in Asia. (Note that not all agree with this interpretation of the data, see Tryon and Bailey). Studies of ancient DNA extracted from Neanderthal fossils suggest our species may have occasionally interbred with them (Green et al., 2010). Our increasing global impact continues today, as cultural innovations such as agriculture and urbanization shape the landscape and species around us.

The evolution of our species from an ape-like Miocene ancestor was a complex process. Our lineage is full of side branches and evolutionary dead ends, with species like the robust australopiths that persisted for over a million years before fading away. Some human traits, like bipedalism, evolved very early, while others, like large brains, did not evolve until relatively recently. Still other traits, like molar size, evolved in one direction only to be pushed back later by changing ecological pressures. Rather than a powerful ship charting a straight course toward some pre-determined destination, the evolution of our lineage — indeed, of any species' lineage — fits the image of a lifeboat tossed about by the shifting seas of environmental change, genetic luck, and geological chance. One wonders where the next six million years might take us.

References and Recommended Reading

Anton, S. C. Natural history of Homoerectus. American Journal of Physical AnthropologyS37, 126-70 (2003)

Blumenschine, R. J. et al. LatePliocene Homo and hominid land usefrom Western Olduvai Gorge, Tanzania. Science299, 1217-12121 (2003)

Brunet, M. et al. New material of the earliest hominid from the UpperMiocene of Chad. Nature434, 752-755 (2005)

Dart, R.A. Australopithecusafricanus: the southern ape-man of Africa. Nature115, 195-199 (1925)

DeGiorgio, M. et al. Out ofAfrica: modern human origins special feature: explaining worldwide patterns ofhuman genetic variation using a coalescent-based serial founder model ofmigration outward from Africa. PNAS USA106, 16057-16062 (2009)

Falk, D. et al. Early hominid brain evolution: a new look at oldendocasts. Journal of Human Evolution38, 695-717 (2000)

Green, R.E. A draft sequence of the Neandertal genome. Science328, 710-722

Harcourt-Smith, W. E. & L.C. Aiello. Fossils, feet and theevolution of human bipedal locomotion. Journal of Anatomy204, 403-416 (2004)

Hublin, J.J. The origin of Neanderthals. PNAS45,169-177 (2009)

Kimbel, W. H. et al. Systematic assessment of a maxilla of Homo from Hadar, Ethiopia. American Journal of Physical Anthropology103, 235-262 (1997)

Kunimatsu, Y. et al. A new Late Miocene great ape from Kenya and itsimplications for the origins of African great apes and humans. PNAS USA104, 19661-19662. (2007)

McHenry, H. M. Body size and proportions in early hominids. American Journal of Physical Anthropology87, 407-431 (1992)

Pickford, M. et al. Bipedalism in Orrorin tugenensis revealed by itsfemora. Comptes Rendus Palevol 1, 1-13 (2002)

Relethford, J. H. Genetic evidence and the modern human originsdebate. Heredity100, 555-563 (2008)

Rightmire, G. P. Out of Africa: modern human origins special feature:middle and later Pleistocene hominins in Africa and Southwest Asia. PNAS USA106, 16046-16050 (2009)

Rightmire, G.P. Homo in theMiddle Pleistocene: Hypodigms, variation, and species recognition. Evolutionary Anthropology17,8-21 (2008)

Roebroeks, W. & P. Villa. On the earliest evidence for habitualuse of fire in Europe. PNAS USAEpub ahead of print (2011)

Senut, B. et al. First hominid from the Miocene (Lukeino Formation,Kenya). C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences332, 137-144 (2001)

Ward, C. V. et al. Complete fourth metatarsal and arches in the footof Australopithecus afarensis. Science331, 750-753 (2011)

Ward, C. V. Interpreting the posture and locomotion ofAustralopithecus afarensis: where do we stand? American Journal of Physical AnthropologyS35, 185-215(2002)

White, T. D. et al. Ardipithecusramidus and the paleobiology of early hominids. Science326, 75-86 (2009)

Young, N. M. et al. The phylogenetic position of Morotopithecus. Journal of Human Evolution46, 163-184 (2004)

Zollikofer, C. P. et al. Virtual cranialreconstruction of Sahelanthropustchadensis. Nature434, 755-759 (2005)

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Learning Objectives

By the end of this section, you will be able to do the following:

• Describe the derived features that distinguish primates from other animals
• Describe the defining features of the major groups of primates
• Identify the major hominin precursors to modern humans
• Explain why scientists are having difficulty determining the true lines of descent in hominids

Order Primates of class Mammalia includes lemurs, tarsiers, monkeys, apes, and humans. Non-human primates live primarily in the tropical or subtropical regions of South America, Africa, and Asia. They range in size from the mouse lemur at 30 grams (1 ounce) to the mountain gorilla at 200 kilograms (441 pounds). The characteristics and evolution of primates are of particular interest to us as they allow us to understand the evolution of our own species.

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 hands and feet that are adapted for climbing, or brachiation (swinging through trees using the arms). These adaptations include, but are not limited to: 1) a rotating shoulder joint, 2) a big toe that is widely separated from the other toes (except humans) and thumbs sufficiently separated from fingers to 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, claws that have been modified into flattened nails, typically only one offspring per pregnancy, and a trend toward holding the body upright.

Order Primates is divided into two groups: Strepsirrhini (“turned-nosed”) and Haplorhini (“simple-nosed”) primates. Strepsirrhines, also called the wet-nosed primates, include prosimians like the bush babies and pottos of Africa, the lemurs of Madagascar, and the lorises of Southeast Asia. Haplorhines, or dry-nosed primates, include tarsiers ((Figure)) and simians (New World monkeys, Old World monkeys, apes, and humans). In general, strepsirrhines tend to be nocturnal, have larger olfactory centers in the brain, and exhibit a smaller size and smaller brain than anthropoids. Haplorhines, with a few exceptions, are diurnal, and depend more on their vision. Another interesting difference between the strepsirrhines and haplorhines is that strepsirrhines have the enzymes for making vitamin C, while haplorhines have to get it from their food.

Figure 1. A Philippine tarsier. This tarsier, Carlito syrichta, is one of the smallest primates—about 5 inches long, from nose to the base of the tail. The tail is not shown, but is about twice the length of the body. Note the large eyes, each of which is about the same size as the animal’s brain, and the long hind legs. (credit: mtoz (http://creativecommons.org/licenses/by-sa/2.0), via Wikimedia Commons)

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 fragmented. These proto-primates remain largely mysterious creatures until more fossil evidence becomes available. Although genetic evidence suggests that primates diverged from other mammals about 85 MYA, the oldest known primate-like mammals with a relatively robust fossil record date to about 65 MYA. Fossils like the proto-primate Plesiadapis (although some researchers do not agree that Plesiadapis was a proto-primate) 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.

The first true primates date to about 55 MYA in the Eocene epoch. They were found in North America, Europe, Asia, and Africa. 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.

Anthropoid monkeys evolved from prosimians during the Oligocene epoch. By 40 million years ago, evidence indicates that monkeys were present in the New World (South America) and the Old World (Africa and Asia). New World monkeys are also called Platyrrhini—a reference to their broad noses ((Figure)). Old World monkeys are called Catarrhini—a reference to their narrow, downward-pointed 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 both arboreal and ground-dwelling species. The arboreal habits of the New World monkeys are reflected in the possession of prehensile or grasping tails by most species. The tails of Old World monkeys are never prehensile and are often reduced, and some species have ischial callosities—thickened patches of skin on their seats.

Figure 2. A New World monkey. The howler monkey is native to Central and South America. It makes a call that sounds like a lion roaring. (credit: Xavi Talleda)

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. When walking quadrupedally, monkeys walk on their palms, while apes support the upper body on their knuckles. Apes are more intelligent than monkeys, and they have larger brains relative 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) ((Figure)).

Figure 3. Primate skeletons. All great apes have a similar skeletal structure. (credit: modification of work by Tim Vickers)

The very arboreal gibbons are smaller than the great apes; they have low sexual dimorphism (that is, the sexes are not markedly different in size), although in some species, the sexes differ in color; and they have relatively longer arms used for swinging through trees ((Figure)a). Two species of orangutan are native to different islands in Indonesia: Borneo (P. pygmaeus) and Sumatra (P. abelii). A third orangutan species, Pongo tapanuliensis, was reported in 2017 from the Batang Toru forest in Sumatra. Orangutans are arboreal and solitary. Males are much larger than females and have cheek and throat pouches when mature. Gorillas all live in Central Africa. The eastern and western populations are recognized as separate species, G. berengei and G. gorilla. Gorillas are strongly sexually dimorphic, with males about twice the size of females. In older males, called silverbacks, the hair on the back turns white or gray. Chimpanzees ((Figure)b) are the species considered to be most closely related to humans. However, the species most closely related to the chimpanzee is the bonobo. Genetic evidence suggests that chimpanzee and human lineages separated 5 to 7 MYA, while chimpanzee (Pan troglodytes) and bonobo (Pan paniscus) lineages separated about 2 MYA. Chimpanzees and bonobos both live in Central Africa, but the two species are separated by the Congo River, a significant geographic barrier. Bonobos are slighter than chimpanzees, but have longer legs and more hair on their heads. In chimpanzees, white tail tufts identify juveniles, while bonobos keep their white tail tufts for life. Bonobos also have higher-pitched voices than chimpanzees. Chimpanzees are more aggressive and sometimes kill animals from other groups, while bonobos are not known to do so. Both chimpanzees and bonobos are omnivorous. Orangutan and gorilla diets also include foods from multiple sources, although the predominant food items are fruits for orangutans and foliage for gorillas.

Figure 4. Lesser and great apes. This white-cheeked gibbon (a) is a lesser ape. In gibbons of this species, females and infants are buff and males are black. This young chimpanzee (b) is one of the great apes. It possesses a relatively large brain and has no tail. (credit a: MAC. credit b: modification of work by Aaron Logan)

Human Evolution

The family Hominidae of order Primates includes the hominoids: the great apes and humans ((Figure)). 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 six 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. A number of marker features differentiate humans from the other hominoids, including bipedalism or upright posture, increase in the size of the brain, and a fully opposable thumb that can touch the little finger. Bipedal hominins include several groups that were probably part of the modern human lineage—Australopithecus, Homo habilis, and Homo erectus—and several non-ancestral groups that can be considered “cousins” of modern humans, such as Neanderthals and Denisovans.

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.

Figure 5. Hominin phylogeny. This chart shows the evolution of modern humans.

Very Early Hominins

Three species of very early hominids have made news in the late 20th and early 21st centuries: Ardipithecus, Sahelanthropus, and Orrorin. The youngest of the three species, Ardipithecus, was discovered in the 1990s, and dates to about 4.4 MYA. Although the bipedality of the early specimens was uncertain, several more specimens of Ardipithecus were discovered in the intervening years and demonstrated that the organism was bipedal. Two different species of Ardipithecus have been identified, A. ramidus and A. kadabba, whose specimens are older, dating to 5.6 MYA. However, the status of this genus as a human ancestor is uncertain.

The oldest of the three, Sahelanthropus tchadensis, was discovered in 2001-2002 and 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, and it is unclear how this fossil fits with the picture given by molecular data, namely that the line leading to modern humans and modern chimpanzees apparently bifurcated about six million years ago. It is not thought at this time that this species was an ancestor of modern humans.

A younger (c. 6 MYA) species, Orrorin tugenensis, is also a relatively recent discovery, found in 2000. There are several specimens of Orrorin. Some features of Orrorin are more similar to those of modern humans than are the australopithicenes, although Orrorin is much older. If Orrorin is a human ancestor, then the australopithicenes may not be in the direct human lineage. Additional specimens of these species may help to clarify their role.

Early Hominins: Genus Australopithecus

Australopithecus (“southern ape”) is a genus of hominin that evolved in eastern Africa approximately four million years ago and went extinct about two million years ago. This genus is of particular interest to us as it is thought that our genus, genus Homo, evolved from a common ancestor shared with Australopithecus about two million years ago (after likely passing through some transitional states). 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 in modern humans and more similar 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, were found in Laetoli, Tanzania and dated to 3.6 million years ago. They showed that hominins at the time of Australopithecus were walking upright.

There were a number of Australopithecus species, which are 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 ((Figure)a) had smaller canines and molars compared to apes, but these were larger than those of modern humans. Its brain size was 380 to 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 and dated to 3.24 million years ago ((Figure)). 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.

Figure 6. Australopithicene and modern human skulls. 1985 ford f250 460 carburetor manual. The skull of (a) Australopithecus afarensis, an early hominid that lived between two and three million years ago, resembled that of (b) modern humans but was smaller with a sloped forehead, larger teeth, and a prominent jaw.

Figure 7. Lucy. This adult female Australopithecus afarensis skeleton, nicknamed Lucy, was discovered in the mid-1970s. (credit: “120”/Wikimedia Commons)

Australopithecus africanus lived between two and three million years ago. It had a slender build and was bipedal, but had robust arm bones and, like 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. bahrelghazali is unusual in being the only australopith found in Central Africa.

A Dead End: Genus Paranthropus

The australopiths had a relatively slender build and teeth that were suited for soft food. In the past several years, fossils of hominids of a different body type have been found and dated to approximately 2.5 million years ago. These hominids, of the genus Paranthropus, were muscular, stood 1.3 to 1.4 meters tall, and had large grinding teeth. Their molars showed heavy wear, suggesting that they had a coarse and fibrous vegetarian diet as opposed to the partially carnivorous diet of the australopiths. Paranthropus includes Paranthropusrobustus of South Africa, and Paranthropusaethiopicus and Paranthropusboisei of East Africa. The hominids in this genus went extinct more than one 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.

Early Hominins: Genus Homo

The human genus, Homo, first appeared between 2.5 and three million years ago. For many years, fossils of a species called H. habilis were the oldest examples in the genus Homo, but in 2010, a new species called Homo gautengensis was discovered and may be older. Compared to A. africanus, H. habilis had a number of features more similar to modern humans. H. habilis had a jaw that was less prognathic than the australopiths and a larger brain, at 600 to 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.

Link to Learning

Watch this video about Smithsonian paleontologist Briana Pobiner explaining the link between hominin eating of meat and evolutionary trends.

H. erectus appeared approximately 1.8 million years ago ((Figure)). 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, which are sizes similar to those of modern humans. Its degree of sexual dimorphism was less than in earlier species, with males being 20 to 30 percent larger than females, which is close to the size difference seen in our own species. H. erectus had a larger brain than earlier species at 775 to 1,100 cubic centimeters, which compares to the 1,130 to 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.

Figure 8. Homo erectus. Homo erectus had a prominent brow and a nose that pointed downward rather than forward.

Humans: Homo sapiens

A number of species, sometimes called archaic Homo sapiens, apparently evolved from H. erectus starting about 500,000 years ago. These species include Homo heidelbergensis, Homo rhodesiensis, and Homo neanderthalensis. These archaic H. sapiens had a brain size similar to that of modern humans, averaging 1,200 to 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 species survived until 30,000 to 10,000 years ago, overlapping with modern humans ((Figure)).

Figure 9. Neanderthal. The Homo 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. erectus migrated out of Africa and into Asia and Europe in the first major wave of migration about 1.5 million years ago. It is thought that modern humans arose in Africa from H. erectus and migrated out of Africa about 100,000 years ago in a second major migration wave. Then, modern humans replaced H. erectus species that had migrated into Asia and Europe in the first wave.

This evolutionary timeline is supported by molecular evidence. One approach to studying the origins of modern humans is to examine mitochondrial DNA (mtDNA) from populations around the world. Because a fetus develops from an egg containing its mother’s mitochondria (which have their own, non-nuclear DNA), mtDNA is passed entirely through the maternal line. Mutations in mtDNA can now be used to estimate the timeline of genetic divergence. The resulting evidence suggests that all modern humans have mtDNA inherited from a common ancestor that lived in Africa about 160,000 years ago. Another approach to the molecular understanding of human evolution is to examine the Y chromosome, which is passed from father to son. This evidence suggests that all men today inherited a Y chromosome from a male that lived in Africa about 140,000 years ago.

The study of mitochondrial DNA led to the identification of another human species or subspecies, the Denisovans. DNA from teeth and finger bones suggested two things. First, the mitochondrial DNA was different from that of both modern humans and Neanderthals. Second, the genomic DNA suggested that the Denisovans shared a common ancestor with the Neanderthals. Genes from both Neanderthals and Denisovans have been identified in modern human populations, indicating that interbreeding among the three groups occurred over part of their range.

Section Summary

All primate species possess adaptations for climbing trees and probably descended from arboreal ancestors, although not all living species are arboreal. Other characteristics of primates are brains that are larger, relative to body size, than those of other mammals, claws that have been modified into flattened nails, typically only one young per pregnancy, stereoscopic vision, and a trend toward holding the body upright. Primates are divided into two groups: strepsirrhines, which include most prosimians, and haplorhines, which include simians. Monkeys evolved from prosimians during the Oligocene epoch. The simian line includes both platyrrhine and catarrhine branches. Apes evolved from catarrhines in Africa during the Miocene epoch. Apes are divided into the lesser apes and the greater apes. Hominins include those groups that gave rise to our own species, such as Australopithecus and H. erectus, and those groups that can be considered “cousins” of humans, such as Neanderthals and Denisovans. Fossil evidence shows that hominins at the time of Australopithecus were walking upright, the first evidence of bipedal hominins. A number of species, sometimes called archaic H. sapiens, evolved from H. erectus approximately 500,000 years ago. There is considerable debate about the origins of anatomically modern humans or H. sapiens sapiens, and the discussion will continue, as new evidence from fossil finds and genetic analysis emerges.

Review Questions

Which of the following is not an anthropoid?

1. Lemurs
2. Monkeys
3. Apes
4. Humans

Which of the following is part of a clade believed to have died out, leaving no descendants?

1. Paranthropus robustus
2. Australopithecus africanus
3. Homo erectus
4. Homo sapiens sapiens

Which of the following human traits is not a shared characteristic of primates?

1. Hip structure supporting bipedalism
2. Detection and processing of three-color vision
3. Nails at the end of each digit
4. Enlarged brain area associated with vision, and reduced area associated with smell

Free Response

How did archaic Homo sapiens differ from anatomically modern humans?

Show Solution

Archaic Homo sapiens differed from modern humans by having a thick skull and a prominent brow ridge, and lacking a prominent chin.

Why is it so difficult to determine the sequence of hominin ancestors that have led to modern Homo sapiens?

Show Solution

The immediate ancestors of humans were Australopithecus. All people past and present, along with the australopithecines, are hominins. We share the adaptation of being habitually bipedal. The earliest australopithecines very likely did not evolve until 5 million years ago. The primate fossil record for this crucial transitional period leading to australopithecines is still sketchy and somewhat confusing. By about 2.5 million years ago, there were at least two evolutionary lines of hominins descended from early australopithecines.