Human evolution

Human evolution

Living humans evolved from ancestral apes during the last 5 to 7 million years. Our ancestors and relatives, called hominins, remained limited to Africa for two thirds of their history. With chimpanzee-sized bodies and brains, early hominins diversified into several lineages with different dietary strategies. One of these found a path toward technology, food sharing and hunting and gathering, giving rise to our genus, Homo approximately 2 million years ago. As populations of Homo spread throughout the world, they gave rise to regional populations with their own anatomical and genetic distinctiveness. Within the last 100,000 years, a massive dispersal of humans from Africa absorbed populations of archaic humans elsewhere in the Old World. In the time since this emergence from Africa, humans have continued to disperse, interact, and evolve. The rise of agricultural subsistence shifted human ecology, fueling our ongoing evolution.

Paleoanthropologists have uncovered thousands of fossil specimens representing human ancestors and collateral relatives. Archaeologists study traces of the behavior of hominins during the last half of human evolution, giving details about diet and social organization. Today, geneticists can add evidence from whole-genome comparisons of living humans, primates, and some ancient hominins.

We can roughly consider human evolution in three parts. The first, running from 7 million up to around 4 million years ago, saw the origination of the hominin lineage and the initial appearance of our bipedal pattern of locomotion. The second part, from 4 million up to around 1.8 million years ago, was the age of the australopithecines. This group of species reflect a stable set of adaptations in body size and locomotion, with a substantial dietary and geographic diversity. From the australopithecines emerged our own genus, Homo, whose history dominates the last 1.8 million years of our evolution. The spread of Homo throughout the world, along with many later dispersals and population expansions, lay the foundation for today’s human populations.

Origin of the hominins

Chimpanzees and bonobos are our closest relatives among living primates. Whole genome comparisons suggest that our common ancestors with these apes lived between 4 million and 7 million years ago Innan:Watanabe:2006, Patterson:2006. Our common ancestors with gorillas lived a bit earlier, within the last 10 million years Hobolth:speciation:2007, and orangutans sometime before 12 million years ago Hobolth:2011. Hence it is during the period between 10 million and 4 million years ago that paleontologists look for the immediate precursors of our lineage.

A rich record of fossil apes has been recovered from the Miocene geological epoch, which lasted from 23 million to 5.2 million years ago. Before 15 million years ago, all known apes lived in Afro-Arabia. Early in the Middle Miocene, some apes dispersed into Asia and Europe, including the Asian ancestors of orangutans. Miocene apes ranged extensively in body size and adaptive niche, and evolved a diversity of locomotor strategies. Many were pronograde quadrupeds on the ground and in trees, essentially like living Old World monkeys like macaques and baboons. A few had shoulders and vertebral columns that indicate orthograde posture or climbing, but no early apes are known to have had the long arms and below-branch suspensory capability of today’s great apes Nakatsukasa:2009. Vertical, orthograde posture was once thought to be ancestrally shared by humans and apes like gibbons and orangutans; it is now clear that many aspects of this body plan evolved convergently in the African and Asian apes Begun:Kivell:2011, Crompton:2008.

Living humans and known fossil hominins are obligate bipeds, with pelvis, foot and vertebral adaptations that impede effective quadrupedal gait and climbing. The origin of hominins is entangled with this unique adaptation, but the earliest members of our lineage surely did not have the full package of adaptations found in later hominins. All living apes can move bipedally, and some Miocene apes such as Oreopithecus may have specialized on terrestrial bipedality Kohler:Moya-Sola:1997. Recognizing the beginnings of the hominin adaptation to bipedality has been central to identifying early hominins, whose identity remains subject to debate Wood:Harrison:2011.

The first candidate fossils for early hominins share a suite of dental resemblances with later members of our lineage, including small canine teeth, low-crowned molar teeth and thick molar enamel. Some paleoanthropologists suggest that such dental traits are shared much more broadly with other Miocene lineages, and may not reflect hominin affinities sahelanthropus. Skeletal adaptations to upright posture and bipedal stance provide strong evidence that later fossils, after 4.2 million years ago, are human relatives. For earlier fossils, evidence of posture and stance is more equivocal. Sahelanthropus tchadensis is the earliest known, at around 7 million years ago in north-central Africa. Represented by a nearly complete skull and jaw, it shows an orthograde placement of the skull atop the spinal column Brunet:2002. Orrorin tugenensis, from western Kenya dating to 6 million years ago, also has femora consistent with bipedal weight-bearing Pickford:Orrorin:2002. Ardipithecus kadabba 5.5 million years old from Ethiopia, combines the dental features with a toe bone that suggests bipedality Haile-Selassie:2001.

Ardipithecus ramidus, dating to 4.4 million years ago from Ethiopia, comprises a large fossil sample including one nearly complete skeleton White:paleobiology:2009. From its limb proportions, grasping feet and apelike hands, Ardipithecus was a habitual quadruped. But several of its features are similar to hominins, including a shortened pelvis, a cranial base and spine indicative of orthograde posture, and toe and foot anatomy suggestive of bipedal weight-bearing. The teeth and jaws of Ar. ramidus, like those of earlier Ar. kadabba, are among its most hominin-like features. It may be the earliest well-documented member of our lineage, alternatively, it may be a stem member of the chimpanzee, gorilla or human-chimpanzee common ancestral lineage.

Australopithecines

From this diversity of apes arose the first species to show clear evidence of a commitment to terrestrial bipedal locomotion, Australopithecus anamensis. Between 4.2 and 3.9 million years ago, this species existed in East Africa Kanapoi:date. After this time, the same region was inhabited by Australopithecus afarensis, which is represented from more than a dozen fossil-bearing localities representing hundreds of known specimens up to 2.9 million years ago. The teeth of these two closely similar species show several temporal trends, toward a derived pattern of larger postcanine tooth size and functional change in the canine-premolar cutting anatomy. Because of these trends, most paleoanthropologists regard Au. anamensis and Au. afarensis as successive members of a single evolving lineage Kimbel:anamensis:2006.

During this time interval, other lineages of hominins may also have been present, including Kenyanthropus platyops from Kenya and Australopithecus bahrelghazali, from Chad, both between 3.3 and 3.5 million years ago Leakey:Maeve:Lomekwi:2001, Brunet:1995. These are possibly distinct from Au. afarensis because of cranial and dental peculiarities, but in each case the specimen is fragmentary. A partial foot skeleton from Woranso-Mille, Ethiopia, also may represent another lineage with a distinct locomotor strategy, possibly linked to earlier Ardipithecus Haile-Selassie:foot:2012.

Diversity is much clearer among the hominins near the Plio-Pleistocene boundary. After 2.8 up to 2.3 million years ago, South Africa was the home of Australopithecus africanus, also represented by large fossil samples and in most respects similar in cranial anatomy and teeth to Au. afarensis. By 2.5 million years ago, the robust australopithecines appeared in East and later in South Africa. “Robust” refers to the chewing mechanics of these hominins, which combined powerful jaw muscles with extraordinarily large molar and premolar teeth. They had approximately the same body size as other australopithecines, but clearly had a different dietary strategy. Australopithecus robustus was a South African form, following Au. africanus. Australopithecus boisei was the apex of this trend toward dietary specialization, which constitutes the majority of hominin fossils from East Africa between 2.5 and 1.5 million years ago.

Australopithecines share several aspects of their teeth with humans, but other traits were not shared with any living ape or humans. As a genus, the molar teeth were low-crowned and thick-enameled, the molars were large and premolars strongly converge on the size and form of the molars. Taken together with cranial and mandibular features reflecting powerful chewing, these teeth reflect a diet that imposed challenges substantially different from those facing recent chimpanzees, gorillas or humans. Based on stable isotope evidence and dental microwear, early hominins had diets broadly similar in composition to each other. They ate more grass-derived carbon than living apes or savanna-living primates like baboons, possibly including some meat, insects such as termites, and stems, seeds and underground corms Sponheimer:2005. An exception was Au. boisei, which relied heavily on grasses or sedges, possibly wetland resources such as the pith or underground parts of papyrus Cerling:boisei:2011.

The australopithecines were obligate bipeds, meaning that their skeletal adaptations to bipedality precluded effective quadrupedal movement. Their feet had a first toe aligned with the other toes, minimal opposibility or grasping ability, and arches similar to the feet of living people. Their knees were angled to promote effective weight support during bipedal stance, and did not rotate to grasp or climb with the feet. In contrast to ape pelves, humans and australopithecines have short hipbones that make a broad, bowl-shaped structure to support the viscera. A broader hip and shorter ischium enabled effective muscle control of the lower limbs during bipedal walking and running. Our bipedal form of locomotion is not as fast chimpanzee or gorilla knuckle-walking, but it is highly energetically efficient.

Australopithecine skeletons retained many characteristics related to arboreality. With female masses around 35 kg and males up to 50 kg, they approximated living chimpanzees in body size. The most complete skeletal individuals, such as the “Lucy” skeleton of Au. afarensis, had statures of 100 to 140 cm, much shorter than the average of any recent human population. Australopithecines had small brains, approximately 450 ml on average compared to 1350 ml in living humans. Au. anamensis is found in wooded paleoenvironments, as are the earlier, more apelike fossils that may represent our lineage White:anamensis:2006. Relatively long, heavily muscled arms, curved toes and finger bones, and a long clavicle and apelike shoulder blade all suggest that climbing remained important to Au. afarensis and Au. africanus, even as these hominins moved into more open grassland settings. Some individuals having flat feet DeSilva:Throckmorton:2011 or possibly retained some grasping ability of their toes Haile-Selassie:foot:2012, Clarke:STW573:1995. Some varieties, including Au. africanus, had arms relatively much longer than living humans Haeussler:McHenry:2007. Still, with hands and legs ill-suited for suspension or above-branch quadrupedal walking, early hominins must have climbed in a manner analogous to recent humans.

Early Homo

By 1.8 million years ago a very different kind of hominin had emerged. Homo erectus was the size and stature of recent human hunter-gatherer populations, bigger than any known australopithecine McHenry:2000. The skulls of Homo erectus also contained disproportionately larger brains than australopithecines, initially between 600 and 900 ml. As the relative size of the brain increased, the relative sizes of the teeth markedly declined. The earliest clear fossil evidence of Homo erectus occurs at Dmanisi in the Republic of Georgia and Modjokerto, Java, with additional fossil discoveries in East and South Africa prior to 1.6 million years ago Ferring:2011, Swisher:1994a. In each of these areas, remains of Homo erectus existed along with evidence for with stone tool manufacture and transport of stone. The evidence indicates that Homo erectus relied on a higher-quality diet including meat, which imposed greater demands on technical abilities and social organization, but created opportunities for dispersal and range expansion Anton:2002.

At present, identifying the population that gave rise to these early humans is one of the most engaging problems in the study of human evolution. Stone tools are known from several sites in Ethiopia and Kenya before 2.5 million years ago, and cutmarks on animal bone indicate that these earliest tools were often used for butchering animals. Between the earliest stone tools and the appearance of clear examples of Homo lie nearly 700,000 years of time, during which the ancestors of Homo erectus must have been emerging, but the identity of these ancestors remains mysterious. Between 1.9 and 1.5 million years ago, a number of fossil crania and a handful of partial skeletons may represent a species known as Homo habilis. The crania have larger brains than typical for australopithecines, ranging from 500 to 750 ml, and their teeth and jaws are smaller than earlier australopithecines. Still, only the earliest specimens of this sample could be ancestors of later Homo, and scholars disagree about how many species these early specimens represent. Australopithecus sediba is an exceptionally interesting sample, dating to 2 million years ago from Malapa, South Africa Pickering:Robyn:Malapa:2011. Two very complete skeletons of this species combine humanlike teeth and hands with the body proportions, brain size and possible arboreal adaptations of earlier hominins. Whether this species could be ancestral to Homo erectus or Homo habilis depends on the status of a handful of fossil fragments from East Africa that have in the past been assigned to our genus. Until more is known about the anatomy of these forms, it will be difficult to test hypotheses about their relationships.

The expansion of brain size from Australopithecus to Homo affected many aspects of life history and behavior. Neural tissue imposes a high metabolic cost, which humans met by adopting dietary and behavioral strategies that provide high caloric returns Aiello:Wells:2002. The first postnatal year of human brain development includes a rapid expansion of brain size and concomitant shape changes, in contrast to ontogenetic trajectories of other primates Zollikofer:PoncedeLeon:2010. Neural development in humans extends across a long childhood, with late sexual maturation and an adolescent growth spurt. These ontogenetic patterns appeared in concert with increasing brain size in Pleistocene humans. An increase in hunting and scavenging compared to other primates yielded a net increase in diet quality but imposed several risks, such as competition with large carnivores, unreliability of game, and long training necessary for skill development. Humans mitigate these risks by food sharing, sexual division of labor and gathering of plant foods and animal resources including honey. Hunter-gatherer social groups are relatively egalitarian, with decision-making regulated by a coalition of many group members. In this setting, learning of social rules and communication about social norms are fundamental determinants of survival and reproduction Boehm:1993. This social environment selected for larger brains with more sophisticated communication and inference about the intentions of other social actors Dunbar:social:2003. Whereas australopithecines had vocal tracts similar in form to chimpanzees and gorillas, early Homo had both vocal and auditory changes to support humanlike sound production and reception Martinez:hyoid:2008.

After their origin, early humans diversified into regional populations with some morphological differences. In East Asia, Homo erectus occupied a range from north China to Java, which during periods of low sea level was connected to the Asian mainland. Across this area, populations developed regional variation in the shape of the browridge and forehead, extent of muscle development of the jaw and neck, and shape of the teeth. Some of these people made a deepwater crossing to the island of Flores by 1 million years ago, where they may later have evolved into a late-surviving isolated dwarf population, called Homo floresiensis Brumm:Wolo:2010. In Africa, the fossil record is sparser but supports the idea that ancient humans increased in variability in the period after 1.2 million years ago. The West and South Asian archaeological records show that these regions were also occupied by early human populations, but preserve scant fossil remains. Europe was inhabited by 1.2 million years ago, but the skeletal record represents chiefly the last 800,000 years.

Everywhere they lived, humans used stone tools. The basics of production involved the procurement of stone raw material either from rocky outcrops or from the rounded cobbles in streambeds. People were selective about material, choosing fine-grained stone with predictable fracture dynamics, which they sought and transported over kilometers. Removing a sharp flake by itself yields a reliable cutting edge; removing several flakes from a rock, or “core”, can shape an edge suitable for chopping or piercing bone. This basic technological pattern, called “Oldowan”, was used by human societies from the beginnings of stone flaking. After 1.6 million years ago, mainly in Africa and later Europe and West Asia, people shaped core tools into symmetrical tools with long edges, called handaxes. The resulting Acheulean stone industry was remarkably persistent. Along with stone, archaeologists know that Pleistocene humans often used fire, wooden spears and other implements, and sometimes tools made of bone.

By 300,000 years ago, humans throughout the Old World had increased in brain size to a range between 800 and 1300 ml. Most paleoanthropologists refer to these remains to different species than Homo erectus. In Africa and Europe, they are often called Homo heidelbergensis, while many scientists call them “archaic Homo sapiens”. These people began to experiment with different technical forms, including a process of stone tool manufacture known as a “prepared core” technique. The result was a greater control over the shape of end products, sometimes yielding blades and points that were attached (hafted) onto spears as compound tools. These stone industries are called Middle Stone Age (MSA) traditions in Africa, Middle Paleolithic outside Africa.

Neandertals and the origin of modern humans

Genetic evidence has greatly clarified our understanding of the human populations of the last 250,000 years. Archaeology and skeletal remains help to complete the story, adding perspective on the causes and timing of the key events. This was a time of vast migrations and mixture of distant populations with each other.

By 250,000 years ago, MSA people had developed regional tool industries with little evidence of interregional movement or exchange McBrearty:2000. These populations gave rise to the present genetic diversity of living Africans, which exceeds that in other regions of the world Hammer:archaic:2011. A small skeletal sample represents these MSA populations from across Africa. These represent the earliest humans with modern anatomical characteristics including a high forehead, face tucked beneath the front of the braincase, and rounded cranial vault with its maximum breadth across the parietal bones. The functional import of these changes is not yet understood, but they seem to reflect a basic ontogenetic shift in infancy.

The later MSA, after 120,000 years ago remained regionally differentiated. In both Southern Africa and the Maghreb, people collected shells and marked objects, including natural pigments and ostrich eggshell Texier:Diepkloof:2010. In Mozambique, people gathered large stores of wild grains, in Ethiopia they transported obsidian over hundreds of kilometers Negash:2011.

Some African population emerged into West Asia, by 105,000 years ago, and encountered the Neandertals. Neandertals were the inhabitants of Europe and western Asia between 200,000 and 30,000 years ago. Beginning from a common anatomical background with modern humans, they evolved a number of traits that appeared nowhere else: long, barrel-shaped skulls with a rearward projection called the “occipital bun”, thick curving long bones with large joints, and “hyperarctic” body proportions in the European part of their range. Neandertals were a small population dispersed over a large space, and much more than contemporaries in Africa, depended much more on meat from large prey animals Bocherens:2005. The Neandertals were a minor component of the overall Pleistocene human population, but their skeletal and archaeological remains are numerous, so that we understand ancient lifeways more through their remains than those of other populations.

A sign of the weaknesses of the skeletal record is the Denisova genome, from the Altai Mountains of southern Siberia. This genome represents a population that lived at the same time as Neandertals at the eastern extreme of their range, but was substantially distinct from the known Neandertal genetic sample Reich:Denisova:2010. Living people in Australia and New Guinea derive around 5 percent of their ancestry from a population similar to this individual. Neandertals themselves contributed between 2 and 4 percent of the ancestry of present populations throughout the world, except within Africa itself Green:draft:2010. These genetic results may help to explain morphological features that reflect regional continuity of human populations in Europe, East Asia and Australasia. The spread of Africans within the last 100,000 years accounts for more than 90 percent of the ancestry of living people elsewhere, but a small multiregional component of ancestry has remained in the face of this and subsequent migrations.

Recent human evolution

After modern human populations became established throughout the world, evolution continued to shape our biology. With a genome derived more than 90 percent from Africa, the earliest migrants into Europe and northeast Asia found themselves poorly suited for the temperature excursions and lower insolation of these regions. The tropical regions of Asia had a similar physical geography but very different floral and faunal communities than Africa. Watercraft allowed people to colonize Australia, Melanesia, and other island regions, and facilitated the migration of people from the Bering land bridge into the southern parts of the Americas before 14,000 years ago.

As humans dispersed throughout the world, they also increased vastly in numbers. At the end of the last glaciation, people expanded their dietary breadth to a greater number of plant and animal species, a process called the Broad Spectrum Revolution. Some experimented with planting and keeping seed crops, others began to manage herd animals more intensively. Over many generations, both these processes led to domestication of former wild species, settlement of many human groups into villages and cities, and the rise of political and economic elites. Pastoralists sustained large populations on formerly less hospitable plains and steppes, sometimes migrating over long distances. Civilization was one result of this agricultural revolution; warfare and serfdom were others.

Human skeletal traits and genes changed during the last 20,000 years at a rate unmatched by earlier skeletal periods accel. Humans became more gracile as cranial muscle attachments and structures such as the browridge became lighter. After the introduction of agriculture, smaller teeth and jaws became common, and many people fail to develop third molars, or “wisdom teeth”, entirely Frayer:dental:1977. Along with such evolutionary changes, skeletal samples document the catastrophic health effects resulting from agriculture and village life.

Pathogens have been among the most obvious causes of recent human evolution. More than twenty different alleles that protect to some extent from falciparum malaria are known from different human populations Hedrick:2011, many of which have arisen within the last few thousand years. Many pathogens have exerted effects on other genes, both long-standing gene variants such as blood antigens and MHC variants, and new mutations. Diet is another important cause of recent evolutionary changes, as some human groups have specific genetic adaptations to starchy grains Perry:amylase:2007 and milk consumption Enattah:2008. The physical environment has exerted its own selection on populations at high altitude, with selection affecting oxygen transport Simonson:2010, and high latitude, with recent strong selection on genes associated with pigmentation Norton:2007.

Industrial populations of the last 200 years have undergone further radical changes in longevity, residence patterns and family size. Pre-industrial societies in Western Europe and North America were high fertility and mortality populations in which quantitative traits were changing under selection Courtiol:2012. Industrialization preceded a demographic transition to higher mean longevity and lower fertility, reducing the force of selection on traits associated with mortality. Nevertheless, the biology of modern populations continued to change, with documented selection on quantitative traits of medical and biometric interest Stearns:measuring:2010. The future direction of human evolution cannot be predicted from our past history. But the pace of recent evolution suggests that our species may have many more changes before us.