Human Evolution and Genetics
Introduction
Within the last seven million years, the human lineage originated from ancestral primates. Our ancestors evolved a unique pattern of innovations, such as upright walking, complex tool manufacture, a hunting and gathering lifestyle, and language. Each of these human characteristics built upon the anatomical and genetic legacy of primate ancestors, following a distinctive path to the present-day.
Genetic information about living people and other primates has added substantial detail to our knowledge of human evolution. The sequencing of the human genome
Humans among the primates
Comparisons of the genomes of living primates suggest that they last shared a common ancestor sometime during the Late Cretaceous period, around 80 million years ago (Ma)
Monkeys, apes and humans are grouped together as anthropoid primates. Early anthropoids appeared during the Late Eocene, ca. 40 Ma
Living and fossil apes and humans are called hominoids. Living apes, including chimpanzees, gorillas, orangutans and gibbons, are adapted to a flexible pattern of movement in the trees. Their shoulders and long arms enable climbing vertically and suspending beneath branches. While living apes can all walk or run bipedally, only humans have anatomical specializations to support efficient bipedal walking. Apes diverged from the cercopithecoids (Old World monkeys) around 30 Ma, but initially remained similar to monkeys in habit, body plan, and ecological breadth. Proconsul was an important fossil hominoid lineage in Africa from 24 to 15 Ma, with several species covering a range of from the size of monkeys like macaques up to chimpanzee-sized or larger
Toward the end of the Miocene, ape diversity declined. South Asian and European apes ultimately became extinct, coincident with climate changes that increased seasonal temperature and rainfall variations and reduced the area of forests
Genetic comparisons of living hominoids show that the common ancestors of chimpanzees, bonobos and humans lived between 4 Ma and 7 Ma
The first hominins
Humans and our fossil relatives, closer to us than to chimpanzees and bonobos, are called hominins. Sometime before 4.2 million years ago, hominins developed the unique form of upright, bipedal walking that humans share today. Paleontologists have recovered an array of fossil apes from before this time, some of which may be ancestors or relatives of hominins. Such apes either do not possess clear evidence of bipedality, or may not have shared the same pattern of bipedality as later hominins. However, they have smaller canine teeth than other living or fossil apes, and some skeletal signs of more vertical posture or time spent on the ground.
Three important samples of fossil hominoids may represent either the earliest members of the hominin lineage or their close fossil relatives: 7 million-year-old Sahelanthropus tchadensis in Chad
A much more complete fossil sample, including a partial skeleton, has been discovered of the 4.3-million-year old species, Ardipithecus ramidus
The australopithecines
Starting after 4.2 million years ago, early hominin fossils form a rich record that documents the diversification of these bipedal apes. The famous “Lucy” skeleton from Hadar, Ethiopia, represents Australopithecus afarensis. Hundreds of other fossil fragments from Ethiopia, Kenya, and Tanzania also belong to this species, which lived between 3.8 and 2.9 Ma. Later, an additional large sample of hominins from South Africa between 3.0 and 2.3 Ma represents Australopithecus africanus. This was the first of the early hominid species to be discovered, by South African anatomist Raymond Dart in 1924
These samples confirm the importance of bipedal locomotion to the early hominid lineage. The shape of the pelvis, knees and feet had evolved to a humanlike form that precluded efficient quadrupedal locomotion. Footprint trails at 3.5 Ma Laetoli, Tanzania also demonstrate their humanlike bipedality. Several pieces of evidence suggest that these australopithecines retained an adaptation to climbing; in particular, this may explain their short legs, small body sizes, powerful arm bones, and curved hand bones
Aside from bipedality, the other major anatomical pattern of early hominids involved the teeth. Australopithecines had large molar and premolar teeth compared to living and fossil apes and humans. These teeth were low-crowned and had thick enamel, apparently adapted to a diet of grinding hard foods such as seeds. Isotopic evidence suggests that their diet was varied, with the main difference from other primates being a high consumption of plants with a C4 photosynthetic cycle, which include grasses and some sedges
A later group of australopithecines greatly emphasized the adaptation to large grinding teeth. These “robust” australopithecines had molar and premolar teeth with as much as four times the area of present-day humans, together with immense jawbones and jaw muscles. Their diet was also varied. presumably included a higher fraction of hard brittle foods, which may have been increasingly important during the drier climates of the Late Pliocene
The first humans
Alongside the robust australopithecines lived the earliest members of our own genus, Homo. Early Homo can be distinguished from contemporary australopithecines by its smaller molar teeth (although still larger than living people) and larger brain size. The transition to large brains and smaller teeth was accompanied by an increased dietary reliance on meat. Because of its high caloric and protein content, meat is more easily digestible and can fuel more substantial brain growth
The archaeological record provides further evidence for a dietary shift, with the earliest-known stone tools occurring in Ethiopia at 2.6 Ma
Before 2 Ma, both fossil and archaeological evidence of Homo remain rare. The first sample of hominin skeletal remains to show clear evidence of the ability to make and use tools is from Malapa, South Africa at 2 Ma
After 1.9 Ma, a species called Homo habilis lived in East Africa and South Africa. With endocranial volumes in the range of 500 to 750 ml, this species represents a clear and substantial increase over other early hominins. The species apparently shared the manipulatory abilities of A. sediba, and details of the the internal surface of the skull suggest similarities with humans
A second species of early Homo, called Homo erectus had larger bodies and especially taller stature. an average of 1.4-1.8 m compared to earlier hominids at 1.0-1.4 m
The use of more open territory and larger home ranges may have enabled Homo erectus to colonize Eurasia for the first time
Pleistocene human evolution
Early in their existence, the populations of Homo erectus in Africa and East Asia started to become different from each other. The form of the cranium, thickness and shape of the browridge, size of neck muscle attachments, and other details differed substantially between regions, though they overlapped. Some researchers view these features as evidence that Homo was divided into different species in different parts of the world. Others consider these morphological differences to be of a similar level to features distinguishing human populations today
After a million years ago, fossil specimens in Africa and Europe represent a change from the characteristics of earlier Homo erectus. The African and early European remains are often referred as “archaic” members of our own species, Homo sapiens, or by another species name, Homo heidelbergensis. It is not clear whether the anatomical evolution was accompanied by biological speciation, or whether it represents an increase in brain size and consequent changes in cranial morphology within a single evolving species. In either case, early European hominids also had morphological features distinguishing them from other regions, including a projecting face and nose, and large sinuses
Humans evolved larger brain sizes across the Pleistocene. The earliest Homo erectus specimens had endocranial volumes averaging around 750 ml; these increased to an average of 1400 ml by 50,000 years ago
The archaeological record provides additional evidence about cognitive evolution. Stone tools became more sophisticated gradually over the Pleistocene. First, the development of bifacially flaked handaxes and cleavers in the Acheulean industry shows that hominids could learn and replicate standardized, symmetrical forms by 1.5 Ma
It is likely that the behavioral complexity after 300,000 years ago required some capacity for spoken language. Language is a fundamental basis for human culture and behavior, because people learn and coordinate their activities by talking to each other. But there is very little anatomical evidence relating to the evolution of language, as the necessary structures — tongue, larynx, and brain — do not fossilize. Still, a few hints exist. At least one \emph{Homo habilis</bib> skull includes a marked enlargement of Broca’s area in the frontal cortex, a brain structure important to planning complex activities and carrying out speech in living humans
Neanderthals, Denisovans and modern humans
The most well-known group of ancient humans are the Neanderthals, inhabitants of Europe and parts of West Asia between 200,000 and 30,000 years ago. The Neanderthals were large game hunting specialists, with sites dominated by the bones of bison, horse, and red deer. Isotopic evidence suggests that their diet included a very high proportion of meat
Recently, genetic evidence directly from Neanderthal skeletal remains has been recovered. Initially, comparisons of the mitochondrial DNA of living people and Neanderthals suggested that the Neanderthals did not contribute to the ancestry of living people
A skeletal finger bone from Denisova Cave, Russia, has preserved a whole genome of an individual from a different population from the Neandertals
African populations of the Middle Pleistocene (780,000-125,000 years ago) were highly diverse in both anatomy and material culture. Africa has a larger habitable land area than other continents, for hunter-gatherers operating with Paleolithic technological strategies
From this diverse population emerged most of the genetic ancestors of present-day humans. Sometime after 150,000 years ago, Africans began to move into West Asia where they could interact with the Neanderthals. This interaction resulted in the disappearance of Neanderthal, Denisovan, and other archaic human populations. These archaic people were in part assimilated into the succeeding population of modern humans, evident from DNA. The spreading population of modern humans became associated with more advanced technology, larger social groups, more rapid colonization and population growth. Late Neanderthals exhibited some of the technological and social innovations of early modern humans, so that the reason for the success of modern humans remains enigmatic.
Continuing human evolution
Humans have increasingly used culture and technology to adapt to our environments. In some ways, our inventions buffer us from natural hazards such as predators, climate, or the risks of foraging activity. Some anthropologists formerly argued that culture has reduced the force of natural selection upon the human population. A common misconception is that humans have not evolved physically or genetically since the origin of modern humans. The post-Pleistocene evolution of our species has in fact been very rapid
After 100,000 years ago, human dispersals and population growth introduced large populations into environments as varied as the Arctic, the Americas, and Australia. Within the last 10,000 years, our interactions with the environment have further been changed by the introduction of agriculture, domestication of animals, life in villages and cities, and other technical and social innovations. Human populations have diversified during this period by founder effects, establishing differences in the frequencies of genes in different regions of the world.
The rapid recent evolution of our species provides an important context for interpreting genetic and physical differences among human populations. For example, human skin and hair pigmentation vary greatly among populations today, a pattern influenced by an array of genetic changes
Even though most humans live today in large industrialized societies, our evolution continues. Long-term studies of the health of individuals within the United States and Europe show that the chances of having children are different depending on several physical and physiological traits, constituting the pattern of natural selection in living people