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The hominids are included in the superfamily Hominoidea, which groups together humans, great apes (chimpanzees, bonobos, gorillas, and orangutans), lesser apes (gibbons and siamangs), and all the fossil relatives (e.g., Oreopithecus, Sivapithecus, Dryopithecus, and Australopithecus). Formerly, the hominoids were divided into hominids (humans) and pongids (great and lesser apes), the last ones of which were later subdivided in pongidae (chimpanzees, bonobos, gorillas, and orangutans) and hylobatidae (gibbons and siamangs). For this reason, the term “hominid” has been traditionally restricted to humans and their extinct relatives (i.e., those fossil taxa belonging to the human phylogenetic lineage). This definition is commonly used even by many anthropologists. However, taxa strictly grouping in hominids are not only humans, but also chimpanzees and gorillas, which form the family Hominidae. The original meaning of the hominid term referred only to the modern meaning of tribe Hominini (i.e., Homo sapiens and other extinct species closely related to humans). In spite of the fact that the terms hominin, hominid, and hominoid are different, Hominini has been considered preferable to describe in this research paper the species of our own evolutionary lineage (tribe Hominina). Fossil (and modern) monkeys and apes (including chimpanzees, bonobos, gorillas, and orangutans) are described in the section “Fossil Primates.”
Description of Modern Humans: Homo sapiens sapiens
The only living species of the family Hominini is Homo sapiens sapiens. Some taxonomists also include in this group the chimpanzees (Pan), subdividing them into two subgroups (Panina and Hominina). Nevertheless, others consider that chimpanzees and humans must be separated into two groups (Panini and Hominini, respectively), and this is the classification used here. Our species, which was defined as Homo sapiens by Carolus Linnaeus in 1758, is characterized by a highly developed brain and capable of complex language, abstract reasoning, and full consciousness of its thoughts, sensations, perceptions, emotions, and self-awareness. Its intelligence allows it to explain and manipulate natural phenomena through philosophy, science, art, and religion. Members of this species are capable of building fires, cooking their food, clothing themselves, and developing numerous complex technologies, including space exploration.
Homo sapiens belongs to the genus Homo, tribe Hominini, subfamily Homininae, family Hominidae, superfamily Hominoidea, infraorder Catarrhini, group anthropoidea or simiiformes, suborder Haplorhini, and order primates. Physically humans tend to be weaker than other similarly sized apes, as gorillas or chimpanzees. Just like apes, H. sapiens has clear sexual dimorphism. The average height in adult humans is 1.40 to 1.80 m in males and 1.30 to 1.70 m in females, and the average weight is 60 to 80 kg in males and 45 to 65 kg in females, although it varies significantly from place to place. H. sapiens has a rounded and large neurocranium, a small and vertical face, a prominent vertical forehead, and an orthognatic face (straight jaw, vertical and flat face), although with slightly protruding jaws. The foramen magnum is placed directly underneath the skull, and brain capacity is 1,250 to 1,850 cm3 with 1,350 cm3 the average. Humans have proportionately shorter palates and much smaller teeth than other hominids and are the only extant primates to have short canine teeth. The dorsal spine is curved (S-shaped), and the foot bones have lost their mobility. Although humans are relatively hairless compared with other hominids, they have notable hair on the top of the head, underarms, and pubic area. The color of their skin and hair is determined by the presence of pigments called melanin, with skin varying from very dark brown to very pale pink, and hair varying from preponderantly black to blond or red.
Genetically, humans are characterized by 23 pairs of chromosomes. Their gestation period is 9 months, and their life span is approximately 84 years for females and 78 years for males in the developed world. The individuals mature sexually at 12 to 15 years, developing physically until 18 years in females and 21 years in males. They are infants from 0 to 3 years, children from 3 to 7 years, juveniles from 7 to 12 years, and adolescents from 12 to 18 years. Humans are omnivorous, capable of consuming both animal meat and vegetation. Their technology has allowed them to colonize all the ecosystems and adapt to all climates, including hostile environments such as Antarctica and outer space. Unlike other apes, humans are capable of fully bipedal locomotion, moving by means of their two posterior limbs and leaving their arms for manipulating objects through their hands. They have opposable thumbs, a character shared with all hominids.
From Linnaeus on, H. sapiens has been traditionally subdivided in terms of races or ethnic groups based on visible traits (especially height, skin color, cranial or facial features, and hair texture). Formerly, some dared to subdivide H. sapiens into subspecies, such as H. sapiens afer (for negroids as paleocongid, sudanid, nilotid [nilotes], and bantid [bantus] races), H. sapiens europaeus (for caucasoids as nordic, mediterranean, alpine, baltic, ladogan, dinaric, armenid, arabid, turanid, dravidic, and iranoafgan races), H. sapiens asiaticus (for mongoloids as nordsiberian, uralian [mongols], oriental, Indonesian, tungid [inuits], ainuid, and Amerindian races), and H. sapiens australasicus (for australoids as australiantasmanian, melanesian, veddoid, and negrito races), and also H. sapiens khoisanii (for capoids as khoid and sanid races [khoikhoi and khoisans]), H. sapiens pygmaeus (for the bambutoid race [pygmies]), H. sapiens aethiopicus (for the aethiopid race), and H. sapiens americanus (for Amerindians). Nevertheless, for scientific as well as social and political reasons, the conception of human race is often controversial. Recent genetic evidence has definitively questioned this taxonomic categorization. The total human genetic variations are approximately 0.5%, of which 85% can be found within any given population, 7% among populations within a race, and only 8% among various races. Human genetic sequences are therefore remarkably homogeneous compared with other species. Much of the genetic variation is found in the regions of the genome affected by the environment, mainly in genes affecting physical appearance, such as skin color. Today, most scientists who study human genotypic and phenotypic variations use concepts such as populations or clinal gradation. The race definitions are imprecise and arbitrary, and generally derived from customs, resulting in many exceptions and much gradation. Therefore, the gradual changes of phenotype in humans over a geographical area only allow us to subdivide H. sapiens into gene clines and populations, not into subspecies or races, as is possible in other species.
Description of Fossil Hominids
Through DNA comparison, geneticists consider that humans (Homo, tribe Hominini) and chimpanzees (Pan, tribe Panini) diverged in evolution 5.5 to 6.5 million years ago. It is noticeable that while very few fossil species related to chimpanzees have been identified, a significant number of fossil species of the human evolutionary lineage have been recognized. This is likely because organic materials fossilize better in savannah-type environments (where our ancestors preferably lived) than in forest environments (where the ancestors of the great apes preferably lived). According to known evidence to date, the tribe Hominini includes seven genera, six of which are fossil (Sahelanthropus, Orrorin, Ardipithecus, Australopithecus, Paranthropus, and Kenyanthropus) and the seventh of which (Homo) has only one living species, ours. For a better and more ordinate description, they have been grouped into primitive hominin fossil species (Sahelanthropus, Orrorin, and Ardipithecus), australopithecines (Australopithecus and relatives), ancestral habilis humans (Homo habilis and relatives), erectine humans (Homo erectus and relatives), archaic sapiens humans (Homo heidelbergensis and relatives), and modern fossil Homo sapiens.
Primitive Fossil Species
Three fossil genera have been included in this group and represent our most primitive relatives: Sahelanthropus, Orrorin, and Ardipithecus. Although the first two (Sahelanthropus and Orrorin) have been provisionally assigned to our Hominini evolutionary lineage, they may be ancestors of chimpanzees as well as humans. They therefore belong to the basal group of both tribes Panini and Hominini, and perhaps should be taxonomically separated in other different tribes. Very few fossil remains of these taxa have been found, but they have allowed paleoanthropologists to partially cover the information gap in the hominid fossil record between 5 and 10 million years ago.
The genus Sahelanthropus includes only one known species discovered in Chad (central Africa): Sahelanthropus tchadensis, so the genus characteristics have been analyzed based on the fossil remains of the species: a nearly complete cranium and a number of fragmentary lower jaws and teeth. It is the oldest discovered hominin, dated at between 7.4 and 6 million years old (late Miocene). Its brain capacity is only 340 to 370 cm3 in volume and similar to that of modern chimpanzees. It has an elongated skull, far-set eyes, a supraorbital ridge, and a vertical face. Although it is yet unknown whether S. tchadensis was bipedal, some paleoanthropologists suggest this due to the fossils of its foramen magnum (a hole at the base of the skull over the spinal column). Sahelanthropus could represent a common ancestor of humans and chimpanzees, since most molecularclock analyses indicate both species (Homo and Pan) diverged 5.5 to 6.5 million years ago (1 to 2 million years after S. tchadensis). Other possibilities are that S. tchadensis was the ancestor of chimpanzees or gorillas—or simply related to humans, chimpanzees, and gorillas, but the ancestor of neither. In any case, this taxon is very related to the common ancestor of all hominids, and it is therefore a very interesting fossil. Even if S. tchadensis was a protogorilla or a protochimpanzee, this taxon would not lose its significance, since few chimpanzee or gorilla ancestors have been found in Africa.
The genus Orrorin also includes an only-known species discovered in Kenya (central East Africa): Orrorin tugenensis. The genus characteristics have been analyzed with the fragmentary fossils (arm and thigh bones, lower jaws and teeth) of that species. O. tugenensis is considered the second-oldest known hominid ancestor, and it could be related to the ancestral-human lineage or even be a direct human ancestor. It has been dated between 5.6 and 6.2 million years old (late Miocene), and its discovery is important because it could be an early bipedal hominin. The characteristics of its femur and humerus suggest that O. tugenensis was skilled at tree climbing but not at brachiation, and could have been able to walk upright bipedally. Its size was similar to the modern chimpanzee, at about 1.40 m tall. Its teeth indicate that its diet was mostly fruits and vegetables, with occasional meat. Orrorin lived in the dry evergreen forest environment of Africa, not on the savannah, as many hypotheses on human evolution had assumed. This fact could indicate that the origin of bipedalism occurred in an arboreal precursor living in a forest and not a quadrupedal ancestor living in an open savannah. Our oldest ancestors, such as O. tugenensis, could have been able to move bipedally over branches with the vertebral column oriented vertically, such as orangutans do today, using their arms for balance and keeping their legs straight. This adaptation moved them away from our closer extant relatives (chimpanzees and gorillas), which are more adapted to tree climbing and to quadrupedal locomotion while on the ground.
The genus Ardipithecus is a hominine genus that lived during the Miocene-Pliocene transition in central East Africa (Ethiopia), between approximately 3.8 and 6 million years ago. Most paleoanthropologists consider it an ancestor of Australopithecus because of a likeness in teeth. Its species was the size of modern chimpanzees, and the structure of its toes and the position of the foramen magnum suggest that it walked upright bipedally. Since it lived in shady forests and not on savannahs, the hypothesis on the origin of bipedalism in an arboreal precursor acquires new evidential support. Two species of Ardipithecus are known: Ardipithecus ramidus and Ardipithecus kadabba, although both were initially described as a subspecies of A. ramidus.
Ardipithecus kadabba is the older of the two Ardipithecus species and lived between 5.2 and 5.8 million years ago, during the Miocene-Pliocene transition. Its canine teeth show primitive features shared with Sahelanthropus and Orrorin, distinguishing them from more recent human lineages. Some paleoanthropologists argue that the presence of a canine cutting complex in this species, which is present in modern chimpanzees but not in A. ramidus and recent humans, indicates a need for relocation in hominid-evolutionary history, and it is reasonable to infer that this species was a primitive hominine lineage which was lost but very close to the most recent common ancestor of Homo and Pan.
On the contrary, the other species, Ardipithecus ramidus, is clearly related to the human lineage. It was initially dated between 3 and 4.4 million years old, but it could have lived as far back as 5.8 million years. Its teeth are intermediate between Ardipithecus kadabba and Australopithecus afarensis. Members of this species were bipedal and forest dwellers, and about 1.15 to 1.20 m tall and 25 to 30 kg in weight.
The Australopithecines together form those extinct hominines that have been sometimes included in the genus Australopithecus. Today, the australopithecines are subdivided to three genera: Australopithecus, Paranthropus, and Kenyanthropus. Some taxonomists consider that Australopithecus and Paranthropus belong to a single genus, Australopithecus, although most prefer to distinguish between both australopithecine groups. Some paleoanthropologists have defined a new taxon to group them: tribe Australopithecini, separating them from the tribe Hominini that would include only Homo, and perhaps Kenyanthropus.
The genus Australopithecus, first described by Raymond A. Dart in 1925, is made up of the gracile australopithecines, which shared several traits with the modern great apes and humans, although it is closely related to the genus Homo. They lived in eastern and northern Africa between 4.2 and 2 million years ago. They are considered the ancestors of the genus Homo, and include, for the first time, a clear bipedal hominine. The species of Australopithecus displays a remarkable sexual dimorphism, being that males are larger than females. The sexual dimorphism in the lumbar spine, seen for the first time in Australopithecus, has been considered an evolutionary adaptation of females to better bear a lumbar load during pregnancy, something that was not necessary in nonbipedal primates. Australopithecus has five known fossil species: Australopithecus anamensis, Australopithecus afarensis, Australopithecus africanus, Australopithecus bahrelghazali, and Australopithecus garhi.
Australopithecus anamensis lived during the early Pliocene between approximately 3.9 and 4.2 million years ago in eastern Africa (around Kenya and Ethiopia). Its fossils (including cranial fragments, upper and lower jaws, teeth, and parts of arm and leg bones) still resemble those of common chimpanzees (mainly the jaws), but their teeth are much more similar to those of humans. It is believed that A. anamensis was primarily a tree-climbing species and arboreal dweller—a behavior retained from the early hominine. Nevertheless, it is known that members of this species walked upright bipedally, and probable that lived in a more open environment. Its dentition indicates that its diet was exclusively herbivorous and frugivorous. This species shares many traits (teeth and jaws) with Ardipithecus ramidus, which may be its direct ancestor, but the arm and leg bones are already more similar to those of humans.
Australopithecus afarensis is a gracile australopithecine that lived between 2.9 and 4.2 million years ago (late Pliocene) in eastern Africa (around Kenya and Ethiopia). It had a considerable sexual dimorphism in size, being that adult males were 1.45 to 1.55 m in height and 45 to 70 kg in weight, but adult females were 1.05 to 1.15m in height and 25 to 40 kg in weight. A. afarensis had a brain capacity of 380 to 540 cm3 (470 cm3 on average), and its face was prognathic (the mandible protruded farther than the maxilla). Its canines and molars were still relatively lager than those of modern humans, but they were very reduced when compared with the common chimpanzees and other apes. Its postcranial skeleton (mainly its pelvis) strongly indicates that it was bipedal, although it was probably partly arboreal and tree climbing. Some consider that A. afarensis was almost exclusively bipedal, since its feet features made it difficult if not impossible to grasp branches with the hind limbs. Footprints associated with A. afarensis showing bipedal locomotion have been found at Laetoli in Tanzania. However, its diet was frugivorous, so its arboreal activity was likely great. Its capacity to transmit sounds would not be greater than that of the current chimpanzees. It is considered to be an ancestor of Australopithecus africanus and the genus Paranthropus, and its own more direct ancestor is A. anamensis.
A species closely related to A.afarensis is Australopithecus bahrelghazali, an enigmatic fossil since it is the only gracile australopithecine found in central Africa (concretely in Chad). The only found fossil (mandible fragment and teeth) belongs to an individual dated at 3.6 million years old, but it is probable that the species lived between 3 and 4 million years ago in the late Pliocene.
Australopithecus africanus is the most famous gracile australopithecine. It lived between 2.5 and 3.3 million years ago in southern and eastern Africa. A. africanus shares many traits with the older A. afarensis, but its fossils indicate that it was significantly more like modern humans. It was sexually dimorphic, being that adult males were 1.35 to 1.45 m in height and 40 to 60 kg in weight, and adult females were 1.1 to 1.2 m in height and 25 to 30 kg in weight. Its skull was higher and more rounded and the face was less prognathic (flat and vertical) and shorter than its ancestors. Its teeth were also smaller, with smaller canines than those in other hominoids such as chimpanzees. The brain capacity of A. africanus was 410 to 520 cm3 (455 cm3 on average). Its cranium resembled that of the chimpanzee, but it shared human similarities in its teeth, eye orbits, and most importantly, foramen magnum, which indicates a humanlike posture. The discovery of A. africanus allowed paleoanthropologists to show that a large cranial capacity had succeeded bipedal locomotion in the human evolutionary timeline. It was considered to be a direct ancestor of Homo, although others believe A. africanus did not have descendants or evolved into Paranthropus instead of into Homo. Stone tools have not been found associated with A. africanus, which indicates that its intelligence was still very limited.
Australopithecus garhi is a gracile australopithecine whose fossils were found in Ethiopia in rocks of 2.5 million years ago. It probably lived between 2 and 3 million years ago. Its cranium and face were very similar to A. afarensis and A. africanus, although its molar and premolar teeth had some similarities with those of Paranthropus, since they are larger than those of the other gracile australopithecines. The brain capacity of A. garhi was about 450 cm3 on average, similar to other australopithecines. It was considered to be the missing link between the genus Australopithecus and the genus Homo, although it is believed that it was only a competitor species to the true ancestral species of Homo and therefore not a direct human ancestor. However, 2.5- or 2.6-million-year-old stone artifacts (closely resembling Oldowan technology) were discovered together with A. garhi fossils, indicating the first tools are older than the appearance of the genus Homo, which was not previously believed. If this is confirmed, then A. gahri (and Austrolapithecus) was the first toolmaker, relegating H. habilis (and Homo) to second place.
The genus Paranthropus (Broom, 1938) groups the robust australopithecines that lived between 1.1 and 2.7 million years ago, and it includes three known species: Paranthropus aethiopicus, Paranthropus boisei, and Paranthropus robustus. They were bipedal hominine that descended from gracile Austrolapithecus. They were very well-muscled australopithecines, with a more massively craniodental build, and tended to have a gorilla-like sagittal crest on the cranium which anchored massive temporalis muscles of mastication. Paranthropus coexisted and shared a habitat with some Homo species, such as H. habilis, H. ergaster, and even H. erectus. Its brain capacity was larger than that of the gracile australopithecines, and stone tools have been found associated with Paranthropus fossils. However, there is considerable debate whether or not those tools were made by them or by contemporaneous Homo, so the true Paranthropus intelligence is still under debate. They represent a hominine evolutionary branch distinctly diverging from the human lineage.
Paranthropus aethiopicus was a robust australopithecine that lived in eastern Africa (Kenya) between 2.2 and 2.7 million years ago (transition between Pliocene and Pleistocene). It is therefore the earliest example of Paranthropus and shares many primitive traits with A. afarensis. For this reason, P. aethiopicus is considered to be a direct descendant of A. afarensis and the ancestor of the other robust australopithecines. It had a prognathic face (but flatter than other hominins) and a large sagittal crest. The cranial capacity of P. aethiopicus was approximately 410 cm3 on average. This species lived in a mixed savannah and woodland environment, probably in a more arid habitat than gracile australopithecines, and had a strictly herbivorous diet.
Paranthropus boisei (originally called Zinjanthropus boisei) is the most famous and largest robust australopithecine. They lived in eastern Africa (Tanzania, Kenya, Ethiopia, and eastern Congo) between 1.2 and 2.6 million years ago (early Pleistocene). Like the other australopithecines, they exhibited great sexual dimorphism, with adult males being 1.35 to 1.45 m in height and 55 to 80 kg in weight, and adult females being 1.05 to 1.15 m in height and 35 to 45 kg in weight. The skull was prognathic, with a vertical, long, and broad face; a robust mandible; and a pronounced sagittal crest. Curiously, the shape of their foramen magnum is more similar to Homo than that of the other australopithecines. The brain capacity of P. boisei was 500 to 550 cm3 (520 cm3 on average). Their cranial characteristics are indicative of a herbivorous diet consisting of hard or tough foods (tubers, nuts, and seeds).
Paranthropus robustus is the other well-known robust australopithecine that lived in southern Africa (South Africa) between 1.2 and 2 million years ago (early Pleistocene). Adult males were 1.2 to 1.3 m in height and 45 to 70 kg in weight, and adult females were 1 to 1.10 m in height and 35 to 45 kg in weight, indicating major sexual dimorphism. Its cranium was characterized by a heavy chewing apparatus (pronounced sagittal crest, and large jaws and jaw muscles), and teeth (molars extremely robust, and molarized premolars) that were well adapted to serve in the dry savannah environment. The cranial capacity of P. robustus was approximately 450 to 530 cm3 (500 cm3 on average).
Finally, the australopithecines also include a genus called Kenyanthropus, of which only one species is known: Kenyanthropus platyops. It lived in eastern Africa (Kenya) between 3.2 and 3.5 million years ago, although some paleoanthropologists consider that it might have lived up to 2 million years ago. K. platyops had a small brain capacity, but it also had high cheekbones and a flat face that relates it closely with the human lineage. This species has been considered to be the missing link between Australopithecus and Homo, since it has intermediate characteristics between the typical gracile australopithecines and typical humans. Concretely, its traits are intermediate between Australopithecus afarensis and Homo rudolfensis, relegating Australopithecus africanus to a second place in human-evolution history. However, this fossil is very problematic, since it is so distorted by matrix-filled cracks that meaningful morphologic traits are next to impossible to assess with confidence.
Ancestral Humans: Homo habilis
Paleoanthropologists have always considered that the appearance of Homo coincides with the first evidence of stone tools (Oldowan industry) at the beginning of the Lower Palaeolithic, 2.5 million years ago. However, it is not clear that ancestral humans invented these first tools since this lithic industry has been associated with more primitive australopithecines (Australopithecus garhi). Three ancestral human species have been found: Homo habilis, Homo rudolfensis, and Homo georgicus.
Homo habilis is a primitive human that lived in eastern and southern Africa (Tanzania, Kenya, Ethiopia, eastern Congo, and South Africa) between 1.6 and 2.2 million years ago (early Pleistocene). H. habilis was short, with adult males being 1.15 to 1.25 m in height and 35 to 50 kg in weight, and adult females being 0.95 to 1.05 m in height and 30 to 35 kg in weight. It had long arms compared with modern humans, but it also had a clear reduction in the prognathism of its face (which was broad and vertical), suggesting its close relationship to modern humans. Its foot bones show less mobility than its predecessors. Its cranium was more rounded with wide-set eyes, and its foramen magnum was placed in a more central position than in the other australopithecines. The cranial capacity of H.habilis was 590 to 680 cm3 (650 cm3 on average). H. habilis fossils are often accompanied by primitive Oldowan stone tools. The intelligence and social organization of H. habilis was probably more sophisticated than in the other australopithecines or chimpanzees. For a long time, it was considered the first species of Homo to appear, descending from a species of australopithecine (perhaps from A. africanus or from another unknown species related to A. afarensis). However, this honor could be snatched away by H. rudolfensis, not so much by age as by taxonomic interpretation. It is also believed to be the ancestor of the more sophisticated Homo ergaster, but there is strong debate over whether or not H. habilis is a direct human ancestor. Its small size, likeness to australopithecines in body morphology, and primitive attributes (e.g., narrow birth canal and legs longer than arms) have led some paleoanthropologists to propose excluding habilis from the genus Homo and, instead, including it in the genus Australopithecus, naming Australopithecus habilis.
Homo rudolfensis is an ancestral human, originally considered to be a member of H. habilis, which lived in eastern Africa (Kenya and Malawi) between 1.9 and 2.4 million years ago (late Pleistocene). It has remarkable differences with respect to H. habilis, with a flatter face (vertical, long, and broad), wider molar and premolar teeth, and more complex roots and crowns. Adult males were probably 1.30 to 1.40 m in height and 40 to 50 kg in weight, and adult females were 1.10 to 1.20 m in height and 30 to 40 kg in weight. The cranial capacity of H. rudolfensis was probably 550 to 750 cm3 (650 cm3 on average), and, like H. habilis, its fossils are associated with primitive Lower Palaeolithic stone tools. According to its fossil record, it may be the oldest species of the genus Homo. Nevertheless, as in the case of H. habilis, there is debate over whether or not H. rudolfensis belongs to the genus Homo (some paleoanthropologists include this species in the genus Kenyanthropus), and over which one of these is the more probable ancestor of the later human species, or if it was some third species yet to be discovered. It has been suggested that H. rudolfensis evolved from Kenyanthropus platyops, while H. habilis seems to have evolved from Australopithecus garhi, raising debate on the real taxonomic position of both species and which is the evolutionary lineage that ended in the modern human.
Homo georgicus is an ancestral human species that lived in the Caucasus region (Georgia) 1.8 million years ago, according to the fossils known to date. Initially it was considered to belong to Homo ergaster, but size and morphological differences led paleoanthropologists to consider defining a new species, apparently more primitive than this one. It is the most primitive hominine species discovered outside of Africa. This species presents a strong sexual dimorphism. It is thought that adult males were 1.40 to 1.50 m in height and 45 to 55 kg in weight, and adult females 1.10 to 1.20 m in height and 30 to 40 kg in weight. Its skull was similar to Homo ergaster, but with a smaller face and more prognathic. The cranial capacity of H. georgicus was 600 to 680 cm3 (650 cm3 on average). They probably were capable of making tools, and some specialists have proposed that the Acheulean industry existing 1.4 to 1.6 million years ago in Israel is theirs. They ate animal meat, which was important for their survival during the winter season. H. georgicus has been considered a human group descended from H. habilis, which probably evolved in Africa and was capable of emigrating to Eurasia. This species may be the ancestor of two later species: African Homo ergaster and Asian Homo erectus. Nevertheless, its taxonomic position is still debatable.
Ancestral Humans: Homo erectus
Many paleoanthropologists include the diverse species considered ancestral humans (i.e., the human type intermediate between ancestral habilis humans and modern sapiens humans) as belonging to a single species: Homo erectus. However, most prefer to distinguish at least two species: the African Homo ergaster and the Asian Homo erectus. This discussion concerning the taxonomy of this human group is still open, depending on whether H. erectus is considered a geographically widespread species found in Africa, Asia, and even Europe, or considered an exclusively Asian lineage that evolved from the less cranially derived African H. ergaster. In addition, two other very derived species have been proposed: Homo soloensis and Homo floresiensis, and they deserve special mention.
Homo erectus and its relatives were the first early human species able to constitute hunter-gatherer societies, procuring plants and hunting animals from the wild, without significant recourse to the domestication of either. They used more diverse and sophisticated stone tools than their predecessors, first using the Oldowan industry and later the Acheulean industry from 1.65 million years ago. More important, they were the first humans to make creative use of fire. Moreover, they may have communicated with a protolanguage much more developed than the basic communications used by chimpanzees, but without the developed structure of the modern human language. Their brain capacity generally coincides with the more sophisticated tools found together with its fossils and with the type of societies that they formed.
Homo ergaster is considered the African Homo erectus, but its morphological characteristics suggest that it be considered a new species. It lived in eastern and southern Africa (Ethiopia, Kenya, Tanzania, and South Africa) between 1.4 and 1.9 million years ago (early Pleistocene). It had a more reduced sexual dimorphism than the previous hominine. Adult males were approximately 1.8 to 1.9 m in height and 75 to 90 kg in weight, and adult females were 1.7 to 1.8 m in height and 60 to 70 kg in weight. The body proportions (longer legs and shorter arms) of H. ergaster were, for the first time, similar to those of modern humans. It had a smaller and more orthognatic face, a more rounded cranium, and a smaller dental arch. The cranial capacity of H. ergaster was 700 to 850 cm3 (800 cm3 on average). They made various tool types, such as hand axes and cleavers, belonging to the Acheulean industry 1.6 million years ago. Moreover, evidence (such as charred animal bones, fossils, and traces of camps) suggests that they made use of fire. It is believed that H. ergaster is a descendant of ancestral humans, such as H. habilis or H. rudolfensis.
Lately, Homo erectus has been considered an exclusively Asian species that lived between 0.2 and 1.8 million years ago (late and middle Pleistocene). Since it is the most successful and long-lived species of the Homo genus, it is generally thought to have a considerable number of subspecies. Although physical characteristics must therefore be described for each subspecies, H. erectus was characterized (on average) by a sexual dimorphism slightly greater than that in modern humans, adult males being approximately 1.60 to 1.70 m in height and 60 to 70 kg in weight, and adult females being 1.50 to 1.60 m in height and 45 to 55 kg in weight. Its face was almost orthognatic, with minimal jaw projection. It also had a long and low skull with a pronounced supraorbital ridge. The cranial capacity of H. erectus was between 750 and 1,250 cm3 (950 cm3 on average), or even more, although this depended on the particular subspecies.
The first fossils of this species were found in the central part of the island of Java (Indonesia) and called Pithecanthropus erectus. Later, other similar fossils were found in China near Beijing and called Sinanthropus pekinensis. Today, some paleoanthropologists subdivide H. erectus into two great informal groups: Indonesian pithecanthropines and Chinese sinanthropines. Only one pithecanthropine subspecies has been considered [Homo erectus erectus], while five subspecies are suggested for the sinanthropine group: [Homo erectus wushanensis, Homo erectus yuanmouensis, Homo erectus lantianensis Homo erectus hexianensis, and Homo erectus pekinensis].
The oldest H. erectus specimens are found among the sinanthropines, the oldest of them being the Wushan Man (H. e. wushanensis) discovered in Longgupo (Chongqing, China), which lived at least 1.8 million years ago. The second-oldest member of the sinanthropines is the Yuanmou Man (H. e. yuanmouensis) identified in Yuanmou (Yunan, China), whose fossils are associated with stone tools, pieces of charred animal bones, and ash from campfires; they probably lived 1 to 1.7 million years ago. These oldest sinanthropines had a brain capacity of approximately 750 to 990 cm3 (870 cm3 on average) and an estimated stature of about 1.60 to 1.70 m in males. Members of H. e. lantianensis, popularly known as the Lantian Man, were found in Lantian (Shaanxi, China), and probably in Gongwangling (near Lantian), though these were promoted to a subspecies (Homo erectus gongwanglingensis); they lived between 0.4 and 1 million years ago. This last group probably also includes sinanthropine fossils found in Yunxian (Hubei, China)—sometimes considered to be another different subspecies (Homo erectus yunxianensis) and dated as living between 0.4 and 0.7 million years ago. The brain capacity of this intermediate erectine group varied between 800 and 1,200 cm3 (1,000 cm3 on average). Another sinanthropine subspecies is Homo erectus hexianensis, which was discovered in Hexian (Anhui, China); it was dated at 250,000 to 300,000 years old, and its brain capacity was 1,025 cm3. Finally, fossils of H. e. pekinensis, originally called Sinanthropus pekinensis, were found at Zhoukoudian (near Beijing, China), associated with stone tools and evidence of the use of fire, and dated between 460,000 and 230,000 years old; their brain capacity was already 900 to 1,250 cm3 (1,100 cm3 on average), and males had an estimated stature of about 1.55 to 1.65 m.
Homo erectus erectus included pithecanthropines that lived on the island of Java (Indonesia) between 0.7 and 1.6 million years ago. Their brain capacity was 800 to 950 cm3 (900 cm3 on average). The most famous of them is Java Man of Trinil (eastern Java), initially called Pithecanthropus erectus, who lived between 0.7 and 0.9 million years ago. The oldest members of this group were found in Sangiran (central Java), and initially called Meganthropus palaeojavanicus due to their supposed— but unfounded—giant size; they lived between 0.9 and 1.6 million years ago. Many paleoanthropologists considered this specimen an old subspecies of erectus (i.e., Homo erectus palaeojavanicus), and others even considered it to be a new species.
The last pithecanthropines awakened strong controversy over their real taxonomic position and their relationship with the evolutionary history of modern humans. Generally, they are regarded as a subspecies of H. erectus and named as Homo erectus soloensis, informally javanthropines. They are an anomalous pithecanthropine due to their age and characteristics. While most subspecies of Homo erectus disappeared according to the fossil record roughly 250,000 years ago, they persisted up until 30,000 years ago. Some of them reached a cranial capacity of more than 1,400 cm3, and their culture was unusually advanced. They are the largest of the pithecanthropines, adult males being approximately 1.75 to 1.85 m in height and 70 to 80 kg in weight, and adult females being 1.6 to 1.7 m in height and 55 to 65 kg in weight. These individuals were first considered as a subspecies of Homo sapiens and thought to be the ancestors of the modern aboriginal Australians. Today, some paleoanthropologists separate this anomalous group into a new species, Homo soloensis, considered as the culmination of the Asian erectine lineage. The first fossil remains come from the Ngandong-Solo River (Java, Indonesia) and were originally nicknamed Solo Man, classified as Javanthropus soloensis, and dated at only 50,000 to 25,000 years old. In the same region, other similar and more complete human fossils were found, including individuals with a brain capacity of 1,050 to 1,250 cm3 and dated at 300,000 to 100,000 years (they were classified as the subspecies Homo erectus ngandonensis). Populations of similar humans have been found throughout southeastern Asia (China, Vietnam), and even in India, Jordan, and Australia. In China, the most famous fossils of this probable species were found in Dali (Shaanxi, China), known initially as Dali Man and Homo daliensis, and in Jinniushan (Liaoning, China). These human fossils were dated between 300,000 and 200,000 years old. Their supraorbital ridge was less robust and this cranium more rounded than the oldest pithecanthropines, with a brain capacity of 1,100 to 1,250 cm3. They could belong to the oldest group of this species, and could be included as a subspecies: Homo soloensis daliensis. Most controversial are the remains of similar “javanthropines” found in Narmada (Hathnora, India), dated from 150,000 to 200,000 years old and with a brain capacity of 1,150 to 1,420 cm3; they have been considered another subspecies of erectus (Homo erectus narmadensis) but seem to belong to this human group. Most problematic are the relationships of javanthropines with the 50,000- to 60,000-year-old human fossils found in Malakunanja and Nauwalabila (northern Australia). Interesting is the question related to the origin of Homo soloensis. Its last specimens seem to have coexisted with Homo sapiens in southeastern Asia. Many paleoanthropologists find strong relationships between this group and European H. heidelbergensis, and consider that they are in fact H. heidelbergensis or another archaic human relative that migrated to Asia, replacing the older H. erectus.
A lot more problematic is the last group of humans related to erectine humans: Homo floresiensis. It is possibly a new species remarkable for its small body and brain capacity with respect to other humans, and its survival until very recent times. A complete subfossil skeleton of Homo floresiensis, nicknamed “Hobbit” for its minute size, was found on the island of Flores (Indonesia) and dated to be only 18,000 years old. Other diminutive individuals, who have been identified and associated with small stone tools, lived between 13,000 and 94,000 years ago. There are suspicions that this small human species may have survived longer in other parts of Flores to become the source of the Ebu Gogo stories, told among the local people, in which small, hairy, language-poor humanoid creatures dwelled in caves on the island (similarly, on the nearby island of Sumatra, there is notice of another small mythological humanoid known as Orang Pendek). They were 1.05 to 1.10 m in height, and probably 25 to 30 kg in weight, considerably shorter than the physically smallest populations of modern humans such as pygmies and negritos (1.35–1.55 m). They also had small brains, with a cranial capacity of 380 cm3, lower than in chimpanzees and in primitive australopithecines. Despite this, their remains are associated with sophisticated stone tools of the Upper Palaeolithic, indicating an advanced behavior. The origin and nature of this human group is a controversial issue today within the scientific community. Some anthropologists have suggested that these individuals were microcephalic modern humans or affected by endemic cretinism. Others have related H. floresiensis to primitive gracile australopithecines (as A. afarensis), not previously thought to have expanded beyond Africa, that survived up to modern times. However, their cranium and body features resemble those of H. erectus, with which they seem to have a phylogenetic relationship. The erectine origin of H. floresiensis is the more accepted hypothesis, despite the size difference. To explain the small size of this species, it has been suggested that populations of H. erectus underwent strong insular dwarfism (caused by the limited food environment on their islands), a form of speciation also seen on Flores in several species, including a dwarf Stegodon (a group of elephant-type proboscideans). Since Flores remained isolated even in the recent glacial periods characterized by low sea levels, the discoverers of H. floresiensis suggest that this species, or its erectine ancestors, could have reached the isolated island of Flores only by water transportation, perhaps arriving on bamboo rafts about 100,000 years ago.
Archaic Homo Species
Numerous vaguely defined taxa, most of which are not widely accepted, are included in the group informally known as archaic sapiens humans. Among them are Homo antecessor (defined by Bermúdez de Castro, Arsuaga, Carbonell, Rosas, Martínez, and Mosquera in 1997), Homo heidelbergensis, Homo neanderthalensis (nicknamed Neanderthals), Homo rhodesiensis, and Homo helmei.
Homo antecessor is an archaic sapiens species that lived in Europe between 0.8 and 1.2 million years ago. It is considered the earliest known human form of Europe, although individuals of Homo georgicus from Georgia are older. Its physical traits are intermediate between the oldest Homo ergaster and the most modern Homo heidelbergensis. The sexual dimorphism of H. antecessor was the same as in its European successors H. heidelbergensis and H. neanderthalensis, but less than in its probable ancestor H. ergaster or H. georgicus. Adult males were approximately 1.70 to 1.80 m in height and 70 to 90 kg in weight, and adult females were 1.60 to 1.70 m in height and 60 to 70 kg in weight. Hands and feet of H. antecessor were similar to those of modern humans, therefore indicating modern locomotor and manipulatory behaviors. Its individuals had a rounded cranium with a protruding occipital bun, a single supraorbital ridge, and a long, vertical face with hollow cheeks (in contrast to the flat face of H. ergaster and H. erectus). It had less robust mandibles, smaller molars, and premolars that were more molarized than its predecessors. Its teeth had primitive characteristics that resembled H. ergaster, but its dental eruption pattern had the same developmental stages as H. sapiens. The cranial capacity of H. antecessor was 1,000 to 1,150 cm3 (1,050 cm3 on average). Its fossils are associated with a great variety of tools, including hammer-stones and retouched flakes. Certain evidence, such as cuts where flesh had been flensed from the bones, indicates that H. antecessor practiced cannibalism.
- antecessor is very probably the ancestor of Homo heidelbergensis in the Homo neanderthalensis lineage. Since H. antecessor also has intermediate characteristics between H. ergaster and H. sapiens, it is also considered a link between both species. Although the only known fossils of H. antecessor come from Atapuerca (Spain), some other specimens in Europe and northeastern Africa may also belong to this species. One of these is the fossil human identified in Ceprano (Italy), called Homo cepranensis, and nicknamed Ceprano Man. It lived between 0.8 and 0.9 million years ago, and its features seem to be also intermediate between H. ergaster and H. heidelbergensis. More problematic are the African specimens Homo mauritanicus and Homo uxoris. Both specimens have been considered to be modern African Homo erectus or Homo ergaster, the first one dated at 0.7 million years old and the second one at 0.7 to 0.9 million years old. By priority, some claim that the H. antecessor name should be changed to H. mauritanicus, but the morphological differences, at least in their mandibles (the only part found of mauritanicus to date), are too great to consider them to be of the same species.
All of these specimens, in addition to H. georgicus and older Homo erectus, open the debate on the origin of H. antecessor. There are two hypotheses: a northernAfrican origin and an Asian origin. According to the Asian hypothesis, the Caucasian H. georgicus is considered the probable ancestor of both the European H. antecessor and the south Asian H. erectus (and perhaps even of the African H. ergaster). In this case, H. georgicus would be the first human to leave Africa, being the link between H. habilis and all the later erectine humans (including antecessor), requiring that specimens similar to H. georgicus be found in Africa. Nevertheless, according to the fossil data known to date, there are great morphological and age differences that prevent establishing a clear phylogenetic relationship between H. georgicus and H. antecessor. The other hypothesis suggests that H. antecessor evolved from the African H. ergaster, although it is not known whether this event occurred in Africa (and therefore it is an immigrant to Europe) or in Europe.
Homo heidelbergensis is considered a descendant of H. antecessor and the direct ancestor of Homo neanderthalensis. It lived in Europe between 0.7 and 0.15 million years ago. They were very tall and corpulent, and had a sexual dimorphism similar to modern humans, with adult males being approximately 1.75 to 1.85 m in height and 70 to 90 kg in weight, and adult females 1.65 to 1.75 m in height and 55 to 70 kg in weight. The physical characteristics of its body (pelvis and limbs) were similar to Homo neanderthalensis. Their skulls were elongated, with a depressed, wide nasal bridge and a prominent supraorbital ridge. They showed significant sexual dimorphism in their teeth and jaws, but in general they had wider molars and thicker incisors than did H. neanderthalensis.
The cranial capacity of H. heidelbergensis was 1,100 to 1,450 cm3 (1,275 cm3 on average), overlapping the average of modern humans. They used core techniques to make tools, which were very varied. Their stone-tool technology is very close to the Acheulean industry used by H. erectus, although more advanced (including the possible use of red ochre as paint). Some accumulation of individuals at specific postmortem sites could indicate ritual burial, but there is not enough evidence. It is possible that H. heidelbergensis, like its descendant H. neanderthalensis, acquired a primitive form of language. Since there were similar archaic sapiens humans living in Africa, such as Homo rhodesiensis, there is a controversy over whether or not the H. sapiens lineage evolved from an African H. heidelbergensis, or this species is exclusively European descending from H. antecessor and being the ancestor of only H. neanderthalensis.
Homo neanderthalensis, nicknamed Neanderthals, lived between 25,000 and 150,000 years ago almost exclusively in Europe, but also in parts of western and central Asia, including the Near East and the Siberian Altay mountains. They have been considered for a long time to be a subspecies of H. sapiens, but genetic evidence has shown that these are two different species, although they share 99.5% of their DNA. It has been calculated that both species shared a common ancestor about 500,000 years ago, and this ancestor could have been H. heidelbergensis. Nevertheless, other more recent studies indicate that the common ancestor lived about 800,000 years ago, suggesting that H. antecessor (and not H. heidelbergensis) was the true ancestor of both human lineages. This conclusion is more harmonious with the paleontological evidence and morphological divergence found in the fossil record. The postcranial skeleton of the Neanderthals was heavier and stronger than that of modern humans, with a more robust bone structure, although it was generally shorter. Adult males were approximately 1.6 to 1.7 m in height and 70 to 80 kg in weight, and adult females 1.5 to 1.6 m in height and 50 to 60 kg in weight. Their skulls were low, flat, and elongated, with an occipital bun, a projecting midface, and a thick supraorbital ridge.
The cranial capacity of H. neanderthalensis was 1,200 to 1,750 cm3 (1,420 cm3 on average). The assertion that Neanderthals had a brain capacity much larger than modern humans has persisted a long time among paleoanthropologists, but it is probably an effect of the statistical deviation in the analysis of the fossil record of Neanderthals. Similar studies in fossil Homo sapiens specimens (e.g., Homo sapiens idaltu) concluded that they had a 1,490 cm3 cranial capacity. Nevertheless, their intelligence might have been very advanced. They used more advanced tools than H. heidelbergensis and H. erectus, having created a predominantly flint industry known as the Mousterian of the Middle and part of the Old Palaeolithic. They made sophisticated stone flakes, hand axes, and spears. According to the reconstructions of their vocal tract (with the presence of a hyoid bone), and neurological (with hypoglossal canal size similar to modern humans) and genetic evidence (with the presence of the same version of the FOXP2 gene as in modern humans, which plays a role in human language), it is considered that the Neanderthals had an elaborate protolanguage that was more musical than the language of modern humans. Moreover, they were almost exclusively carnivorous, being considered the apex predator (hunting large animals, such as the mammoths) for making good use of their stone flakes. They intentionally buried their dead in graves with goods, used pigment ochre, and practiced ritual cannibalism or defleshing. All these ritual treatments of the dead probably denote the development of a religious ideology.
It is widely accepted that H. neanderthalensis evolved from H. heidelbergensis in Europe and then became extinct, to be replaced or absorbed by modern humans traveling from Africa. Their last populations lived in southern Iberia, around the coast of Gibraltar, 24,000 years ago. Neanderthals and modern humans coexisted in Europe for millennia, surely intermixing their cultures, but very probably the species never genetically interbred, which has been proposed, since they were not interfertile, as they were separate species. Their last populations adopted the more advanced Châtelperronian culture (Upper Palaeolithic), a culture considered to have belonged first to our species Homo sapiens, which later passed this culture on to the Neanderthals.
Homo rhodesiensis, nicknamed Rhodesian Man and sometimes classified as Homo sapiens arcaicus, is a possible direct ancestor of Homo sapiens that lived in Africa between 125,000 and 450,000 years ago (and probably even longer ago). Their fossil remains show some primitive traits shared with H. ergaster and H. antecessor, and other traits that allow us to link them to H. heidelbergensis, but they also had other traits that relate them to H. sapiens. Adult males were approximately 1.60 to 1.70 m in height and 60 to 70 kg in weight, and adult females 1.50 to 1.60 m in height and 50 to 60 kg in weight. They had an elongated skull with a large supraorbital ridge, and high and narrow cheekbones. Their cranial capacity was 1,250 to 1,350 cm3 (1,310 cm3 on average).
- rhodesiensis belonged to the Acheulean industry of stone tools (Lower Palaeolithic), but perhaps its first appearance is related to the beginning of the second phase of the Acheulean 600,000 years ago, in which the tools became thinner, more symmetric, and more trimmed than in older groups (ascribed to H. ergaster or to African H. antecessor). Most current paleoanthropologists believe this archaic human group to be within the variability of H. heidelbergensis. Nevertheless, others consider that there are three archaic human lineages: the African H. ergaster–H. antecessor–H. rhodesiensi–H. sapiens lineage, the European H. antecessor–H. heidelbergensis–H. neanderthalensis lineage, and the Asian H. ergaster–H. erectus–H. soloensis lineage. No direct linkage of these species can so far be determined.
Some paleoanthropologists have suggested that, within the African record, there is room for another intermediate species within the H. sapiens lineage: Homo helmei, also known as Africanthropus helmei or Florisbad Man. Another name given to this group has been Homo njarasensis. It is anatomically intermediate between H. rhodesiensis (or H. heidelbergensis) and H. sapiens, and is considered as the direct ancestor of the first subspecies of human moderns, Homo sapiens idaltu, and therefore the origin of our species. This group is considered sometimes like the African version of H. neanderthalensis. It lived between 130,000 and 300,000 years ago. The cranial capacity of H. helmei was already 1,250 to 1,550 cm3 (1,390 cm3 on average), and its supraorbital ridge was very reduced. Its stone tools belonged to the Levalloisian, Mousterian, and Aterian cultures of the Middle Palaeolithic of Africa (which include microliths, as well as bone tools and carvings). Most paleoanthropologists do not consider this species as valid, but its possible validity is based primarily on the emergence of the Mode 3 technology (Mousterian and Aterian), which could be associated with a speciation event within the human lineage.
Fossil Evidence: Origin and Evolution of Homo sapiens
It is broadly accepted that modern Homo sapiens has only one extant subspecies (Homo sapiens sapiens) and only one fossil subspecies (Homo sapiens idaltu). The earliest H. sapiens fossils were found in the Omo River and at Herto (Ethiopia), respectively dated at 160,000 and 195,000 years old, and they are assigned to the subspecies H. s. idaltu. They lived in central East Africa (Ethiopia, Eritrea) between 125,000 and 195,000 years ago. They had a brain capacity of 1,450 cm3, and their supraorbital ridge is still slightly prominent. They are considered anatomically and chronologically intermediate between archaic humans (Homo helmei) and more recent, fully modern humans (Homo sapiens sapiens).
Modern H. s. sapiens is first present in the fossil record at the Kibish Formation and Mumba (Tanzania), dated from 130,000 years ago, and at Border Cave and Klasies River Mouth (South Africa), dated from 110,000 to 120,000 years ago. These are only slightly earlier than early H. sapiens at Skhul and Qafzeh (Israel). The Skhul human fossils were dated from 80,000 to 120,000 years ago, and those of Qafzeh from 92,000 to 115,000 years ago. These early humans may have been descendants of the first migrants to leave Africa. They have become classified as a new subspecies, Homo sapiens palestinus, although this taxon is not widely accepted. Nevertheless, these humans seem to have become extinct or retreated back to Africa 80,000 years ago, possibly to be replaced by Neanderthals escaping the colder regions of Ice Age Europe.
All other modern human fossils found outside of Africa are of more recent times. The oldest fossils of modern humans outside of Africa are those of Mungo (Australia), nicknamed as Mungo Man. They have been dated at 42,000 years old. In Asia, 40,000-year-old fossils of H. s. sapiens have been found in Ordos (Mongolia), and others 32,000 years old in Naha-Okinawa (Japan) and 27,000 years old in the Zhoukoudian upper cave (China). Finally, the oldest modern humans found in Europe come from Pestera cu Oase (Romania), Compe Capelle (France), Mladec and Predmostí (Czech Republic), and Cro-Magnon (France)—all between 23,000 and 36,000 years old (they and the rest of early-European modern humans are nicknamed Cro-Magnons or cromagnons).
DNA analysis indicates that modern humans originated in Africa about 180,000 or 200,000 years ago. According to the recent African origin (RAO) and out-of-Africa theory, H. s. sapiens developed 180,000 years ago, possibly in East Africa, being the capoids or the khoisan form (formerly classified as H. s. khoisanii), the oldest representative of our subspecies. This first lineage is the mtDNA haplogroup L0, and it has been nicknamed the “mitochondrial Eve.” Khoisan mitochondrial divergence has been dated no later than 110,000 years ago.
Three main lineages of modern humans diverged between 80,000 and 120,000 years ago. The first lineage (mtDNA haplogroup L1 and Y-DNA haplogroup A) colonized southern Africa (ancestors of bambutoids as pygmies, formerly classified as H. s. pygmaeus). The first lineage to branch from haplogroup A has been nicknamed the “Y-chromosome Adam.” Those early human fossils found at Border Cave and Klasies River Mouth (South Africa) perhaps belong to this old lineage. The second lineage (mtDNA haplogroup L2 and Y-DNA haplogroup B) appeared 80,000 years ago and settled in central and western Africa (ancestors of negroids as paleocongids, nilotids and bantids, formerly classified as H. s. afer). Finally, a third lineage (mtDNA haplogroup L3) remained in eastern Africa (perhaps ancestors of aethiopids, formerly classified as H. s. aethiopicus).
This last lineage (L3) was significant because it was the first H. s. sapiens to have left Africa, crossing the Red Sea 70,000 years ago. Previously, other early modern humans had ventured out of Africa briefly, as indicated by 90,000year-old human fossils found in Israel (H. s. palestinus), but they became extinct. These people probably were more related to the first lineage (L1) than to the second (L2) and third (L3) African lineages.
It is known that humans are genetically highly homogeneous, which may have resulted from the Toba catastrophe 70,000 to 80,000 years ago. A supervolcanic event at Lake Toba (Sumatra) could have reduced the world human population to 10,000 individuals or even less, extinguished H. s. idaltu in Israel, and created a bottleneck in the human evolution of H. s. sapiens. According to this hypothesis, humans left Africa for the first time after the Toba supervolcanic event, migrating to Arabia and the Middle East. From that time, human survivors began to have a fully modern behavior, including the ritual of burying the dead.
The mitochondrial L3 lineage crossed the Red Sea in two waves: the first wave occurring 70,000 to 80,000 years ago across the narrow span of water between the Horn of Africa and the Arabian Peninsula, and the second wave occurring 60,000 to 70,000 years ago, crossing the Red Sea more toward the north and settling in the Near and the Middle East. The first ones formed an old Arabian group (mtDNA haplogroup M and Y-DNA haplogroup M130) 60,000 years ago, which headed along the southeast coast of Asia (India, Sri Lanka, Indonesia, Filipinas, Malasia, and Polynesia) reaching Australia 50,000 to 55,000 years ago. The haplogroup M is nicknamed the Coastal Clan and is considered an ancestral east Eurasian lineage that derived from the australoid people, such as the australianstasmanians, melanesians, veddoids, and negritos (those formerly classified as H. s. australasicus). The 42,000year-old Mungo Man (Australia) is the oldest fossil probably belonging to this migrant lineage. They surely supplanted and replaced the last erectine humans (Homo soloensis) from southeastern Asia and Australia.
The second group derived from those first African emigrants (L3) went north and radiated in the Near and the Middle East, appearing as the mtDNA haplogroups N and R, and Y-DNA haplogroup M89. Descendants of these two groups moved out and explored the surrounding areas (Turkey, Caucasus, and central Asia), initiating a second great migration out of Africa and eventually colonizing the whole world. Because almost all of the mitochondrial lineages found in Europe and Asia descend from N and R groups, these people are considered the Eurasian Clan.
Europe was colonized 45,000 years ago by migrants from central Asia, the Near East, and the Middle East, slowly displacing the Neanderthals. They were people who emerged from the Next and the Middle East (mtDNA haplogroups N and R), and that bore mtDNA haplogroups J, T, and meaningfully pre-VH. Most people moved north across the Caucasus, bringing their lineages into Europe during the middle Upper Palaeolithic. These lineages were represented in Europe for the first time by the CroMagnons, heralding the end of the era of the Neanderthals in Europe. Descendants of early Europeans began to split off and form their own groups, represented by the mtDNA haplogroup HV (derived from pre-VH). Around 15,000 to 20,000 years ago, colder temperatures made living conditions nearly impossible for much of the Northern Hemisphere. Early Europeans retreated to the warmer climates of Iberia, Italy, and the Balkans. Their population sizes were drastically reduced, and much of the genetic diversity that had previously existed in Europe was lost. After the ice sheets began their retreat 15,000 years ago, this people moved north again and recolonized Europe. Some of these lineages (mtDNA haplogroups V and H, derived from HV 15,000 to 30,000 years ago) expanded toward Western Europe, others (mtDNA haplogroups K and U, derived from R 50,000 years ago) towards northern Europe, and finally others (mtDNA haplogroups I and W, derived from N 30,000 years ago) toward Eastern Europe.
Other groups derived from R, the mtDNA haplogroup U6, moved to northern Africa. After the last glacial maximum, expansions led people across the Strait of Gibraltar, allowing for some gene flow between North Africa and southwestern Europe.
Radiating out from central Asia and the Middle East, the mtDNA haplogroups B and F and Y-DNA haplogroups M89 and M9 (appearing 40,000 to 60,000 years ago) conquered the Far East (south and east Asia) 40,000 years ago. They formed the different southwestern Asian mongoloid groups that, together with Siberian groups, were formerly classified H. s. asiaticus. Fossils found in Ordos (Mongolia) that are 40,000 years old, and in the Zhoukoudian upper cave (China) that are 27,000 years old, are evidence of this new wave of human migrants toward southwestern Asia, replacing and/or mixing with the australoid M haplogroup in the south Asian coastals. Descendants of early southern and eastern Asians began to split off and form their own groups, reaching the Philippines and Indonesia, and finally Melanesia, Polynesia, and Micronesia in more recent times.
Radiating out from the central Asian homeland and derived from N, R, and/or M 50,000 to 60,000 years ago, the mtDNA haplogroups Z, A, B, X, C, and D colonized Siberia approximately 30,000 years ago. They formed the group known as the Siberian Clan. Some of these lineages (A and B) also reached Korea, Japan, and southeastern Asia during this epoch. Human fossils 32,000 years old found in Naha-Okinawa (Japan), and nicknamed Yamashita-Cho Man, belong to this lineage. The haplogroup Z migration heading west, out of Siberia, came to an abrupt end around the Ural mountains and Volga River (Russia). Finally, several other Siberian lineages colonized America for the first time.
America was occupied by Asian people bearing mtDNA haplogroups A, B, D, and X, and the Y-DNA haplogroups M130 and M217; they crossed the Bering Strait from Siberia into Alaska during the last glacial maximum, when a land bridge, called Beringia, united the continents of Asia and America. Most parts of these haplogroups arrived from Siberia, except for the M217 group, which arose from east Asia. These groups were the ancestors of all Amerindians (sometimes classified as H. s. americanus). The reduced genetic diversity found in America indicates that these lineages arrived only 15,000 to 20,000 years ago. In fact, only the Y-DNA haplogroup M3 (derived from Siberian M242) is entirely American. Since a branch of haplogroup X found in Amerindians (such as Sioux and Navajo) is almost entirely absent from Siberia, an alternative hypothesis of the migration route toward America has been suggested: the Solutrean hypothesis, asserting that stone-tool technology of the Solutrean culture in prehistoric Europe (France, Spain) may have influenced the development of the toolmaking culture of the Clovis Paleo-Indians in America. This hypothesis suggests that peoples from Europe may have been among the earliest settlers in the Americas about 13,500 years ago.
Homo sapiens sapiens began its existence between 180,000 and 200,000 years ago, but its lineage began to evolve more than 4 million years ago. After hundreds of thousands of generations, the hominids have trended toward being less physically skulled but showing a disproportionate encephalization. According to this trend, future humans could have larger heads and shorter legs and arms, retaining each time more and more traits previously seen only in juveniles (neoteny). Surely our future adults will resemble the morphology of our present babies. Nevertheless, future human evolution may not be governed by the same principles that govern the evolution of other animals, since humans differ from other species by their advanced social organization and language, as well as the use of advanced technology, controlled energy, and clothes. Many paleoanthropologists claim that the human brain has not changed for 150,000 years. On the contrary, genetic research shows that certain genes, related to brain size, did change between 6,000 and 35,000 years ago, suggesting that humans are still evolving.
A species gradually evolves through natural selection into a new species, but this usually occurs in small, geographically isolated populations. This type of evolution is very slow for a large and widespread species such as humans. Therefore, this type of evolution is not very probable for humans, since the tendency for genetic intermingling among humans is very large, and the mixing will increase in the future.
Today, there is no existing species that might compete or threaten the human species, as the Neanderthals were outcompeted by H. sapiens. However, a species could arise that would be a rival of our own species: an explicit, new species, artificially created through genetic manipulation of ourselves or another species, or based on computers and cybernetics (artificial intelligence).
We have a high adaptability to changing environments and, excepting for a cosmic catastrophe, we will probably survive through the future climatic turnovers. Nevertheless, humans are capable of modifying their environment, so there is a risk that humans themselves will modify their environment in such a way that they cannot survive in it any longer. If earth would suffer a global disaster in the future, whether due to our fault or by other causes, humans could try to find a way out: the colonization of space. Today, it is a theme mainly of science fiction, but there are already several space programs that have, as their objectives, autonomous self-sufficient human habitation and the establishment of space colonies on the moon or on Mars. Space-colonization technology could, in theory, allow human expansion at a high, but subrelativistic, speed toward interstellar colonization, permitting self-contained habitats with life spans from decades to centuries. If the isolation of this type of interstellar colony were prolonged long enough in time, then the birth of a new human species could become possible through the basic principles of evolution (natural selection, adaptation, genetic drift, etc.)— but in an extraterrestrial environment.
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