Homininae

Homininae
Various members of Homininae (both extant and extinct).
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Infraorder:	Simiiformes
Superfamily:	Hominoidea
Family:	Hominidae
Subfamily:	Homininae Gray, 1825
Tribes
Gorillini Hominini and see text

Homininae is a subfamily of Hominidae that includes humans, gorillas, chimpanzees, and some extinct relatives; it comprises all hominids that arose after the split from orangutans (Ponginae). The Homininae cladogram has three main branches, which lead to gorillas, chimpanzees, and humans. There are several extant species of chimpanzees and gorillas (see Taxonomy), but only one human species remains, although several sub-species of Homo sapiens still existed 30,000 years ago.

History of discoveries and classification

See also: List of human evolution fossils

Until 1980, the family Hominidae contained only humans, with the great apes in the family Pongidae. Later discoveries led to a revision of classification, with Hominidae uniting the great apes (now in the sub-family Ponginae) and humans (in the sub-family Homininae). Further discoveries indicated that gorillas and chimpanzees are more closely related to humans than they are to orangutans, leading to their current placement in Homininae as well.

The subfamily Homininae can be further subdivided into the tribes Gorillini (gorillas) and Hominini (bonobos, chimpanzees, and humans). The early Late Miocene Nakalipithecus nakayamai, described in 2007, and perhaps also its contemporary Ouranopithecus, are basal members of this clade, not assignable to either the Gorillini or Hominini tribes. They suggest that the Homininae tribes diverged not earlier than about 8 million years ago (see Human evolutionary genetics).

Today, chimpanzees and gorillas live in tropical forests with acid soils that rarely preserved as fossils. No fossil chimpanzees or gorillas have been reported. However, four chimpanzee teeth, about 500,000 years old, have recently been discovered in the rift valley, where many fossils from the human lineage (hominins) have previously been found. This shows that some chimpanzees lived close to Homo (H. erectus or H. rhodesiensis) at the time; the same is likely true for gorillas.

Paranthropus and gorillas

Paranthropus are a family of robust Australopithecines that lived from 2.7–1.2 million years ago that share many similarities with gorillas. Paranthropus have gorilla-like (prominent) sagittal crests and very strong jaw muscles (Temporalis muscle) that reach up to attach to both sides of the crest. Paranthropus also have characteristic wide cheek bones (zygomatic arches) projecting forward of the nasal opening. The large postcanine dentition, thick enamel, and robust mandibles indicate a diet of hard or tough foods. Pranthropus did not show sexual dimorphism as strongly as gorillas, where males can reach twice the weight of females.

Paranthropus coexisted with members of Homo, such as H. habilis and H. ergaster. As far back as 2.7 million years agom, P. aethiopicus also coexisted with other Australopithecine species, such as A. africanus and A. afarensis; because Paranthropus were taller and heavier than the others, they are known as the robust australopithecines. It is widely believed that the Paranthropus lineage died out, leaving no descendants.

Evolution

Two specimens of Australopithecus sediba approximately 1.3 meters tall.with Lucy (centre) superimposed on a generalized australopithecine background

It is commonly believed that the australopithecines are more closely related to humans than to African apes. This view is hardly compatible with the biomolecular data which place the Homo/Pan split at the beginning of the australopithecine period. Nothing in the fossil hominid morphology precludes the possibility that some australopithecines were ancestral to gorillas or chimpanzees and others to humans. Based on human evolutionary genetics, the common ancestor of humans and chimpanzees had as many pre-human traits as chimpanzee-like traits. An australopithecine 4 million years old (such as Ardipithecus ramidus 4.4 mya) is close to the last common ancestor (5-6 mya); it is thus difficult to place in a precise chimpanzee or human lineage because the two diverged not long before. Some of the most recent Paranthropus fossils are 1 million years old, about one tenth the time since the purported time of divergence of humans and gorillas (7-9 mya). Many of the Paranthropus traits are very much like those of gorillas. In contrast, it is more difficult to place 3- or 4-million-year-old australopithecine fossils in the chimp or human lineages. Most A. africanus fossils seem to be in the human lineage, whereas those of A. afarensis are in the chimp lineage, probably with the exception of "Selam" (3.3 mya), which has been placed in A. afarensis, but has more Homo traits than "Lucy" (3.2 mya), for instance, and might better be placed in A. africanus. According to Adrienne Zihlman, Bonobo body proportions closely resemble those of Australopithecus. The recent discovery of Australopithecus sediba is remarkable. As shown on the figure (Prof. Lee R. Berger) the body proportions of A. sediba are similar to those of "Lucy"; A. sediba could possibly be considered as a late form of A. afarensis.

Mainstream views on Australopithecines evolution can be found in the "human evolution" page, but similar conclusions, suggesting Paranthropus as ancestors of gorilla, have been reached by at least two other biologists, independently: the author of the "Paranthropus aethiopicus" page of the "Online Biology Dictionary" and Richard Dawkins in his book "The Ancestor's Tale". According to this theory, chimps and bonobos are descended from Australopithecus gracile type species while gorillas are descended from Paranthropus robustus P. boisei or P. aethiopicus. These apes may have once been bipedal, but then lost this ability when they were forced back into an arboreal habitat, presumably by those australopithecines who eventually became us. In short, the ancestors of chimpanzees and gorillas are A. afarensis and Paranthropus, respectively.

Taxonomic classification

Hominoid family tree

Subfamily Homininae

• Tribe Gorillini
  • Genus Gorilla
    • Western gorilla, (Gorilla gorilla)
      • Western lowland gorilla, (Gorilla gorilla gorilla)
      • Cross River gorilla, (Gorilla gorilla diehli)
    • Eastern gorilla, (Gorilla beringei)
      • Mountain gorilla, (Gorilla beringei beringei)
      • Eastern lowland gorilla, (Gorilla beringei graueri)
• Tribe Hominini
  • Genus Pan
    • Common chimpanzee, (Pan troglodytes)
      • Central chimpanzee, (Pan troglodytes troglodytes)
      • West African chimpanzee, (Pan troglodytes verus)
      • Nigeria-Cameroon chimpanzee, (Pan troglodytes ellioti)
      • Eastern chimpanzee, (Pan troglodytes schweinfurthii)
    • Bonobo (pygmy chimpanzee), (Pan paniscus)
  • Genus Homo
    • Human, (Homo sapiens)
      • Con synchronisms, (Homo sapiens species)

Brain size evolution

There has been a gradual increase in brain volume (brain size) as the ancestors of modern humans progressed along the timeline of human evolution, starting from about 600 cm³ in Homo habilis up to 1500 cm³ in Homo sapiens neanderthalensis, whereas the brain size along the chimpanzee and gorilla lines remained almost unchanged (see figure). Thus, in general there is a good correlation between brain volume and intelligence. However, modern Homo sapiens have a brain volume slightly smaller (1250 cm³) than Neanderthals, women have a brain slightly smaller than men and the Flores hominids (Homo floresiensis), nicknamed hobbits, had a cranial capacity of about 380 cm³ (considered small for a chimpanzee), about a third of the Homo erectus average. It is proposed that they evolved from H. erectus as a case of insular dwarfism. In spite of their smaller brain, there is evidence that H. floresiensis used fire and made stone tools at least as sophisticated as those of their proposed ancestors H. erectus. In this case, it seems that for intelligence, the structure of the brain is more important than its size. "As large as you need and as small as you can" to summarize the opposite evolutionary constraints on human brain size.

Evolution of family structure and sexuality

Evolution of Sexuality in Homlininae.

See also: Human Sexuality

Sexuality is related to family structure and partly shapes it. The involvement of fathers in education is quite unique to our species, at least when compared to other Homininae. Concealed ovulation and menopause in women both also occur in a few other primates however, but are uncommon in other species. Having sex in private also distinguishes us from Bonobos, Chimpanzees and Gorillas. Testis and Penis size are related to family structure: monogamy or promiscuity, or harem, in humans, chimpanzees or gorillas, respectively. The levels of sexual dimorphism are generally seen as a marker of sexual selection. Studies have suggested that the earliest hominins were dimorphic and that this lessened over the course of the evolution of the genus Homo, correlating with humans becoming more monogamous, whereas gorillas, who live in harems, show a large degree of sexual dimorphism (see Paranthropus). Concealed (or "hidden") ovulation means that the phase of fertility is not detectable in women, whereas chimpanzees advertise ovulation via an obvious swelling of the genitals. Women can be partly aware of their ovulation along the menstrual phases, but men are essentially unable to detect ovulation in women. Most primates have semi-concealed ovulation, thus one can think that the common ancestor had semi-concealed ovulation, that was inherited by gorillas, and that later evolved in concealed ovulation in humans and advertised ovulation in chimpanzees. Menopause also occurs in rhesus monkeys, and possibly in chimpanzees, but does not in gorillas and is quite uncommon in other primates (and other mammal groups). Menopause contributes to an increased longevity of women.

Evolution of bipedalism

See also: Human skeletal changes due to bipedalism

Recent studies of Ardipithecus ramidus (4.4 million years old) and Orrorin tugenensis (6 million years old) suggest bipedalism, it is thus possible that bipedalism evolved very early in hominins and was reduced in chimpanzee and gorilla when they became more specialized. Australopithecus and early Paranthropus may have been bipedal, but then lost this ability when they were forced back into an arboreal habitat, presumably by early Homo. Very early hominins such as Ardipithecus ramidus may have possessed an arboreal type of bipedalism that later independently evolved towards knuckle-walking in chimpanzees and gorillas and towards efficient walking and running in modern humans (see figure). It is also proposed that one cause of Neanderthal extinction could have been a less efficient running than Cro-Magnon.

Prominent researchers

• Zeresenay Alemseged, an Ethiopian paleoanthropologist living in San Francisco and expert on human evolution; discovered Selam.
• Lee R. Berger, an American paleoanthropologist working in South Africa; discovered fossils of large mammals including hominin, A. sediba.
• Robert Broom, a Scottish physician and palaeontologist whose work in South Africa led to the discovery of Paranthropus and of "Mrs. Ples".
• Michel Brunet, a French paleoanthropologist and professor, discovered Toumaï in Chad.
• Yves Coppens, a French paleontologist, discovered the first Paranthropus (P. aethiopicus) and Lucy.
• Raymond Dart, an Australian palaeoanthropologist, discovered the Taung Child in South Africa, the first Australopithecus africanus.
• Charles Darwin, the British naturalist who documented with considerable evidence that species originate through evolutionary changes.
• Richard Dawkins, a British geneticist, researcher and writer on a wide range of topics involving evolution.
• Donald Johanson, an American paleontologist, with many discoveries including Lucy, the emblematic Australopithecus afarensis.
• Louis Leakey, an African archaeologist and naturalist whose work was important in establishing hominin evolution in Africa.
• Mary Leakey, a British archaeologist and anthropologist whose discoveries in Africa include several Australopithecines and the Laetoli footprints.
• Svante Pääbo, a Swedish biologist specializing in evolutionary genetics, leader of the sequencing of the Neanderthal and Denisovan genomes.
• Brigitte Senut, a French paleoanthropologist and professor, with many discoveries including Orrorin.
• Sarah Tishkoff, an American geneticist whose recent work has greatly improved our understanding of African evolutionary genetics
• Philip V. Tobias, a South African palaeoanthropologist, one of the world's leading authorities on the evolution of humankind.
• Erik Trinkaus, a prominent American paleoanthropologist and expert on Neanderthal biology and human evolution.
• Adrienne Zihlman, an American anthropologist, professor at UCSC, working on primate anatomy and on the role of females in evolution.

See also

• Human evolution
• Human evolutionary genetics
• List of human evolution fossils
• Pan prior
• The Third Chimpanzee

References

• "Human Evolutionary Genetics" Jobling M.A., Hurles M., Tyler Smith C. 2004, Garland Science, New York