| 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 | |
Homininae is a subfamily of Hominidae, which includes humans, gorillas and chimpanzees, and some extinct relatives; it comprises all those hominids, such as Australopithecus, that arose after the split from orangutans (Ponginae). Our family tree, which has 3 main branches leading to chimpanzees, humans and gorillas, could be more coherent than previously thought. Today there are several species of chimpanzee and gorillas (see Taxonomy) but only one human species remains, although several sub-species of humans appeared about a million years ago and still existed 30,000 years ago. The proposed evolutionary tree makes sense also when one considers brain size: there is a constant increase in brain volume along the human line, whereas the brain size remains almost unchanged in the chimpanzee and gorilla lines. Bipedalism seems to be an early acquisition in Homininae, which was present in early Australopithecines; it evolved independently into knuckle-walking in both chimpanzees and gorillas, and into efficient walking and running in modern humans. Evolution of family structure and sexuality is more complex and cannot be followed by paleontological studies of fossils. However physical characteristics and sexual dimorphism can provide some clues.
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Until 1980, the family Hominidae contained only humans, with the great apes in the family Pongidae.[1] Discoveries led to a revision of classification, with the great apes (now Ponginae) and humans (Homininae) united in Hominidae.[2] Further discoveries indicated that gorillas and chimpanzees are more closely related to humans than they are to orangutans, hence their current placement in Homininae.[3]
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 gorilla or the chimpanzee-humans lineage. They suggest that the Homininae tribes diverged not earlier than about 8 million years ago (see Human evolutionary genetics). A hominin is a member of the tribe Hominini, a hominine is a member of the subfamily Homininae, a hominid is a member of the family Hominidae, and a hominoid is a member of the superfamily Hominoidea.
Today, chimpanzees and gorillas live in tropical forests with acid soils where bones are hardly preserved as fossils. It is considered that no fossil chimpanzees or gorillas have been reported. However, four chimpanzee teeth, about 500 ky old have recently been discovered in the rift valley, where many fossils from the human lineage (hominins) have previously been found.[4] This discovery shows that, at the time, some chimpanzees lived close to Homo (H. erectus or H. rhodesiensis). It is likely that the same could be true for gorillas. For an history of Australopithecines and Homo fossils (see List of human evolution fossils).
Paranthropus are a family of robust Australopithecines that lived from –2.7 to –1.2 mya. They have many similarities with gorillas. Paranthropus have gorilla-like cranial crests: the sagittal crest is running at the top of the skull, the very strong jaw muscles (Temporalis muscle) go all the way up to attach on both sides of this crest. Paranthropus also have characteristic wide cheek bones (zygomatic arches) projecting forward of the nasal opening and marks along the skull from strong chewing muscles. In addition, large postcanine dentition, thick enamel, and robust mandibles indicate a diet of hard or tough foods. Today Gorillas show a strong sexual dimorphism: males reach an height of 180 cm and a weight of more than 200 kg, twice that of females (140 cm, 100 kg). Sexual dimorphism in Paranthropus was not as strong as in today gorillas, but Paranthropus were taller and heavier than other Australopithecines living at the same time, hence their name: Robustus, and showed a more pronounced sexual dimorphism.[5] Between 2.7 million and 1.2 million years ago Paranthropus shared the earth with some early examples of the genus Homo, such as H. habilis and H. ergaster. As far back as 2.7 Mya P. aethiopicus co-existed with A. africanus and A. afarensis. It is widely believed that the Paranthropus lineage died out, leaving no descendants. Although these many similarities between Paranthropus and gorillas were noted in the original papers describing Paranthropus discoveries, the logical conclusion that they could be gorilla ancestors was not proposed.
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.[6] Based on human evolutionary genetics, the common ancestor of humans and chimpanzees had as many pre-human traits as chimpanzee-like traits.[7] 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.[8] The recent discovery of Australopithecus sediba is remarkable.[9] 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.
Subfamily Homininae
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.[10] 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.[11]
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.[12][13] 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).[13] Menopause contributes to an increased longevity of women.
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[14] 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.
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