A Hominin is any member of the primate tribe Hominini, a classification that generally is considered to include only humans (genus Homo), chimpanzees (Pan), and their extinct ancestors. In recent classifications, Hominini is a tribe of the subfamily Homininae (humans, chimpanzees, gorillas) of the family Hominidae (humans, chimpanzees, gorillas, and orangutans).
Chimpanzees and humans are placed together in Hominini because of their remarkable anatomical and biochemical similarities and because research suggests chimpanzees are Homo's closest living relatives. Indeed, chimpanzees and humans share over 98 percent genetic similarity (see chimpanzee article).
However, in addition to numerous physical differences—chimpanzees rarely have heart attacks, are resistant to malaria caused by Plasmodium falciparum, and do not go through menopause (Wood 2006), for example—it should be noted that humans define themselves not only according to morphology and DNA structure, but also in terms of culture, psychology, intelligence, behavior, religion, and other aspects. In such ways, there is a striking gap between humans and chimpanzees. (See Chimpanzees and humans, Hominidae, and Homo sapiens for uniqueness of humans.)
Primate classification has undergone many revisions over the years, from the 1960s when humans were the only extant species in the family Hominidae, until today, when it is common to place chimpanzees, gorillas, and orangutans in Hominidae as well. There are even other taxonomic schemes, such as placing extant and extinct chimpanzees and gorillas in the family Panidae, orangutans in the historical group Pongidae, and humans in Hominidae.
The creation of the taxon Hominini is the result of the current idea that the least similar species of a trichotomy should be separated from the other two. Thus, orangutans are separated into the subfamily Ponginae while the other great apes are placed in subfamily Homininae. By various methods (protein differences, DNA differences, etc.), it is determined that the branching point between humans and chimpanzees is more recent than between chimpanzees and gorillas, thus suggesting that chimpanzees are the closest extant relatives of Homo. (Mayr 2001)). The gorillas are separated into tribe Gorillini, while extant and extinct humans and chimpanzees are placed in tribe Hominini.
The anatomical and biochemical similarity between chimpanzees and humans is indeed striking. Various studies show that they have about 98 to 99.4 percent of their DNA in common (Wildman et al. 2003, Wood 2006). For example, comparisons between chimpanzees and humans in terms of protein sequences, allele differences, and DNA heteroduplex melting points show more than 98 percent identity (King and Wilson 1975; Wood 2006). Ebersberger et al. (2002) found a difference of only 1.24 percent when he aligned 1.9 million nucleotides of chimpanzee DNA and compared them with the corresponding human sequences in the human genome (Wood 2006). Using a 4.97 million nucleotide portion of DNA from human chromosome 7 and comparing to chimpanzee orthologies yielded only 1.13 percent mismatches (Liu et al. 2003). Other biochemical comparisons can be seen in the article on chimpanzees.
In the 1996 proposal of Mann and Weiss, the tribe Hominini included the separate subtribes of Panina and Hominina. The genus Homo, and, by inference, all bipedal apes, is by itself only in the subtribe Hominina, while Pan is in the Panina subtribe.
Chimpanzees are so similar to humans that some scientists have proposed that the two chimpanzee species, troglodytes and paniscus, belong with sapiens in the genus Homo, rather than in Pan. Of course, this again considers only anatomical and genetic differences, rather than a comprehensive view that includes social, psychological, religious, and other factors.
Through a study of proteins, comparison of DNA, and use of a molecular clock (a method of calculating evolution based on the speed at which genes mutate), scientists believe thePan/Homo split happened about 5 to 8 million years ago (Mayr 2001, Physorg 2005). One study, utilizing 167 nuclear protein-coding genes, showed the split to be roughly 5 to 7 million years ago (Kumar et al. 2005), while another analysis, utilizing 20 million aligned base pairs, revealed that human–chimpanzee speciation occurred less than 6.3 million years ago and probably more recently, such as 5.4 mya (Patterson et al. 2006). This later study compared key sequences of genes rather than looking at average genetic differences between human and chimp. Interestingly, this later study claims that study of chromosome X shows features that could best be explained if the chimpanzee and human lineages initially diverged, but then later exchanged genes before final separation.
Kumar et al. (2005) note that hypotheses "about the timing of human-chimpanzee divergence demand more precise fossil-based calibrations." However, it is interesting to note that no fossil species on the Pan side of the split have been determined; all of the extinct genera are ancestral to Homo, or are offshoots of such. Mayr (2001), for example, notes that no hominid fossils (in the narrow sense of the word as including humans and their relatives) nor fossil chimpanzees have been found between 6 and 13 million years ago. However, both Orrorin and Sahelanthropus existed around the time of the split, and so may be ancestral to both humans and chimpanzees.
In 2002, a 6–7 million year old fossil skull nicknamed "Toumaï" by its discoverers, and formally classified as Sahelanthropus tchadensis, was discovered in Chad and is possibly the earliest hominid fossil ever found. In addition to its age, Toumaï, unlike the 3–4 million year younger gracile australopithecine dubbed "Lucy," has a relatively flat face without the prominent snout seen on other pre-Homo hominids. Some researchers have made the suggestion that this previously unknown species may in fact be a direct ancestor of modern humans (or at least closely related to a direct ancestor). Others contend that one fossil is not enough to make such a claim because it would overturn the conclusions of over 100 years of anthropological study. While some scientists claim that it is merely the skull of a female gorilla, others have called it the most important hominin fossil since Australopithecus.
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