Thursday, September 18, 2014

Our Tribe

Humans are part of a subfamily called Homininae made up of chimpanzees, gorillas (of which there are two extant species that share a genus, and one extinct species in a separate genus), humans, and a number of extinct species which are or are closely related to human ancestor species.

At least until recently, the next level of classification called a "tribe" was called Hominini which included two extant sub-tribes (one made up of chimpanzees and one made up of humans and extinct species ancestral to them), as well two other extinct species Sahelanthropus tchadensis and Orrorin tugenensis, who appear to be more closely related to humans and chimpanzees than they are to gorillas , but who cannot clearly be classified as part of either a chimpanzee clade, or a clade that includes humans.

Apparently, the modern trend is to define this "tribe" in a way that excludes chimpanzees (all of genus Pan), but includes Sahelanthropus tchadensis and Orrorin tugenensis, as well as subtribe Hominina which includes extinct genus Ardipithecus, extinct genus Kenyanthropus, extinct genus Praeanthropus, extinct genus Australopithecus, extinct genus Paranthropus, and the extant and extinct species of the genus Homo that includes modern humans.

There are arguably as many as 26 formally classified extinct species in seven genuses that are human ancestors or closely related to human ancestors, in addition to arguably three archaic subspecies of the species Homo Sapiens that are extinct in addition to extant modern humans.  But, since many of these species classifications are based upon incomplete skeletal remains from a single location, or from genetic traces without enough skeletal material to meaningfully describe them, these classifications are at best contingent and subject to reassessment as more data becomes available. The map below from the Wikipedia article on Ardipithecus illustrates the situation:

[Proconsul in the map above is the 24 million year old ancestor of gibbons and great apes including chimpanzees, gorillas and modern humans.  Aegyptopithecus is a 34 million year old ancestor of monkeys and great apes alike.  The other five species on the map post-date the divergence of chimpanzees and modern human ancestors, but predate Homo Erectus.]

The prevailing view these days is to lump Kenyanthropus and Praenthropus into Australopithecus.  There is currently division between whether Paranthropus, which are relatively recent fossils with somewhat less archaic features in some respects should be included in Australopithecus as well.  The six million year old Orrorin is arguably more human-like than Australopithecus (e.g. Lucy) despite being about 3 million years earlier.  Sahelanthropus tchadensis, which is about 7 million years old, has been classified variously as a common chimpanzee-human ancestor, as part of the genus Ardipithecus (which is about 4.4 million years old), and as the ancestor not of humans but of gorillas.  The oldest fossil representatives of the genus Homo (i.e. Homo habilis) are at most 2.33 million years old.

However you choose to classify the various species, there is little debate that animals in the genus Pan, made up of one species with four subspecies of common chimpanzees (Pan troglodytes of the Western, Nigeria-Cameroon, Central and Eastern varieties, in geographic order, with a substantial gap between Western chimpanzees and the continuum of the other three subspecies, with some zoologists preferring to split Eastern chimpanzees into separate Northern and Southern subspecies rather than lumping them into a single subspecies), and one species of bonobos (Pan paniscus) aka pygmy chimpanzees, are more closely related to modern humans than any other species of animals that still exist today.  There are roughly 170,000-300,000 common chimpanzees and 30,000-50,000 bonobos alive today.  Common chimpanzees have 23 chromosomes, bonobos have 24 chromosomes and modern humans have 26 chromosomes, but the chromosome divisions are such that particular genes in modern humans usually have corresponding genes, sometimes on a different chromosome, in chimpanzees and bonobos.

The split between ancestors of modern humans and the genus Pan is estimated to date to four to six million years ago, which is also around the time that a lot of "missing link" species that are probably ancestors of modern humans or related to ancestors of modern humans more closely than to chimpanzees, even if they are not directly ancestral to modern humans, start to appear in the fossil record.  There is evidence, albeit weak, to suggest that the split probably took place near the Eastern edge of the modern range of chimpanzees in the vicinity of the East African Rift Valleys, which is the only place where current fossil evidence of archaic human ancestors and chimpanzees overlaps.  But, this could be due to the poor conditions for fossil preservation of remains that can be discovered by archaeologists today in jungles, and a lack of exploration in other parts of Africa due to war and poverty.

The split between Pan troglodyes and Pan paniscus is estimated to date to about a million years ago, although circumstantial evidence suggests that 1.5-2 million years ago (as a result of the formation of the Congo River) may been a more accurate date.  Chimpanzees live North of the Congo River, while Bonobos live South of the Congo River.  Thus, chimpanzees and bonobos are modestly more closely related than humans are to Homo Erectus (which emerged as a species about two million years ago), but less closely related than humans are to Neanderthals (which emerged as a species perhaps 500,000 years ago).  The species Homo Sapien came into being around 250,000 years ago or so, humans left Africa around 125,000 to 100,000 years ago, and non-African humans began to differentiate into distinct sub-populations around 75,000 years ago.  Neanderthals went extinct around 29,000 years ago, and probably at least one species of archaic hominins in Asia, and another in Africa probably walked the Earth as recently as 12,000 years ago.

There is a slim possibility that there may be a few dozens archaic hominins still alive in some remote corner of the early such as deep in some Indonesian jungle, or remote part of the Himalayas, although xenobiologists who have looked for them have so far come up empty, and any such species would have to be at a minimum a moribund relict population, even if they are not entirely extinct right now.

Chimpanzees and bonobos are generally viewed as more closely related to each other than they are to modern humans, although neither of the two species is decisively more closely related to modern humans than the other.

While chimpanzees and bonobos are close genetic relatives, in an evolutionary sense, however, they are behaviorally very different.  Chimpanzees are not only bigger on average than bonobos, but also have much greater size differences between the larger males and the smaller females.  They also murder each other (usually members of other bands of chimpanzees) on a fairly regular basis.  In contrast, bonobos are the epitome of the slogan "make love, not war".

Bonobos are exceedingly promiscuous by the standard of humans or other great Apes. They routinely engage in sex for pleasure rather than reproduction in every way that a porn star could imagine, are polyamorous, and frequently bisexual in the non-reproductive sexual activity.  They are at least three to twenty-five times less likely to murder each other as chimpanzees, are smaller than chimpanzees, and have far less size disparity between males and females.  The bonobo murder rate is of the same order of magnitude as the murder rate in contemporary human communities, not more than 20 per 100,000 individual per year, while the chimpanzee murder rate is at least 73 per 100,000 individuals per year.

A new study in the journal Nature, substantiates these stereotypes and also shows similarities between the parties of murder in chimpanzees and those of gang murders among humans today. As the abstract to the article explains:
Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing. . . . we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades. Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos. We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio). Variation in killing rates was unrelated to measures of human impacts.
Thus, murders are much more common in chimpanzee communities (with 99 confirmed and 53 suspected killings in fifty years in eighteen communities studied) than in bonobo communities (with no confirmed killings and one suspected killing in fifty years in four communities). The percentage of chimpanzee murder suspects who were male is almost identical to that of human murder suspects. And, the pattern of killing predominantly other males in neighboring communities in groups, is very similar to the pattern of gang killed in American ghettos during high crime periods.

Humans are behaviorally and physically, intermediate between are Pan troglodytes and Pan paniscus evolutionary relatives.  Human males are bigger than human females, but not as excessively as Pan troglodytes or as slightly as Pan paniscus.  We are less promiscuous than bonobos, but more promiscuous and sexual and varied in our non-reproductive use of sexuality than chimpanzees.  We are less violent and murderous than chimpanzees are towards each other, but when we do murder, we often fall into patterns similar to that of chimpanzees.  We show the same kind of xenophobia and fear of outsiders as chimpanzees do, but define our communities much more broadly than any of our primate evolutionary relatives.

All of species in subtribe Hominina, Sahelanthropus tchadensis, Orrorin tugenensis, Pan troglodytes and Pan paniscus share a common ancestor who was probably also intermediate behaviorally between modern chimpanzees and modern bonobos.

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