(These fossils and reconstrctions, like other forms discussed in this chapter, can be viewed on in the Essential Fossils folio. (Link in separate window.)|
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This text was drafted by award-winning MMW TAs Tara Carter and Beth Peterson, (Anthropology) in the summer of 2008, supported by the MMW-based research funds left in his account on the death of Prof. Donald F. Tuzin, long an instructor and supporter of MMW. The text was substantially modified by Professor David K. Jordan in summer, 2011, and again in 2013 to fit it to the needs of MMW-11.
This text may be freely reproduced for non-commercial educational purposes.
Human beings are members of the primate order. So are prosimians, monkeys, and apes. While the primate order is diverse, there are some characteristics that most primates share and that are used to define a primate relative to other mammals. These include:
Primates are generally characterized as highly social animals and they tend to live in social groups that include more than one adult male, which is somewhat rare in other mammals.
Most primates rely heavily on social learning, which occurs during their long developmental and maturation period. Social learning allows them to become competent members of their species, relying on social interaction and relationship to solve problems. In the context of early primate evolution, it is important to recognize that the intense sociality of humans has very deep roots in our evolutionary history.
This essay will provide a brief chronicle of primate evolution, the major splits in the primate family tree, and what we know of the earliest possible hominids, as well as the idea of the “Last Common Ancestor” and why we will never know who that was.
It is useful to imagine related forms in a kind of family tree. And since we are interested primarily in the line of descent that leads to ourselves, it is convenient to speak of other lines as “branching off” from our own, although saying it that way can be misleading if it inclines us to imagine that somehow our own descent line is “straight” while others are deviations from it. From an evolutionary standpoint, of course, no descent line “branches off” of any other. Lines simply separate. Still, “branching off” is a convenient shorthand.
The first primate-like animals, called Plesiadapiformes, emerged during the Paleocene about 65 million years ago with the first definite primates appearing in the fossil record around 55 million years ago.
By about 50 million years ago, the branch that would become modern prosimians split off. Modern prosimians include lemurs, lorises, and tarsiers. (They are cute little creatures. Take a study break and do a quick web search for pictures.)
By around 40 million years ago, the branch that would become the platyrrhines or New World Monkeys, split off. The New World Monkeys include owl monkeys, squirrel monkeys, and capuchins.
The branch that would lead to the cercopithecoids or Old World Monkeys, split from the line that would give rise to us and the other apes (hominoids) about 25 million years ago. The Old World Monkeys include the baboons, macaques, and guenons. The hominoids, sometimes collectively called apes, include the smaller or “lesser” apes (gibbons and siamangs) and the larger or “great” apes (orangutans, gorillas, chimpanzees, bonobos, and humans).
The line leading to the gibbons and siamangs split from our line about 17 million years ago.
About 14 million years ago, the branch leading the orangutans split. About 7 million years ago, the branch that gave rise to gorillas split from ours and not too long after, about 5-6 million years ago, the branch that gave rise to chimpanzees and bonobos split from the branch that led to us, , the so-called hominids. (Remember that “hominoids” is the more inclusive —and longer— word, including lots of apes. “Hominids” is the less inclusive —and shorter— word, referring to a much smaller range of animals. Obviously all hominids are hominoids, but not all hominoids are hominids.)
We are, evolutionarily speaking, closely related to all of the Great Apes but particularly to chimpanzees and the closely related bonobos. (Bonobos were formerly called “pygmy chimpanzees.”) In fact, molecular evidence has demonstrated that chimpanzees and bonobos are more closely related to humans than they are to gorillas. That is, chimpanzees and bonobos share more genetic material with humans than they do with gorillas. Depending on the type of genetic testing done, humans share about 98% of our DNA with chimps and bonobos.
We did not evolve from chimpanzees but rather we share an ancestor with them, a mysterious form referred to as the Last Common Ancestor (LCA) of the hominid line. About 6 million years ago, there was an animal form —the LCA— one of a group of closely related animals, that would have had some characteristics of chimpanzees (and bonobos) and some characteristics of modern humans. For some reason, a division occurred in that species which, over time, led to new species evolving. Most of those species went extinct; but some ultimately led to the animals we know today as chimpanzees and bonobos; and others led to an extinct genus called Australopithecus and to humans.
The term hominid refers to living humans and all of our bipedal ancestors and relatives after the split with the line leading to chimpanzees and bonobos. (Some researchers actually include chimps and bonobos in the hominid category, and then distinguish the subdivision leading to us as hominines. That usage is defensible, perhaps even preferable, but is not used here.)
Because the fossil record is incomplete —it is not easy to become a fossil— and it is very difficult to determine conclusively how to classify a small number of fossils from closely related species. Therefore we will never really know who or what the Last Common Ancestor was. However, a careful examination of all available evidence both of living primates and of extinct ones, tells us a good deal about what the LCA was probably like.
The earliest hominids appeared in the late Miocene or early Pliocene geological epoch, about 7-5 million years ago, so that would be around the time of the split between our line and chimpanzees/bonobos. (To open a chart of geological periods in a separate window, click here.) It is quite likely that other hominid species split from the branch before the line leading to us, i.e. there were probably early hominid species that are not in our direct descent line but in closely related branches. In the absence of fossil evidence, many or most such lines are unknown to us.
Stable carbon isotope analysis is one way that researchers can begin to reconstruct what kind of plant life was around at different periods. There are several different carbon isotopes and plants differ in the proportion of the different carbons that make them up. By analyzing the proportion of different kinds of isotopes in fossil remains of animals it is possible to develop at least a partial picture of their diet, and even to reconstruct parts of the environment, like rainfall and seasonal variation. Research in these areas has demonstrated that during the late Miocene, much of the world was likely experiencing a cooling trend that resulted in African forests becoming less contiguous, developing clear spaces within and between them.
This process of fragmentation of forests, that is, the emergence of more grasslands and transitional areas between grasslands and forests, presented a different kind of challenge to Miocene primates than did contiguous tropical forests. In this environment, some hominoids stayed in the forest or in the wetter areas of the fringes and became the ancestors of African great apes. But some may have moved more into the transitional areas and the drier grasslands. These would have become ancestors of the earliest hominids. That is the direction in which the current evicence points.
Recent evidence suggests that the late Miocene environmental shifts were not as dramatic as once thought. However, the changes tended to result in local fluctuations that influenced hominid evolution. The earliest hominids adapted to small-scale fluctuations through increasing flexibility in their behavior. This shift to life in transitional areas and grasslands would lead to changes in adaptive strategies, locomotor behavior, increased tool use, dietary specialization, and changes in social organization, allowing them to flourish in a variety of environments.
The earliest hominids —the forms most likely to be on our family tree just slightly on our side of the Last Common Ancestor— exhibited some characteristics of the Miocene apes (hominoids) and some that are more similar to what we see in the later hominids. What we expect to find, more or less by definition, is that:
What we share with chimps we both probably inherited from the LCA and should expect to find in all fossil forms between the LCA and modern people or between the LCA and modern chimps. (Unfortunately, we have not yet discovered any fossil forms clearly assignable to the line between LCA and modern chimps.)
Some features that we may detect in Miocene forms will turn out to have vanished on our line or the ape line or both.
Some features (like bipedalism) will turn out to have appeared only on our line (not necessarily in all of the extinct fossil forms that might turn up) or only on the chimp line (again, not necessarily in all of the extinct fossil forms that might turn up).
1. Sahelanthropus tchadensis is the oldest known fossil hominid. A complete cranium was discovered in northern Chad (and named Tomaï) and has been dated to about 7 million years ago . Some researchers suggest that Sahelanthropus should be classified as a hominoid, but not a hominid.
(These fossils and reconstrctions, like other forms discussed in this chapter, can be viewed on in the Essential Fossils folio. (Link in separate window.)
2. Orrorin tugenensis fossils were discovered in the Tugen Hills in east Africa near Lake Baringo, Kenya. The fossil evidence consists mostly of dental remains but also includes significant lower limb bones. Based on careful analysis of the lower limb bones, Orrorin, probably dating to about 6 million years ago, was almost certainly bipedal and, thus, more securely classified as a hominid than Sahelanthropus.
3. Currently, there are two fossil hominid species assigned the genus Ardipithecus, the last, and latest, genus of early hominids. The Ardipethecus fossils were all found in the Middle Awash region of Ethiopia and are divided into two species: Ardipithecus ramidus and Ardipithecus kadabba.
One interesting fact to note about the Ardipithecus finds is that, that, while the general global cooling trend during the late Miocene/early Pliocene resulted in fragmentation of forests and increased savanna grasslands, as we noted, these fossils were found in what had been a more heavily forested region. Most later hominid fossil forms found in Africa seem to have lived in grassland habitats. Current evidence seems to suggest that the split between the branch leading to chimps/bonobos and the branch leading to us occurred in a forest. It is possible, although skepticism is warranted, that the Ardipithecus fossils represent the very first hominids to diverge away from the ape line. While this conclusion is extremely tentative, the possibilities these fossil hominids represent are truly exciting.
This may be a good time to review the charts in the introduction to this set of essays (link).
A review quiz is available for this essay.