This is a chapter from my book The Emergence and Nature of Human History, Volume One. The main section of the book is a chronology of the events that led to the emergence of human consciousness. The chronology employs Carl Sagan's device of condensing the Universe's age down to one year, with the Big Bang occurring on 1 January. The section also measures time through the use of an imaginary timeline one million meters in length.
Because of the tremendous importance of primate evolution in this chronology, I examine this subject particularly closely. The chapter is very long, so I have divided it into three parts.
I am not a scientist; I am a (very minor) historian and former history teacher. But I am writing a "deep history" of the world, and so I was forced to venture into subjects which are typically the province of physicists, chemists, biologists, and geologists. If I have made egregious errors of any kind, or even minor ones, please correct me.
If you are interested in reading other parts of the chronology, just look at my diary list starting with "The Beginning of the Physical Universe".
Evolution of the Anthropoids
From where did Suborder Haplorhini emerge, and when did this happen? The Americas, Europe, Antarctica, and Australia have been definitively ruled out as places of anthropoid origin. For many years the dominant view has been that Africa was the birthplace of anthropoids. Given the many spectacular anthropoid fossils discovered in that continent, and the fact that the first representatives of the human genus were apparently of African origin, such a view is understandable. However, a significant number of paleontologists argue that Asia, not Africa, gave rise to the haplorhines, and they cite highly significant finds in many locations, particularly China, Burma, and India to support their contentions. These scientists argue that the presence of anthropoids in Africa was the result of Asian species that made their way to the African continent and eventually gave rise to all the lineages of African anthropoids, including the one that ultimately produced us. Moreover, the advocates of an Asian origin of the haplorhines postulate a much earlier origin of the suborder than do those scientists supporting an African birthplace.
EVIDENCE OF ANTHROPOIDS IN ASIA
In 1994, a group of paleontologists working in China announced the discovery of what they believe to be a basal anthropoid, which they named Eosimias—“dawn monkey”. Eosimias had several traits that seem primitive in combination with others that appear to be unusually advanced. It is the opinion of paleontologist Chris Beard, who has worked extensively with the remains of Eosimias, that this primate represents an animal distinct from both omomyoids and adapoids, and that, in fact, it represents the true base of the anthropoid tree. The specimens uncovered by Beard and his colleagues have been dated at 45 million ybp, and consist of several significant parts of the anatomy: complete lower jaws with dentition, upper canine teeth and sections of maxilla (upper jaw) that give us important clues about the face and eyes, and parts of the ankle that give clues to Eosimias’s monkey-like way of moving through branches.32 Some eight examples of Eosimias have been uncovered, five of them in China, and together they are part of a family known as the Eosimiidae.33
In 2005 a team of paleontologists announced that several dozen fossils, mostly teeth, representing two previously unknown genera from the anthropoid families Amphipithecidae and Eosimiidae, had been uncovered in Pakistan. These specimens are from the Oligocene Epoch, and have been dated at around 31 million ybp. The significance of these finds, which at the time were the oldest of their kind yet discovered in South Asia, is expressed by these researchers in this statement, which makes reference both to various families (all of which have “dae” or “pithecidae” in their names) and genera (with the roots “simias” or “pithecus”):
The results of our various phylogenetic analyses…primarily based on morphological characters…consistently point toward the monophyly of a large clade, including Asian Eosimiidae, Amphipithecidae, Arabo–African Oligopithecidae, Propliopithecidae, African Proteopithecidae, Parapithecidae, and South American platyrrhine primates. Assuming this clade to be the Anthropoidea clade, from the present evidence, eosimiids and amphipithecids (and by extension Phileosimias and Bugtipithecus, respectively) are stem anthropoids and, as such, support the hypothesis that Asia was the ancestral homeland of the Anthropoidea clade.34
In 2008 a team of researchers announced the discovery in India of an eosimiid of very small size, roughly the dimensions of a mouse lemur with a weight of perhaps no more than 75 grams. Its genus has been named
Anthrasimias by its discoverers, and it is estimated to have lived 55 million ybp. This pushes anthropoid origins in Asia back 10 million years. The discoverers of
Anthrasimias contend that the eosimiid line in Asia means that the Omomyoidea and Adapoidea were sister clades of Anthropoidea, and that neither therefore could have given rise to it.
35 The implication here is clear: Anthropoidea emerged very soon (in geological terms) after most scientists believe the primate order itself evolved.
There has been a long debate in paleontology surrounding the family Amphipithecidae, mentioned above. Specifically, scientists have argued about its true phylogenetic status, and whether it was an anthropoid family. Based on recent evidence from the Pondaung Formation of Burma, the researchers involved in analyzing these specimens (including Beard) contend that the remains are of an animal of a new genus and species, Ganlea megacanina. It appears that Ganlea had very large lower canine teeth, ones that showed a pattern of wear typical of an animal that is using its teeth to break open hard objects to get the food, such as seeds and fruits with tough skins, inside of them, The dentition in general is that of a basal anthropoid, and according to the authors of the study announcing it, Ganlea demonstrates that the amphipithecids were indeed anthropoids. Perhaps just as significantly, its feeding behavior and ecological setting were highly similar to those of the New World Monkeys we see in the Amazon basin today.36 So somewhere around 37-38 million years ago in the late middle Eocene of southeast Asia, a primate was moving about the tree branches living like a monkey—a member of the Suborder Haplorhini.
In general, scientists favoring the Asian hypothesis of anthropoid origins can point to the antiquity of such crucial finds, and the monkey-like traits of these specimens, to bolster their argument that Anthropoidea’s earliest members emerged in southern and eastern Asia and that the African lineages are the result of migrations into Africa, not the evolution within Africa of indigenous types.
EVIDENCE OF ANTHROPOIDS IN THE AFRO-ARABIAN REGION
The African area that has been explored most extensively in the search for anthropoids is the Fayum Depression of northeastern Egypt. This is the richest source of mammalian fossils in Africa. Fayum was tropical in the Eocene/Oligocene Epochs. In this warm and wet climate lived a variety of anthropoids. The most significant families and genera that have been found are:
The Parapithecids. Relatively primitive in dentition and limb features in comparison to other anthropoids, these animals ranged in size from small primates estimated to be no more than about 300 grams in mass such as Qatrania wingi to a larger specimen ranging up to 3000 grams, Parapithecus grangeri. Certain parapithecids are thought to have had large olfactory bulbs in their brains (denoting, perhaps, a keener sense of smell than other primates), and members of the genus Apidium appear to have been excellent leapers.37
The Propliopithecids. More advanced in their dental formula than the parapithecids but more primitive in their cranial and postcranial anatomy than modern Old World monkeys, their most famous representative was Aegyptopithecus zeuxis, which lived about 30 million years ago. Aegyptopithecus was a relatively large animal, estimated to have been around 6700 grams in mass. Despite its small brain, it was probably one of the most intelligent animals on the planet at the time it existed. Many areas of its body are represented in the fossil record. Another major genus of propliopithecids was Propliopithecus, of which only teeth and parts of limbs have been uncovered. Members of this genus appear to have been frugivorous, arboreal, and characterized by prehensile feet. This family is quite possibly very phylogenetically significant, as it may have been at the base of the African primates that produced the modern great apes and humans.38
The Oligopithecids. This includes the genera Catopithecus and Oligopithecus. These primates possessed the modern anthropoid dental formula. They probably ate both insects and fruit (based on an analysis of their teeth). Judging from its eye orbits, Catopithecus appears to have been diurnal, and its limb bones suggest an animal that was an arboreal quadruped.39
The preeminent researcher of African anthropoids is Elwyn Simons. In 2005 Simons and a number of his colleagues announced the discovery of specimens in Egypt of an anthropoid genus known as Biretia. The find was dated at 37 million ybp, and its discoverers and researchers considered it definitive proof that the Haplorhines originated in Africa. In fact, it was their contention that Biretia and a genus designated Algeripithecus represented an ancient African anthropoid clade.40
However, this finding was later dealt a substantial blow. Advocates of an African origin of anthropoids have often pointed to Algeripithecus minutus, which lived about 45 mya in what is now Algeria, to buttress their case. But in 2009 it was announced that on the basis of the discovery of more complete fossils of Algeripithecus, it must be concluded that the animal was definitely not an anthropoid. Algeripithecus and its sister genus Azibius were actually strepsirhines, and the dentition and jaws of these animals show this clearly.41
The evidence, therefore, seems to be shifting in favor of an Asian origin of the anthropoids. However, no conclusive judgment in this matter is yet possible. It would appear that the greatest likelihood is that anthropoids first evolved in Asia relatively soon after the evolution of the primate order itself. Several families of them appear to have colonized Africa, and from these African “immigrant groups” the line of animals that ultimately produced the African great apes emerged.
As far as when specific anthropoid characteristics evolved, a group of researchers working in the area of Haplorhine evolution has put it this way:
We do not know the order in which most of [the] crown anthropoid features evolved. Most features probably appeared in a mosaic fashion in stem anthropoids. Some features may be primitive for Anthropoidea or Primates as a whole and others may have evolved in parallel in multiple crown anthropoid clades. We do know that most of the hard tissue features…are evident in Afro-Arabian fossils of late Eocene age, although several taxa lack some derived features found in living anthropoids... We do not know the character states for many of the presumed Eocene anthropoids from Asia because they are not yet known from adequate cranial materials. Postcranial materials of putative Asian stem anthropoids have been found in isolation, complicating their specific attribution.42
The concept of a developmental mosaic is key because it emphasizes again that evolution and adaptation are not nice, neat, linear, straightforward processes. There are a great many environments exerting a great many selection pressures over a very wide span of time and a huge stretch of geography. It took many millions of years to bring about the advent of the “advanced” anthropoid types. Traits emerge and disappear, the same trait emerges (although not in an identical fashion) in different animals living in similar settings, and behavioral routines evolve that either enhance reproductive fitness or detract from it. The story is only now becoming clearer, and many, many tiles in the mosaic have yet to be found.
The Evolution of Hominoidea
It was these first anthropoids that formed the base of the huge primate clade of the eastern hemisphere, the Catarrhines. It is not yet clear when these earliest anthropoids began to differentiate into animals we would recognize as true monkeys (or monkey-like animals), although the evidence for such primates as Eosimias suggests monkey-like anatomy and behavioral adaptations. Catopithecus browni, a somewhat primitive Fayum primate, is thought to have been a catarrhine, quite possibly among the very oldest. Aegyptopithecus zeuxis has been described as an early catarrhine as well, a mixture of primitive and more specialized features. It is the features of the cranium and teeth that differentiate these animals from the earliest known anthropoids, although the differences can be subtle. There seems to have been a great deal of evolutionary development between the time of Catopithecus and the appearance of Aegyptopithecus.43
There is a significant gap in the African primate fossil record, a seven million year stretch between the late Oligocene Epoch (around 30 mya) and the early Miocene Epoch (around 23 mya). This is particularly frustrating to paleontologists because they wish to identify the earliest true Old World monkey of modern aspect (currently a primate known as Victoriapithecus macinnesi holds that title at 19 million ybp)44 and they want to identify the point at which a tremendously significant genetic split took place: the divergence between Cercopithecoidea, the Old World monkeys, and Hominoidea, the great apes and the humans. The discovery of a fossilized partial cranium of a catarrhine, Saadanius hijazensis, in what is now Saudi Arabia, has triggered a major debate about the timing of this split. Saadanius, dated at between 28 and 29 million ybp, appears to possess a combination of cercopithecoid and hominoid features, suggesting that the split of the great cercopithecoid-hominoid clade took place between 29-28 and 24 million ybp. One of Saadanius’s discoverers characterizes this animal as an ape-monkey intermediate.45 Other scientists, however, strongly dispute this interpretation. One critic has contended that an ancestor-descendent relationship between Saadanius and extant catarrhines has not been demonstrated definitively, and that this specimen could very well represent a sister taxon to the modern catarrhines, a detour on the road to the split between monkeys and apes, not part of the main highway.46 As we have already noted so often, more physical evidence is absolutely necessary to settle these questions beyond dispute.
At this juncture, it may be useful to explain the distinction between a monkey and an ape. Monkeys very often have tails. Apes lack this feature completely. Most monkeys tend to be arboreal in habitat. Apes, while having the physical capacity for climbing, are generally (but not always) terrestrial animals. Monkeys have molars of a different shape than those of apes. With the exception of gibbons, apes tend to be larger than monkeys. Most importantly, perhaps, apes tend to have a higher brain-to-body mass ratio than monkeys. What can molecular evolution tell us about when they branched off from each other?
In 2009 a team of molecular biologists did a broad study of divergences within the primate order. They noted that estimates of the Cercopithecoid-Hominoid divergence time have yielded a very wide range of dates. On the basis of their research, they put the dividing line at about 29.3 million ybp, which, they say, accords with other recent estimates. They further estimate that the split between the Hylobatidae, the lineage containing the gibbons and siamangs, and the Hominidae, the lineage that contains the chimpanzees, bonobos, orangutans, gorillas, and humans, took place around 21.5 million ybp. The authors point out that their calibration points are based on a wide array of fossil evidence representing as much of primate phylogeny as can be accounted for at present.47
The earliest primate thought to have been a possible hominoid was Kamoyapithecus, found in Kenya, and dated from 27.8 to 23.9 million ybp, in the late Oligocene. Paleontologists feel they are on firmer ground with the group of Miocene primates known as proconsulids.48 (See below.) The first primate considered to be a probable true hominoid, and one of the specimens thought to be of key importance in the ultimate evolution of hominids, was Morotopithecus bishopi, the remains of which were first discovered in Uganda in the 1960s. It has been dated at a minimum age of 20.6 million ybp. Analysis of these finds, along with the discovery of additional material in the 1990s, has shed new light on the development of hominoid locomotor abilities and the evolution of hominoid morphology. Parts of the animal’s cranium, dentition, vertebral column, femur, and scapula have been uncovered. Morotopithecus appears to have been arboreal and quadrupedal. It apparently had prominent forelimbs, strong climbing abilities, and the ability to hang off of branches. If further research confirms its similarities to modern apes, Morotopithecus might be seen as a true ancestor of humans.49
The Hominidae and Homininae
As we noted above, the primates classified as members of Hominidae are the great apes, both Asian and African, and humans. In addition to trying to elucidate their ancestry, we are going to look in particular for the ancestors of a subfamily of Hominidae known as the Homininae—the African apes (the gorillas, chimpanzees, and bonobos and their extinct relatives) and the humans (and their extinct relatives). In order to do this, we must focus on the primates of the Miocene Epoch, Morotopithecus being an early example. It was during the Miocene Epoch, from about 23 million to 5 million ybp, that many of the specific features that would ultimately become part of our genus evolved. It was from certain lineages of Miocene hominoids that the direct ancestors of the genus Homo emerged. The challenge for paleontologists and paleoanthropologists has been to place the discoveries that have been made in their proper context. This challenge is a daunting one, to say the least.
A very well-known genus of Miocene ape is known as Proconsul. First discovered in Kenya in 1909, the largest number of specimens has been found on Rusinga Island, in the eastern, Kenyan part of Lake Victoria. There were, by the best calculation, four species of them, ranging in size from that of a large monkey (perhaps 22-25 pounds) to almost as large as a gorilla (approaching 190 pounds in size). All of the specimens recovered existed between 21 and 14 million ybp in what are now Kenya and Uganda. They possessed a 2.1.2.3 dental formula, as all Miocene apes and modern Old World primates do, and surprisingly, their brain-to-body mass ratio was not very much different from that of modern African apes and monkeys of comparable size.50 There is intense debate about Proconsul’s phylogenetic relationship to both the modern apes and humans. Many paleontologists believe Proconsul to be a stem hominoid genus, while others point out that its anatomical features do not show significant synapomorphies [derived traits in two or more taxa that appear to demonstrate common ancestry] with modern hominoids.51
Other significant Miocene ape genera include, but are not limited to:
Dryopithecus. A hominoid genus the remains of which have chiefly been found in Europe, these primates lived from 12-13 mya to about 9 mya. Four species of them are known. They appear to have been frugivorous, suspensory [meaning simply that they could hang from branches and move about by doing so], arboreal, and capable of living in a wide variety of environments. In certain ways these animals displayed crucial hominoid-like traits. They had many cranial and dental similarities to modern African apes, and may in fact be ancestral to them.52
Pierolapithecus. Another primate from the Middle Miocene, about 12-13 mya. This animal was a mixture of primitive and derived traits, the more modern features being found in the thorax and the vertebrae. It seems to have had an ape-like facial structure. Although there are paleontologists who postulate that Pierolapithecus may be ancestral to both apes and humans, their arguments are not yet widely accepted. Pierolapithecus’s remains were found in Spain, but it is thought to have lived in Africa as well.53 It should be noted here that in the Middle Miocene there were still widespread hominoid populations in Eurasia which are thought to have been the product of the travels of African populations.
Kenyapithecus/Nacholapithecus. Paleontologists and paleoanthropologists differ about the role of these primates in hominoid evolution. The first specimen of Kenyapithecus was discovered in the 1960s, and examples were still being unearthed in the 1990s. The oldest members of this clade go back to around 15 million ybp. Kenyapithecus seems to have had the kind of facial features, dentition, and, possibly, method of locomotion common to modern African apes. The discovery of an animal similar in certain ways to Kenyapithecus and yet displaying crucial derived traits has led to the naming of a new genus of Miocene ape, Nacholapithecus. Interestingly, this genus of primate may have had the ability to move itself on the ground, and it may have been capable of something which is of the greatest significance: the ability to assume an orthograde posture—the ability to hold itself upright.54
There are many other Miocene primate genera that have been identified, but gaps in the record (slowly being filled in—see below) have impeded our ability to trace a definitive line of descent leading to true hominins—the animals directly ancestral to humans, extinct varieties of humans, and modern humans themselves. Paradoxically, the more specimens that are unearthed, the more intense the debate seems to be getting. What we can say is that certain primates living in the latter part of the Miocene Epoch of Africa became increasingly terrestrial in their living habits, increasingly able to move themselves along the ground, and increasingly able to hold themselves upright for certain lengths of time.
The Evolution of Bipedalism
It is necessary at this point to examine some of the hypotheses concerning the development of bipedalism—the ability to walk on two feet. There were reptiles that were bipedal in the age of the dinosaurs, and of course the descendants of dinosaurs, the birds, very often move on the ground through bipedal walking. But habitual obligate bipedalism is (or was) found only in the hominins, and it is now one of the distinguishing traits of the genus Homo itself. Some researchers are convinced that it had its origin in a mode of locomotion known as knuckle-walking. [This method, which is employed by contemporary gorillas and chimpanzees, involves using the forelimbs to help stabilize an animal on the ground in conjunction with the movement of the hind limbs.] In this hypothesis, humans were descended from a line of primates that had been both terrestrial and quadrupedal for a very long time.55 Other data indicate, however, that knuckle-walking evolved in the African apes that are sister taxa to humans but not in the line of primates leading to humans. Gorillas and chimpanzees do not knuckle-walk in the same way biomechanically, and there are key differences in the anatomies of gorilla and chimpanzee forelimb structures, particularly those of the metacarpals. It would seem, therefore, that knuckle-walking evolved independently in each lineage. Further, some of the features that previous researchers had said were necessary for knuckle-walking are absent in modern African apes. The conclusion of the authors presenting this research:
The results of this study show that researchers need to reevaluate all posited knuckle-walking features and reconsider their efficacy as indicators of knuckle-walking behavior in extant and extinct primates. In this context, the absence of several posited knuckle-walking features in extant knuckle-walkers (and the presence of some of these features in nonknuckle-walkers) makes it difficult to argue that there is unambiguous evidence that bipedalism evolved from a terrestrial knuckle-walking ancestor. Instead, our data support the opposite notion, that features of the hand and wrist found in the human fossil record that have traditionally been treated as indicators of knuckle-walking behavior are in fact evidence of arboreality and not terrestriality.
The study’s authors do not necessarily reject the hypothesis that knuckle-walking may have evolved in the primates ancestral to humans, gorillas, and chimpanzees, and the differences we see in gorilla and chimpanzee knuckle-walking are adaptations that came after the split with the human lineage. Nevertheless, they think it likely that bipedality evolved from an arboreal, not a terrestrial ancestor.
56 The implications are huge: it means that our ancestors may have stood up regularly
before they adapted themselves to ground living. Bipedalism may have had its uses in the trees. It survived on the ground because it was probably even more useful there.
It is worth noting that bipedalism may have evolved independently in several species, (see below) and that its evolution probably did not proceed in a simple, linear manner. It is further worth noting that the earliest bipedal primates may not have been able to hold themselves fully upright, nor may they have been able to hold themselves upright for prolonged periods of time. We must not assume that our ancestors’ bipedalism was as developed as our own.
As we have seen before, major changes in an animal lineage’s physical adaptations tend to involve a number of factors working in combination and/or influencing each other in a synergistic way. Animals that walk upright have identifiable and distinct anatomical features. Those associated with bipedalism include:
--an increase in the relative length of the legs
--a femur that angles toward the midline of the body and a slightly outward bowing of the knee
--the evolution of a long and curved lumbar (lower) spine
--big toes in line with the midpoint of the body
--a large heel and a stable ankle
--a change in the structure of the ilium and the musculature associated with it, specifically the role of the gluteus maximus as an extensor (straightening muscle) of the thigh and the gluteus medius and gluteus minimus as abductors (extending muscles) of the femur. These abductors act to balance the body during the stride.57 Another key structural feature indicative of bipedalism is the position of the foramen magnum, the opening in the skull through which the spinal cord passes in order to connect to the brain. If the foramen magnum is located at the base of the skull rather than the back of it, there’s a pretty good chance the animal is bipedal.58 We must assume that the evolution of these features happened in different ways in different ancestral primates, in a mosaic-like fashion, and that these features, once they began to come together in certain lineages, must have been mutually reinforcing ones.
Many researchers have naturally focused in particular on the evolution of the foot. The discovery of hominin foot bones by paleontologists and paleoanthropologists gives us major clues about the nature of bipedalism in different primates. The investigators of fossil foot remains believe that there was a great diversity of hominin foot types. It also appears that the feet of several early hominins show a mixture of ape-like and human features, but not always the same mixture. In fact, these differences seem to buttress the belief that bipedalism evolved independently in different lineages of primates, and that there were different modes of primate bipedalism. Different varieties of hominin may have used bipedal locomotion in different kinds of environments, perhaps adapting to a mixture of arboreal and terrestrial settings.59
Of great interest to scientists studying bipedalism’s evolution are the footprints found at the Laetoli Formation in Tanzania. The formation has been dated at 3.6 million ybp. The first footprints at Laetoli were recognized in 1976, and since that time they have been the subjects of painstaking analysis. There are many types of animal footprints in the formation, but of particular interest are footprints that appear to have made by a hominin. There is a difference of opinion concerning the number of individuals who made the prints, and naturally other aspects of the finds have been the subject of debate. A primate known as Australopithecus afarensis (see below) existed at the time the footprints were made, and many investigators believe these prints to be theirs. If they were indeed made by A. afarensis, the evidence indicates these animals may have been fully bipedal, with more flexible, ape-like feet than humans. Other researchers investigating Laetoli suggest that the animal that made the footprints may have been both bipedal and arboreal. Many researchers agree that the animals that made the prints tended to walk with a gait that was similar but not identical to that of modern humans.60 The Laetoli footprints are the earliest direct evidence we have of bipedalism, but most researchers assume its origins go back farther in time.
If we assume that bipedalism, at least in its rudimentary form, existed in arboreal primates, then obviously terrestrial life did not give rise to it. But terrestrial life was tremendously facilitated by it, and the adaptation of bipedalism to terrestrial uses was of enormous significance. What might have been the selection pressures that encouraged the use and development of the bipedal posture?
--Food acquisition. A bipedal animal is a more effective forager, and can transport food more readily over long distances. Early bipeds may have also been scavengers, and the availability of grasping hands for this task may have been crucial. If the first bipeds on land were omnivorous, these capabilities would have been especially advantageous.
--Predator avoidance. The ability to stand upright and see over tall grass to spot potential threats would have been a strong selector for bipedalism.
--Enhanced reproductive success. The ability to transport food by males may have made it possible for females to stay sequestered at “camp” sites and care for infants, thus improving the chances that those infants would survive. It should be pointed out here that bipedalism would have been part of a whole complex of traits enhancing sexual bonding and reproduction.61
We do not think that tool-making was one of the original survival skills arising from bipedalism, since bipedal walking preceded tool-making by at least a million years. But it was the freeing of the hands, in conjunction with a brain capable of conceptualization, that was to be the great builder of human technology-based societies. Bipedalism may not have arisen on the ground, but it was in a terrestrial setting that it found its greatest expression. Its tremendous usefulness made survival possible for the species that put it to its best uses and developed it most fully. Without it, human culture would have been inconceivable.
African Great Apes of the Mid-Late Miocene
We briefly examined some of the important lineages of Miocene apes, and we noted that gaps in the record had impeded our understanding of the evolution of hominins. In the late 20th and early 21st centuries, there was some progress made in filling these gaps, although a definitive picture has yet to emerge. The most important discoveries have included:
--Chororapithecus abyssinicus, a new species claimed on the basis of nine teeth from at least three individuals, discovered in Ethiopia. This find, dated at 10-10.5 mya, appears to be from animals ancestral to the modern gorilla line, although the exact phylogenetic relationship of abyssinicus to gorillas has yet to be determined.62
--Nakalipithecus nakayamai, a new genus and species discovered in Kenya, dated from 9.9-9.8 mya. Based on the animal’s mandible, incisors, and molars, it is also thought to have been gorilla-like in anatomy. It is similar in certain respects to Ouranopithecus, an animal discovered in Greece and thought to be a possible ancestor of the African great apes and humans.63
--Samburupithecus kiptalami, discovered in Kenya and dated to 9.5 mya. This primate is known from its maxilla and dentition, which indicate, as the other finds do, an animal of gorilla-like size. It may have a phylogenetic relationship to the later African apes.64
Obviously, scientists will press the search for possible ancestral hominids further. But these finds are indications that there was a continuous African ape lineage throughout the Miocene Epoch.
The Splitting of the African Ape Lineage
Scientists have sought to determine, through the techniques of molecular genetic analysis used in conjunction with fossil evidence, whether the gorillas or the chimpanzees are the closest relatives to us. The preponderance of evidence is that the gorillas went in their own direction before the chimpanzee-human split. However, the timing of the human-chimp split (or more precisely, the split between hominins and the ancestors of the modern chimpanzees) is a matter of debate. Most estimates have put the split between 7 and 5 million ybp. A study announced in 2005 made an estimate based on two factors: first, an assessment of when the Old World monkeys and apes diverged, and second, the possibility that hominids existed at 6 million ybp. Based on the various assessments of the Old World monkey-ape split, and the effect the timing of this divergence would have had on subsequent branching, these researchers estimated that the human-chimp divergence occurred between 4.9 and 6.6 mya.65 Another group of researchers, comparing sequences of DNA base pairs from four different primate lineages (orangutans, gorillas, chimpanzees, and humans) puts the split between humans and chimpanzees at only about 4 million ybp. They emphasize that this date indicates the complete divergence of the two lineages, implying (if I am interpreting this correctly) that the human-chimp divergence may have begun much earlier than 4 mya:
Our molecular dating estimates are generally in agreement with a large number of studies using different calibration points…[which] found a molecular divergence of HC [human-chimp lineage] at 5–7 Myr, 6 Myr, and 5 Myr, respectively. Speciation, defined as the total cessation of gene flow, is necessarily more recent than these molecular dates, and our value of approximately 4 Myr agrees very well with the time suggested by Patterson et al. for complete cessation of gene flow. It is also in agreement with the oldest fossils generally accepted to belong to the human lineage after the HC split. The autosomal analysis alone cannot be used to determine if the large variance in coalescence times of human and chimp along the genome is due to a large ancestral effective population size or due to prolonged speciation.66
Controversy has arisen over the contention that not only was the split between the human and chimpanzee lineages prolonged, it was marked by interbreeding between the two emerging (but not yet fully diverged) species. The researchers who put forth this hypothesis believe that hominins and ancestral chimpanzees diverged from each other in two stages, and that gene flow between them did not end until the final split occurred, an event, in their view, which occurred no later than 6.3 mya, and probably more recently. These scientists contend that the low rate of divergence of Chromosome X [one of the sex-related chromosomes] and the high rate of divergence of
autosomes [non-sex related chromosomes] between humans and chimpanzees is suggestive, and could be an indicator of interbreeding, or as they also put it,
hybridization.
67 This hypothesis has been vigorously contested, and the authors of a recent study argue that the difference between the divergence of autosomes and X chromosomes in chimpanzee and human populations can be accounted for simply by the
coalescent process [the way in which, going back in time, lineages increasingly narrow as they approach common ancestors] in large ancestral populations of hominins and ancestral chimpanzees. In their view there is no need to postulate a complex hybridization process, and indeed the model of human-chimp speciation that assumes no hybridization occurred best conforms to experimental data.
68
It should be noted here that whether there was interbreeding between ancestral hominins and ancestral chimpanzees or not, the genetic evidence of the relationship between humans and chimpanzees is very strong. (It needs to be stressed, by the way, that chimpanzees are part of a modern genus that has itself undergone changes over the last several million years.) A systematic comparison of the human and chimpanzee protein sets—their proteomes—shows a very marked similarity in orthologous proteins (those that show descent from a common ancestor). In the genomes of the two genera, the total divergence of nucleotides between them was found to be about 1.2%, but of course that average divergence masks greater or lesser divergences in specific areas, and it does not mean that humans and chimps are “98.8% similar.” There are, of course, obvious differences in general appearance between the two, but there are other, less obvious differences. Chimpanzees, for example, do not appear to be vulnerable to Alzheimer’s disease. Chimps and humans have different immune and inflammatory responses, and they have differences in their resistance to parasites. Still, the genetic similarities are striking, and there can be no doubt that chimpanzees are the sister taxon to humans.69
NOTES:
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