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, and so I will be dividing 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".
LIFE IN THE TREES: THE PRIMATES EVOLVE
EARLY IN THE MORNING OF 30 DECEMBER; ABOUT 996,000 METERS UP THE LINE
Now, the end of the year is in sight. The map of the world is gradually becoming one that humans would recognize, even though many changes are still to come. With the extinction of the non-avian dinosaurs, the mammals of the Cenozoic Era will begin to diversify remarkably in form, size, and habitat. They will reshape life on every continent except Antarctica, and will even make their presence felt in the world ocean and in the skies. As we saw in the last chapter, the earliest known eutherian mammal was adapted for climbing as long ago as 160 million ybp. Now, a hundred million years later, there are mammalian populations living among the tree branches of certain forest regions. We will probably never know when arboreal life became the norm for mammals such as these. And in regard to one particular variety of tree-dwellers—the primates—we need to look at the extraordinary physical traits they acquired over the course of their tens of millions of years of existence.
In every evolutionary transition we have examined, the form of a new animal emerges before there is an animal that we can say is definitively of a new type. For example, the earliest tetrapods were not yet fully amphibian, even though they may have had certain amphibian characteristics. There were reptile-like amphibians before there were reptiles. There were non-mammalian synapsids that had a number of mammalian traits. So it was with the primates. In examining the lineage that led to primates, we have examples of animals that showed primate-like traits but which were not necessarily true primates. So when have we arrived at the point where we can say an animal is a true primate? How is a primate defined? Scientists from Carolus Linnaeus, who in the 18th century devised the first truly scientific way of classifying organisms, to modern primatologists, have tackled that question. It is not simply a matter of listing particular features; it is also considering the possession of certain features in combination. There seems to be a consensus on the following points:
• In regard to the skeletal system: the possession of clavicles, the possession of specialized bone structures around the eye (either a postorbital bar or a more-encompassing structure called a postorbital closure, which is typical of monkeys, apes, and humans), a braincase larger in proportion than those of other mammals, incisor, molar, and canine teeth (at some stage of life), with four incisors in the upper jaw. [It should also be noted here that the possession of a petrosal bulla, part of the skeletal structure of the middle ear, is a trait absolutely unique to primates.]
• In regard to the appendages: the possession of nails or claws (tending toward the possession of nails in more “advanced” primates), a distinct big toe (hallux) on the feet, a big toe that is widely separated from the other toes except in the case of humans, hands and feet adapted for grasping (prehension), a tendency toward opposability [the ability to bring a toe or finger in contact with the other toes or fingers for the purposes of grasping an object] in at least one digit on the hands and/or feet. [Opposable thumbs are the most common example in primates.]
• In regard to other general features of the anatomy: a brain size larger in relation to the body than those of other mammals, a shorter snout than most mammals, mammary glands centered in the chest, male genitalia that are pendulous, a well-developed caecum (a section of the large intestine).
• In regard to the senses: a greatly enhanced visual sense, with a tendency toward stereoscopic vision [using both eyes to focus on a single point, which creates the experience of depth-perception] and chromatic [color-perceiving] vision. There is a relatively poor sense of smell (olfaction).
• Other traits: long gestation periods compared to maternal size, relatively slow growth of offspring in relation to maternal size, relatively late sexual maturation, relatively long life-spans.1
Again, it is the possession of these traits in combination that distinguishes primates. Further, the earliest primates did not necessarily possess the most fully elaborated forms of these traits. Primatologists often refer to these as the suite of primate characteristics. How were they acquired?
Hypotheses Regarding the Evolution of Primate Physical Traits
For many years, the dominant hypothesis in primate studies, one that was particularly stressed in anthropology, was the arboreal hypothesis. This set of ideas dates back to the early 20th century, although it has been elaborated on since that time. Basically, this was the argument that such traits as prehensile appendages, stereoscopic vision, and chromatic vision were adaptations to tree-dwelling. This hypothesis also emphasizes a shift in the structure and physiology of the primate brain toward vision at the expense of olfactory capacities. There are seemingly persuasive arguments in favor of this position. Depth perception in the forest maze can indeed be crucial. An adaptation called brachiation, or traveling by swinging from branch to branch, evolved among larger, more “advanced” primates. Depth perception is absolutely essential to this adaptation. Prehensile hands are also well adapted for maintaining one’s grip not just on branches but on life and limb themselves.2 And seeing in color, which is simply the product of the ability to perceive more diverse frequencies of light, is certainly a tremendous advantage in forest life, especially for spotting predators. But are these ideas the only possible explanations for the primate anatomical suite, or are there other, more basic explanations that need to be explored?
An alternative view to the arboreal argument is known as the visual predation hypothesis. First put forth by primatologist Matt Cartmill in the early 1970s, and since modified, its adherents maintain that the source of primate characteristics was the need to see and grab insects for food, insects that were found chiefly on small branches in the lower parts of trees. Indeed, there are many predators in nature with forward-facing eyes, and it is true that the original primates were very likely to have been insectivores. But since there are many predators which lack stereoscopic vision, a variation of the visual predation hypothesis has been offered. Its advocates now contend that nocturnal hunters require forward-facing eyes. Studies of prosimians (the most “primitive” variety of living primates) and comparisons between the eyes of prosimians and those of the earliest primates of the modern type, the euprimates (see below) appear to give credence to this contention. This would seem to argue that the basal primates were a group of nocturnal insectivores moving among slender branches in search of food.3
It should also be noted that, generally, advocates of the visual predation view argue that since there are many mammals that are adapted to arboreal living (such as squirrels) which do not exhibit primate-like features, and have very keen senses of smell, then therefore primate-like traits cannot be attributed to life in the trees per se. Elwyn Simons, a critic of this perspective has asked, rather pointedly, how the adaptations of tree rodents are relevant to the fact that modern primates exhibit the visual and appendicular qualities that they do. [I would ask at this juncture: why does natural selection have to choose only one way of living in a particular environment? All that seems to be necessary is for a way of life to be reproductively advantageous.] He also points out that the anthropoid [ape-like] primate specimens uncovered in the Fayum Depression of Egypt were from animals that were both arboreal and diurnal [daylight-living].4 This would seem to argue that modern anthropoid traits are of great antiquity, and that nocturnal predation wasn’t necessarily the only force that drove primate evolution. There may be other factors at work as well, which suggest that the visual predation hypothesis may not provide a complete explanation.
Still another view is known as the angiosperm radiation hypothesis. Proposed by Robert Sussman, he contends that the radiation of flowering plants (specifically angiosperms in developing rain forests) was the principal driving force in the shaping of primate traits. Sussman maintains that the flowering plants, primates, bats, and those birds that feed off of plants, formed a co-evolutionary relationship with each other, beginning around the boundary of the Paleocene [65.5 million ybp to about 55.8 million ybp] and the Eocene [55.8 million to 33.9 million ybp] Epochs. The angiosperms provided readily available sources of food, and these sources facilitated the rise of all those taxa that fed off of them. Those taxa in turn disseminated seeds, helping angiosperms spread more widely. In this hypothesis, primate traits were shaped by the desire of the earliest primates to reach food that grew on the ends of branches—flowers, fruits, and the insects feeding off of the vegetation. According to Sussman, primates operating in low light conditions needed enhanced visual and manipulation skills.5
There is a synthesis of the visual predation/angiosperm radiation hypotheses emerging. A recent study has suggested that primates developed skills necessary to exploit angiosperms and while doing so grabbed insects in an opportunistic fashion. This foraging pattern was made possible by the ability to move around on small branches while using the appendages to capture prey. The advantage of being able to do these things would have strongly encouraged the evolution of prehensility, and in fact the developments of locomotor skills and prehensile hands seem to have reinforced one another. It is also worth noting that Eocene primates already possessed a high degree of prehensility in their hands.6
It can therefore be argued that the search for insects, fruit, and leaves attracted primate-like animals into the trees and contributed to the evolution of their attributes. Certainly the evolution of precision grip, the ability to grasp and hold very small objects between the thumb and finger, must have been influenced by predation on small insects. But it can also be argued, in my view, that adaptations for predation and fruit-gathering may have been just the beginning of the process of primate development, not its culmination. As primates were attracted to higher elevations in trees, it would seem to me that these habitats must have confronted them with particularly sharp selection pressures. Prehensility would have been of even more crucial importance in such a setting. Stereoscopic and chromatic vision must have been powerfully reinforced by tree-top living, in light of the ubiquity of these features in the primate order. The evolution of forward-facing eyes may have entailed a reduction in peripheral vision, but evidently it was not a fatal one. And we must account for the fact that primates may have started out as nocturnal animals, but the vast majority of them are now awake in the daylight hours. Primate characteristics are in all likelihood traceable to a number of factors operating in a synergistic fashion. In one sense, therefore, it may be said that the various hypotheses concerning the selection pressures that shaped primates are complementary to each other rather than contradictory.
Sister Taxa and Clades
Evolution is a bush, not a ladder. Population A can give rise to an offshoot, Population A1, that can evolve into a separate, distinct species. But Population A doesn’t necessarily become extinct. Often it continues onward. The ancestral form and the descendant form can therefore coexist.7 Only if A1 has gained a strong reproductive advantage over A is A likely to eventually disappear. Further, when two or more kinds of organisms have all branched off from a common (usually extinct) ancestral population, they are known as sister taxa (singular sister taxon) to each other. For example, humans and chimpanzees, having genetically split off from a common ancestor, are sister taxa, and we would say chimpanzees are our sister taxon. At each point going back through time there are junctures of common ancestry giving rise to descendant forms which are sister taxa to each other. Groups of organisms that all have a common ancestor are known as clades. (The study of these branching relationships is called cladistics.) Entire groups of animals, such as the whole of Class Mammalia, can be considered clades. When all these phylogenetic relationships are rendered in graphic form, depicting lines of descent through time and charting evolutionary connections between and among organisms, it gives the appearance of a bushy, treelike, branching object. If it were possible to depict all phylogenetic relationships over the last 3.0-3.5 billion years, beginning with the Last Universal Common Ancestor, (or even from the first metabolic reactions that perhaps helped bring about self-reproducing molecules at 3.8 billion ybp), the bush/tree would be incredibly dense with branches and nodes (the points from which common ancestry begins to branch) and it would be of immense size. In this huge, bushy tree of life, primates are a clade (specifically an order) nested within the larger clade of mammals (a class). It is this clade upon which we are now focused. I must stress that it is genetic analysis that establishes the definition of a clade. So in what animals do we begin to see primate tendencies, and what animals seem to be genetic forebears of the primate line? Further, in what era of prehistory did the momentous events of primate evolution occur?
Approaches to Determining Primate Origins
Two distinct but interdependent methods are used by paleontologists to try to establish the phylogenetic history of a clade. There is the traditional method of evaluating fossil evidence and constructing phylogenetic trees accordingly. Then there are the newer techniques of molecular genetics, which estimate the evolutionary emergence of organisms based on assessments of the genomes of living animals and estimates of the rate of genetic change within a clade over time. Both approaches have their limitations. As we have noted, less than 0.1% of all organic material escapes being recycled into other living matter, making the incidence of fossilization exceedingly rare. In the case of primates, many of the earliest animals were small and delicately built, which compounds our difficulties greatly. When molecular techniques are used, a great deal depends on how the estimate of the genetic distance between types of organisms is calculated. These estimates rest, as we have seen, on calibration rates which start with known points of divergence in the fossil record and other types of data (see below). From these data, rates of genetic change are then extrapolated. As I noted in the chapter on the earliest animal life, there has been controversy surrounding some of the results of this methodology, most notably in estimating the time of the emergence of the first vertebrates. But I also noted, in the chapter on the evolution of mammals, that molecular techniques accurately predicted the period in which the first eutherian mammal had to have evolved, even when the oldest fossil evidence in our possession was from a time tens of millions of years after the estimated point of divergence. So, when used judiciously and with a fossil record that provides crucial evidence of known divergences, molecular techniques can be an effective tool.
In using molecular dating, scientists bear in mind that not all rates of genetic change are identical. In addition to using known divergences from the fossil record, therefore, scientists take into account such variables as the geographic distribution of a taxon (its biogeography), geographic barriers that split members of a taxon from each other and influence their biogeography (a phenomenon known as vicariance), and data from geology on the position of a given landmass in the past. Researchers must also take care to not use calibration rates calculated for one line of animals to date other lines which may be only distantly related. They must also take into account such factors as fossil samples which may be incomplete, in poor shape, or misidentified.8
So we will first examine the fossil record we have in hand, and then examine the molecular estimates, some of which extend the beginning of the primate lineage much deeper into the past than we might expect.
Fossil Evidence of Primate Origins
There is some uncertainty among primatologists about which of the mammals found in the fossil record was the earliest true primate, and there have been vigorous disputes about whether certain animals were or were not part of the primate line. The problem has been a paucity of evidence, and naturally, as we have seen with the other taxa we have examined, new discoveries alter our picture, sometimes dramatically. Based on the fossil record we possess, it appears to many paleontologists that primates evolved out of a line of insectivorous mammals that lived in the late Cretaceous Period, a line that probably also gave rise to flying lemurs (which do not fly and are not true lemurs) and tree shrews. Collectively, the primates, the tree shrews, and flying lemurs all form a clade called Euarchonta, and are descendants of an ancestral or basal euarchontan. As far as which primates came first, the prominent paleontologist Frederick Szalay contends that the paromomyids were the oldest known primate family, on the basis of specimens of the lower jaw, teeth, and basicranium (the bottom part of the skull). The paromomyids were small animals, none of them much bigger than a modern rat. They evolved as a branch of Mammalia during the Paleocene Epoch. Perhaps the best known of the paromomyids was Purgatorius, specimens of which have been discovered in Montana, in the United States. Many species within the paromomyids have been identified, primarily by their dentition, although two skulls have been uncovered. Paromomyids have been discovered in Europe as well as North America, and they extended into the Eocene Epoch.9
There is particular interest in animals known as plesiadapiforms. As late as the 1990s they were being written out of the primate family by some, being consigned to the status of sister taxon.10 But Szalay believes that one particular family, the plesiadapidae were among the earliest primates. The best known genus in this family is Plesiadapis. It is now thought that these primates had diverged from the paromomyids by the early Paleocene at the latest.11 In 2007, the position of the plesiadapiforms within the primate line was strengthened. A team of scientists at the Florida Museum of Natural History, led by paleontologist Jonathan Bloch, announced the discovery of the earliest mostly intact primate skeletons yet discovered, a pair of plesiadapiforms which have been designated Dryomomys szalayi and Ignacius clarkforkensis. On the basis of their exhaustive analysis of these specimens, the researchers believe that the ancestral euarchontan from which primates evolved was insectivorous, arboreal, similar in form to the modern tree shrew, and small in size, perhaps no heavier than 30 grams. They further contend that the radiation of the earliest primates was facilitated by and concomitant with the rapid spread of flowering plants across the continents, which would have provided ample food sources. [These researchers would appear to support the angiosperm radiation hypothesis.] The exploitation of these resources (as we have seen) would have encouraged selection for prehensile hands and feet and the development of locomotor skills. According to Bloch and his partners, diverse kinds of plesiadapiforms radiated through forests for ten million years, and the first euprimates evolved from this radiation by about 62 million ybp, some 7 million years before their first appearance in the fossil record.12
Molecular Estimates of Primate Origins
In the early 2000s a team of scientists offered an estimate of the chronological emergence of primates based on a wide array of molecular analyses of primate phylogeny. It was their opinion that the best estimate was that the primates diverged from other mammals approximately 85 million ybp, some 20 million years before the conventional estimates. These researchers were careful to point out, however, that probably only about 5% to 7% of all the primate species that have ever existed have been discovered, and that the assumption that we have unearthed all the significant finds is an unfounded one.13 Other estimates give highly variable results. One places the earliest possible date of primate divergence from the rest of the mammals at 110 million ybp.14 Another gives an estimated date of around 77 million ybp,15 and another team of researchers puts the range of primate divergence dates at 65-73 mya.16 Several other sources generally support dates in the range of 80-90 million ybp.
Perhaps the most radical hypothesis—and its radicalness does not rule out the possibility that it is accurate—asserts that the best way to ascertain when the primate lineage evolved and then radiated is to calibrate these events to the tectonic record rather than the admittedly very incomplete fossil record. A scientist taking this approach contends that the clade of basal archontans split into the plesiadapiforms, primates, tree shrews, and flying lemurs 185 million years before the present. He argues that it was the break-up of Pangea that triggered these splits. Further, he contends that the two major primate suborders (see below) split from each other in the Early Jurassic Period and the Old World Monkeys and New World Monkeys split as early as 130 mya. He argues that the current fossil and molecular estimates greatly underestimate the antiquity of the primate line, and that a phylogeny based on a widely-dispersed common ancestor makes more sense than the current models. Moreover, this new model solves the thorny problems of primate dispersal across bodies of water.17 If this researcher is correct, the primate line began to diverge into separate clades before the oldest known eutherian mammal, and it began to emerge as an order not too long after (in geological terms) the mammals themselves evolved out of the lineage of non-mammalian synapsids. This hypothesis is still fairly recent, and it remains to be seen whether significant tangible evidence to support it more strongly will be discovered.
The Euprimates Emerge
When paleontologists and primatologists speak of euprimates, they mean animals that resembled what we think of as “true” primates, or, as the term is used, primates of the modern aspect. This means animals that are adapted for leaping (although this adaptation has vanished in humans), eyes that are completely forward-facing, more complex brains, hands and feet that are definitely adapted for grasping (indicated by such features as longer fingers and toes, divergent thumbs on the forelimbs, and divergent big toes on the hindlimbs), and teeth that in the early euprimates were adapted for herbivory rather than insectivory (a key indicator).18 The oldest fossil records we have of the euprimates date back to the late Paleocene/early Eocene, about 56-55 mya. The earliest probable euprimate yet discovered is Altiatlasius koulchii. The samples of this animal, discovered in North Africa, have chiefly been teeth, and its exact relationship to other primates is as yet unresolved. Another North African group known as the azibiids could be in the euprimate clade but the physical evidence we have of them is sparse, and no definitive judgments can be made.19
Despite the finds of possible euprimates in Africa, the place of origin of the euprimates is still a matter of intense research and debate. Their sudden appearance in the fossil records of North America and Europe at the beginning of the Eocene, coupled with the dearth of transitional forms in those areas, suggests to many researchers that euprimates did not originate in those regions. And yet, the very oldest primates, the ones that everyone puts at the base of the primate tree, are of North American origin, which complicates matters. There is evidence to suggest that euprimates might have appeared in Asia at the Paleocene-Eocene boundary [and as we will see below, there are strong advocates for an Asian origin of primates]. But many uncertainties surround the issue of euprimate origins. The difficulty, of course, lies in the enormity of the territory that needs to be explored, and the inherent difficulties associated with discovering fossils, especially ones which are oftentimes so small and delicate in nature. There are many, many areas that have yet to be sampled. The situation is complicated even further by the fact that geographic barriers to the dispersal of euprimate species may have shifted considerably over the many centuries, making their travels hard to trace. There is also disagreement among paleontologists in the matter of how the specimens that have been uncovered should be organized into clades. Since there are promising finds that have been made in Asia, Europe, North America, and Africa, we just cannot yet say on which continent euprimates first evolved, as unsatisfying as that might seem.20
Many scientists have believed that all Eocene euprimates fell into two categories: the adapoids, which traditionally have been called lemur-like animals, and the omomyoids, which have traditionally been called tarsier-like animals.21 Lemurs are “primitive” primates living in Madagascar (see below) while tarsiers are small nocturnal primates with disproportionately large eyes, which live only in Malaysia, Brunei, the Philippines and Indonesia.22 The designations “lemur-like” and “tarsier-like” are somewhat misleading, inasmuch as lemurs are highly diversified animals and a great majority of omomyoids were not really tarsier-like.23 Most scientists now think that the adapoids were the first radiation of one of the two primate suborders, the one known as the Strepsirhini. Living on the margins of the biosphere in generally isolated habitats, the primates of this suborder have an appearance that sets them off from most other primates. Sometimes known as the “lower” primates, they are also frequently referred to as the prosimians. The suborder includes not only lemurs but also the lorises and the galagos. The suborder’s members have smaller and less complex brains than other primates. These animals can possess both claws and nails, and they mark their territory with either scent glands or urine. The females possess a two-chambered uterus (also known as a bicornuate uterus), and they have more than two mammary glands. Many prosimians have a somewhat dog-like facial appearance because of their prominent, often elongated snouts. They have facial and eyebrow whiskers, reminiscent of felines. Their eyes are large, adapted for nocturnal predation, and have a light-reflecting structure at the rear of the eye known as a tapetum lucidum that makes their eyes glow when a light source strikes them. Prosimians tend to have big, flexible ears and a better sense of smell than other primates. All of them have tails, although not always prominent ones.24 If one wanted a glimpse of some of the earliest primates, these animals would give hints of them, because the lemurs and lorises still show marked similarities to the adapiforms, lemurs especially so.25 (Lemurs, by the way, are confined entirely to the island of Madagascar, and the means by which their ancestors reached the island are unclear, although interesting hypotheses are being offered. Lemurs are assumed to be of monophyletic origin and they live in a fairly unique ecological setting, among a group of mammals that are markedly distinct from those of continental Africa.)26
In general, the adapoids were larger than the plesiadapiforms and the omomyoids. Their dentition was primitive compared to that of modern primates. They had long, broad snouts, their eye orbits were encased in bone, and their braincases were larger than those of the earliest primates but smaller than modern lemurs. Their limbs were similar to modern strepsirhines but more strongly built. They had long legs, tails, and torsos, and hands that had nails instead of claws. The adapoids possessed hands with divergent thumbs and prehensile feet. Some of the better known adapoid genera are Cantius, from the early Eocene of Europe and North America, Notharctus from the middle Eocene of North America and Smilodectes, from the early-middle Eocene of North America. There are excellent fossil specimens for these animals, which appear to have been diurnal. Adapis comes from the late Eocene of Europe, and Sivaladapis, from India, was an animal of the late Miocene Epoch [23 million to about 5.3 million ybp]. In addition, among the many other genera of adapoids, there are three from Africa, two of which are from the Oligocene Epoch [33.9 million to 23 million ybp].27 The adapoids and their descendants, despite the marginal niches the strepsirhines occupy today, are a remarkable success story. It is still possible to see in the lemurs and lorises the genetic echoes of the early Paleocene Epoch. There are, in fact, some prominent scientists who suspect the adapoids may have ultimately given rise to the “higher” primates.
The majority of paleontologists have tended to believe that the humble little omomyoids are ancestral to the modern large primates. But a group of scientists working in Asia argues that modern “higher” primates descended from neither adapoids nor omomyoids. If they are right, then much of what we have thought about our deep origins needs to be reassessed. (See below)
Suborder Haplorhini—the Anthropoids
The suborder Haplorhini contains the vast majority of the world’s primates. These primates are more widely known as anthropoids, the “man-like” primates. What are their distinctive traits, and in what ways do these differ from the prosimians?
The senses and sensory organs: Anthropoids do not have a rhinarium, an area of wet, naked skin around or above the nostrils, as do the prosimians. There is a section of bone known as the postorbital septum that separates the eye orbit from the lateral part of the skull, and the eyes themselves face more directly forward than those of prosimians. The retinas of anthropoids possess a macula lutea, in the center of which is a fovea [that section of the retina that has the highest visual acuity and which contains nerve cells called cones that allow for the perception of color]. It is believed that these retinal structures are adaptations that allowed for greater visual acuity during diurnal activities, although anthropoids differ in the degree of their color perception. The visual cortex takes up a larger proportion of the brain than that of the typical prosimian. There is no tapetum lucidum, either. Vision is the dominant means by which the world is examined, with less emphasis on the senses of hearing and smell than is the case with prosimians. The anthropoid inner ear structure is distinctive, as are the blood vessels which supply it.28
The appendages and general bony anatomy: Anthropoid heads tend to be more rounded than those of the “lower” primates. The structure of the foot is distinctive. The great majority of anthropoids have highly flexible hands and feet. Opposability of the thumb (pollex) is very widespread, although the degree of this ability varies significantly, and divergence of the thumb and the hallux is widespread, although not universal. Precision grip is also very common. The arms and legs tend to be more equal in length than in prosimians [although in humans the leg to arm ratio tends to be greater]. The structures of the femur, tibia, and knee differ from those of strepsirhines. The structure of the humerus is distinct in anthropoids.29
The reproductive system: Anthropoid females go through a periodic menstrual cycle. Sexual receptivity in anthropoid females is not tied to the estrus cycle, as in prosimians. Hence, there is more variable sexual receptivity in anthropoid females. There are, however, fluctuations in receptivity based on the period of ovulation. The anthropoid clitoris is relatively small, although there are exceptions to this. Anthropoid females have a single-chambered uterus and the placenta is disc-like in form. Most prosimian males possess spines on the penis; the males of relatively few anthropoid species do. There are two mammary glands only.30
Neuroanatomy: Most important of all, there is a significantly greater relative brain size and complexity in anthropoids compared to that of prosimians. Brain neurons important to facial recognition are linked with the amygdala, which assigns emotional responses to perceptions. It is this connection which may explain the significance of facial expressions in anthropoid social interactions. [It should be pointed out that many anthropoids possess a complex musculature in the face.] The anthropoid encephalization quotient (EQ) [a measure of expected brain mass to body mass] is 2.1, meaning that the anthropoid brain is twice as large on average as that of a given placental mammal of about the same size, although there is great variation in this EQ from species to species. In certain monkeys the EQ is only about 1.05, while in humans it is close to 6. The anthropoid neocortex, the most advanced part of the brain, seems to correlate in size with the size of the social groups typical of various species, although there are many other factors that influence neocortical development, and social group size may be a consequence of neocortical complexity. Brain growth also seems to correlate with the age of the first reproduction and the length of time it takes for an anthropoid to reach full maturation, although it is quite possible that brain complexity and the length of anthropoid juvenility have a reciprocal relationship. Bigger and more complex brains take a longer time to educate, after all. Further, most neuronal development in anthropoids takes place either prenatally or in infancy. As we will see in much greater detail elsewhere, the evolution of the anthropoid brain was not a simple matter.31
Suborder Haplorhini is divided into three infraorders: Tarsiiformes (the tarsiers), Catarrhini (which consists of Superfamily Cercopithecoidea, the Old World monkeys, and Superfamily Hominoidea, the apes and humans,) and Platyrrhini (more commonly called the New World monkeys). The Old World primates are differentiated from those of the New World by the structure of the nose, some differences in cranial structure, and a different dental formula (how many of each kind of tooth are in each quadrant of the mouth). All Old World monkeys, apes, and humans have a 126.96.36.199. formula, meaning that in each quadrant they have two incisors, one canine, two premolars, and three molars—32 teeth.
(A small linguistic note before we go any farther: the names of the genera of the animals ancestral to modern anthropoids very often include the root word pithecus. This root originates from the Greek word pithekos, which simply means ape or monkey. The first part of the animal’s name generally refers to the locale in which it was thought to live, either a particular region or a particular environment. For example, as we will see below, the genus name Australopithecus simply means “southern ape”.)
TOMORROW NIGHT: The Evolution of the Anthropoids; The Evolution of Hominoidea; The Hominidae and Homininae; The Evolution of Bipedalism; The Splitting of the African Ape Lineage
1. Conroy, Glenn C., Primate Evolution, pp. 4-7
2. E. L Simons, “Convergence and Frontation in Fayum Anthropoid Orbits” in Primate Craniofacial Function and Biology, Chris Vinyard, Matthew J. Ravosa, and Christine Wall, editors, pp. 418-421
3. Isbell, Lynne A., The Fruit, the Tree, and the Serpent: Why We See So Well, pp. 38-39
4. Simons, pp. 418-422
5. Robert W. Sussman, “Primate origins and the evolution of angiosperms” in American Journal of Primatology, Vol. 23, 1991. Article first published online 2 May 2005; Conroy, pp. 41-43
6. David Tab Rasmussen, “The Origin of Primates” in The Primate Fossil Record by Walter Carl Hartwig,
7. Templeton, Alan Robert, Population Genetics and Microevolutionary Theory, pp. 148-149
8. Lindell Bromham, and David Penny, “ The modern molecular clock” from Nature Reviews Genetics 4, 216-224, March 2003
9. Frederick S. Szalay, “Paleobiology of the Earliest Primates” in The Functional and Evolutionary Biology of Primates, edited by Russell Tuttle, Aldine Transaction, 2007, pp. 3-11
10. Fleagle, John G., Primate Adaptation and Evolution, Second Edition, pp. 332-333
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Published online October 27, 2009
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