Chapter 10: From Tree Shrew to Ape
Evolution of Early Primates
The proliferation of mammalian forms from the end of the Cretaceous period (ca. 65 Mya) coincided with the extinction of many of the other life forms which roamed over the earth at that time (including dinosaurs). The depopulation of the planet likely opened up many new niches, which accounts for the rapid increase in mammalian species following the Cretaceous-Tertiary boundary. Primates - or their tree shrewlike progenitors - were one of these opportunistic, niche-filling mammals. However, given that a great many mammalian types emerged at this time, how is it that primates acquired the peculiar battery characteristics which would prove so instrumental in the later success of the order?
An early theory of primate evolution, proposed by F. W. Jones (1916), relates the primate characteristics of grasping hands and feet, orbital frontation (increased binocularity of vision), and enhanced cognitive processing capacity to the challenge of arboreal life. Generally known as the Arboreal Theory of primate evolution, this theory explains the enhanced visual-motor systems and cognitive elaboration in primates as products of arboreal life.
However, one major weakness in the argument for the Arboreal Theory is the fact that arboreal mammals exist which do not possess any of these traits, and yet are very successful. An example of this would be tree squirrels, which have very little visual overlap, non-opposable digits, and smallish brains; however, as any walk through a park may attest, squirrels are quite successful in their own right. As response to this, in 1974, Duke University primatologist Matt Cartmill proposed a new theory - the Visual Predation Theory - to account for the evolution of primate characteristics. Instead, he suggested that arboreal predatory behavior accounted for the grasping hands and feet, and particularly the increased visual overlap and brain size (both characteristics observed in terrestrial predators, such as large cats).
However, a weakness in the Visual Predation Hypothesis lies in the fact that prosimians, considered to be closer to the ancestral form of all primates, exhibit lower reliance on visual information for locomotion and predation; rather, they emphasize olfactory and auditory cues in the pursuit of prey. In light of this fact, Washington University primatologist Robert Sussman proposed the Mixed Diet Theory - namely, that an increased exploitation of angiosperms (flowering plants) selected for modern primate characteristics. Enhanced visual acuity, color vision, and characteristics amenable to exploiting terminal branch resources all allowed for efficient acquisition of a resource with an angiosperm-like distribution. Additionally, the emergence of flowering plants in the Paleocene roughly coincides with the emergence of the earliest primate ancestors.
The First Anthropoids
While the earliest potential anthropoid primates may have emerged close to 54 million years ago, the first unambiguous remains date back to approximately 36 Mya, from a region called the Fayum in Egypt. One of the more complete fossils from the Fayum is a monkey named Catopithecus brownii, a diurnal (active during the day), arboreal quadrupeds, and probably fed primarily on insects. One characteristic linking C. brownii with modern day anthropoids is the shared dental formula between it and all modern Old World monkeys (126.96.36.199). Since New World monkeys have different dentitions (188.8.131.52 or 184.108.40.206), C. brownii is not likely to be ancestral to any of the New World species. Later fossil primates from the Fayum - including Aegyptopithecus zeuxis, pictured at left - are grouped into parapithecoids and propliopithecoids. The propliopithecoids (which includes A. zeuxis) are considered to be possible !
progenitors of modern anthropoid primates.
The New World Monkey Enigma
The puzzling ancestry of New World monkeys derives from a low abundance of fossil material, as well as limitations based on knowledge of continental drift. Similar biological diversity and geology suggest that the continent of South America was once a part of Africa. South America eventually broke off of the African mainland (through a geologic process called continental drift), and over the course of millions of years, migrated to its present position. Unfortunately, the separation between South America and Africa occurred over 100 Mya ago, which is much earlier than the first primate fossils on either continent. This leads to several possible scenarios for the ancestry of New World primates:
- African anthropoids crossed the Atlantic somehow, and radiated into new habitats upon reaching South America
- North American primate forms gave rise to the current New World species
- African anthropoids emerged earlier than fossils suggest, and rafted across the Atlantic when it was much smaller
The problem with the first interpretation is the tremendous distance involved in a trans-Atlantic journey (by the late Oligocene it was already over 2000 miles). Large chunks of land mass have been known to break off and essentially "float" across large bodies of water as living cargo vessels, although evidence has yet to emerge in support for this hypothesis on the origin of New World monkeys. The second interpretation is problematic in the sense that North America does not have any evidence of anthropoid species. Thus, descent from a prosimian-like common ancestor would require a remarkable number of evolutionary convergences between New World and Old World anthropoids, a concession many systematists find difficult to accept. The third possibility provides a compromise of sorts between the first two explanations, suggesting that an ancestral primate form might have rafted across the Atlantic, but when it was much smaller. This would push back the divergence dates!
for anthropoid primates beyond those supported by the fossil record, but as an old anthropological adage goes, "absence of evidence is not evidence of absence". Estimations of error in fossil sampling suggest that anthropoids may have actually emerged as early as 52 Mya. An earlier emergence of anthropoid primates might allow for a transoceanic voyage before the distance between South America and Africa became too large.
Emergence of the Hominoids
The hominoids (ancestors of apes and humans) first emerge in the late Oligocene (ca. 27 Mya) in Africa. These early forms are represented by the genus Proconsul (pictured at right). Elements of their dentition, cranium, and postcranial anatomy suggest they were quadrupedal and frugivorous, not unlike earlier fossils. The proconsulids, however, lacked tails, and exhibit limb proportions closer to those of modern apes. By the middle Miocene (ca. 17 Mya) a large number of distinct hominoid species emerged, including Dryopithecus, Kenyapithecus, Oreopithecus, Ouranopithecus, and Sivapithecus; these forms were distributed throughout Africa, Europe, and Asia, suggesting an adaptive radiation of hominoid forms occurred during the warmer Miocene epoch. While some of these species have been closely associated with modern forms (for example, see a comparison between Sivapithecus and modern orang!
utan), none of them appear to have any clear link with the earliest hominids. With the aridification of the late Miocene, the majority of hominoid forms went extinct, leaving behind a few modern day forms (Pan, Gorilla, and Pongo). Of more relevance to the study of human origins, the apparent pressure of a shrinking forest habitat may have driven an ancestral ape to adopt a peculiar form of locomotion; this adaptation, which had profound effects on the trajectory of hominid evolution, is manifested in its earliest members, the Australopithecines.
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