Chapter ten: from tree shrew to ape
Evolution of Early Primates
The proliferation of mammalian forms from the end of the Cretaceous period, around 65 million years ago (mya) coincided with the extinction of many of the other life forms that roamed the earth at the time (including dinosaurs). This depopulation of the planet likely opened up many new ecological niches, resulting in the rapid increase in mammalian species following the Cretaceous-Tertiary boundary. Primates—or, more accurately, their tree shrew–like progenitors—were one of these opportunistic, niche-filling mammals (Fig. 10-1). Given that many mammalian types emerged at this time, how is it that primates acquired the peculiar battery of characteristics that would prove so instrumental in the later proliferation of the order? Additionally, how do we know what we know about life on earth from millions, or even hundreds of millions of years ago? How can characteristics specific to primates, that evolved so long ago, be evaluated and accurately contextualized with the evidence available?
In order to develop theories like those above about the origins of primates—not to mention theories about the origin and proliferation of all animals—fossil evidence is required. Only a very small percentage of all the organisms that ever lived, however, actually become fossils, or the mineralized versions of the bones left behind. In the right conditions though, fossilization does occur, and then the second requirement for accurate paleoanthropological reconstructions is the question of When? Without being able to know when a certain animal lived, we cannot discover its evolutionary or adaptational relationship to those that came before or after.
The most effective way to date fossil specimens involves the use of radiometric methods in calibrating the date of fossils or rocky matrix around them. That is, analysts can measure the process that effects the transformation of one element into another over a certain amount of time. For example, Carbon-14 slowly decays into Carbon-12 (different isotopes, or variants, of the same element) after an animal has died. This process happens at a constant rate, so that measuring the ratio of Carbon-14 to Carbon-12 in a particular fossil can reveal how long that animal has been dead. Other radiometric dating methods use the matrix around fossils, such as potassium-argon (K-Ar) dating, which measures the proportion of these elements in volcanic rock. Other methods involve heating rocks or stone tools to estimate the amount of electrons trapped inside a given piece of flint (thermoluminescence), or counting electrons by subjecting inorganic material like tooth enamel to a magnetic field (electron-spin resonance dating). These values are then compared to those at a given archaeological site. Different radiometric methods work better on different materials and also are more effective for certain time periods. Isotopes that decay quickly, like carbon, are effective only for “younger” fossils, say those from about 40,000 years ago, while isotopes that have a slow rate of decay are better for older sites and fossil specimens. Additionally, paleoanthropologists also use relative methods of dating to supplement radiometric determinations. The environmental context of a particular site, and what other sorts of animals were found nearby, are key to the reconstruction of ancient animal life.
Figure 10.1 Therapsids, reptiles with hair instead of scales (who were also possibly warm-blooded), dominated the Earth before dinosaurs did approximately 250 mya. This is a reconstruction of the therapsid Thrinaxodon, a 12-inch-long animal adapted for a generalized carnivorous diet.
Image Credit: W.W. Norton.
How did primates evolve?
An early theory of primate evolution related the general primate characteristics of grasping hands and feet, orbital frontation (increased binocular vision), and enhanced cognitive processing capacity to the challenge of arboreal life. This theory explained the enhanced visual-motor systems and cognitive elaboration in primates as products of life in the trees.
One major weakness in the argument for this older theory, however, is that some very successful arboreal mammals possess none of these traits. A classic example is tree squirrels, common animals that have little visual overlap, nonopposable digits, and smallish brains. Yet, as any walk through a park may attest, squirrels are successful in their own right.
As a response to this vexing problem with the arboreal theory, in 1974 Matt Cartmill (now of Duke University) proposed a new theory to account for the evolution of primate characteristics. He suggested that arboreal predatory behavior (that is, searching up in trees for food) 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). A weakness in the visual predation theory, though, 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. They emphasize olfactory and auditory cues in the pursuit of prey.
In light of these continuing challenges, Robert Sussman (Washington University) has argued 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 (the highest branches in trees) all allowed for efficient acquisition of a resource with an angiosperm-like distribution. In addition, the emergence of flowering plants in the Paleocene roughly coincides with the emergence of the earliest primate ancestors. These early primate-like animals (called plesiadapiforms) had some fully primate traits and not others. For example, a fossil found by Jonathan Bloch and Doug Boyer (University of Michigan) had a flat nail on an opposable toe, but claws on all its other digits. The claws would have been effective for climbing large tree trunks, but the opposable digit must have assisted in grasping smaller, more terminal branches.
The First Anthropoids
Although the earliest potential anthropoid primates may have emerged close to 50 mya, the first unambiguous anthropoid 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 the monkey Catopithecus brownii, a diurnal (active during the day), arboreal quadruped that 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 (220.127.116.11). Since New World monkeys have different dentitions (18.104.22.168 or 22.214.171.124), C. brownii is not likely to be ancestral to any of the New World species. (For a review of dental formulas see Box 5.1 in Chapter 5.) Later fossil primates from the Fayum, including Aegyptopithecus zeuxis, are grouped into the parapithecoids and the 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 more than 100 mya, 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 (Fig. 10.2).
Figure 10.2 A lighthearted view of the migration of New World
Image Credit: W.W. Norton.
The problem with the first interpretation is the tremendous distance involved in a trans-Atlantic journey (by the late Oligocene that journey would have already been more than 2000 miles). Yet, large chunks of land 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 for the origin of New World monkeys.
The second interpretation is problematic because North America does not have any fossil 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 the ocean 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 errors 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 great.
The Evolution of the Hominoids
The hominoids (ancestors of apes and humans) first emerged in the late Oligocene (ca. 27 mya) in Africa. These early forms are represented by the genus Proconsul. Elements of their dentition, cranium, and postcranial anatomy suggest they were quadrupedal and frugivorous, not unlike earlier fossils. The proconsulids, however, lacked tails and had 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, Oreopithecus, Ouranopithecus, and Sivapithecus. These forms were distributed throughout Africa, Europe, and Asia, suggesting that 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, Sivapithecus and the modern orangutan), none of them appears 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, bipedalism. This adaptation had profound effects on the trajectory of hominid evolution.
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