Early primates began to emerge during this time of great global change at the start of the Paleocene epoch. The distribution of fossil primates on earth makes sense only when one understands that the positions of the continents today differ tremendously from what was found in the past (see Figure 6.3). As noted earlier, during this period North America and Eurasia were connected in the supercontinent called Laurasia. South America, Africa, Antarctica, Australia, and the Indian subcontinent—previously joined together as the supercontinent Gondwanaland—were beginning to separate from one another through continental drift. Africa was separated from Eurasia by a narrow body of water.
On land, dinosaurs had become extinct, and mammals were undergoing the great adaptive radiation that ultimately led to the development of the diverse forms with which we are familiar today. At the same time, the newly evolved grasses, shrubs, and other flowering plants proliferated enormously. This diversification, along with a milder climate, favored the spread of dense, lush tropical and subtropical forests over the earth, including North and South America and much of Eurasia and Africa. With the spread of these huge belts of forest, the stage was set for the movement of some mammals into the trees. Forests would provide our early ancestors with the ecological niches in which they would flourish. Fossil evidence of primatelike mammals from the Paleocene forests has been found in North America and Eurasia. See Figure 6.5 for a full timeline of primate evolution.
One theory for primate evolution, the arboreal hypothesis, proposes that life in the trees was responsible for enhanced visual acuity and manual dexterity in primates. Misjudgments and errors of coordination, leading to
Figure 6.5 This timeline depicts some of the major events of primate evolution.
Figure 6.6 Ancestral features seen in Eocene and Oligocene primates are still seen in prosimians today. Like modern lemurs, these fossil prosimians have a postorbital bar, a bony ring around the eye socket that is open in the back. Anthropoid primates have orbits completely enclosed in bone. Note also the difference in the relative size of the snout in these two groups. Paleoanthropologists make these kinds of comparisons as they reconstruct our evolutionary history.
Falls that injured or killed the individuals poorly adapted to arboreal life, may have been a part of initial forays into the trees. Natural selection would favor those that judged depth correctly and gripped the branches strongly. Early primates that took to the trees were probably in some measure preadapted by virtue of behavioral flexibility, better vision, and more dexterous fingers than their contemporaries.
Primatologist Matt Cartmill further suggests that primate visual and grasping abilities were also promoted through the activity of hunting for insects by sight. His visual predation hypothesis accounts for the observation that other tree-dwelling species and hunting species do not necessarily possess the same combination of visual and manual abilities possessed by the primates. The relatively small size of the early primates allowed them to make use of the smaller branches of trees; larger, heavier competitors, and most predators, could not follow. The move to the smaller branches also gave them access to an abundant food supply; the primates were able to gather insects, leaves, flowers, and fruits directly rather than waiting for them to fall to the ground.
The strong selection in a new environment led to an acceleration in the rate of change of primate characteristics. Paradoxically, these changes eventually made possible a return to the ground by some primates, including the ancestors of the genus Homo.
The first well-preserved “true” primates appeared by about 55 mya at the start of the Eocene epoch. During this time period, an abrupt warming trend began on earth, causing many older forms of mammals to become extinct, to be replaced by recognizable forerunners of some of today’s forms. Among the latter was an adaptive radiation of prosimian primates, of which over fifty fossil genera are known. Fossils of these creatures have been found in Africa, North America, Europe, and Asia, where the warm, wet conditions of the Eocene sustained extensive rainforests. Relative to ancestral primatelike mammals, these early primate families had enlarged braincases, slightly reduced snouts, and a somewhat forward position of the eye orbits, which, though not completely walled in, are surrounded by a complete bony ring called a postorbital bar (Figure 6.6).
During the Eocene, the first signs of anthropoid primates also begin to appear in the fossil record. Until
Visual predation hypothesis A theory for primate evolution that proposes that hunting behavior in tree-dwelling primates was responsible for their enhanced visual acuity and manual dexterity.
Recently, the earliest evidence consisted of the tiny species Eosimias (pronounced “ee-o-sim-ee-us”; Latin for “dawn of the monkeys”), represented by fossils from China, dated to about 45 mya. The Chinese fossils represent several species of tiny, insect-eating animals and are the smallest primates ever documented.76 77 78 79 80 81 Some scientists have challenged whether these tiny fossils are truly anthropoids as they are reconstructed largely from foot bones rather than skulls or teeth. As described in the chapter opener, in 2009, controversy and media attention swirled around the spectacularly well-preserved 47-million-year-old potential anthropoid nicknamed “Ida” 7,8,9,10 Initially discovered thirty years ago through a mining and drilling operation at an old quarry near the southern German hamlet of Messel, Ida’s remains were separated and sold to different collections. In Ida’s time, the Messel region was a tropical forest with a lake and volcano. Fossils from this region are embedded into flaky layers of rock that split open into “plates.” Now that the two plates that constitute Ida have been reunited, scientists can debate whether Ida is indeed the earliest anthropoid or not. This distinction, which would place her on the line leading to humans, captures our imaginations as described in this chapter’s Biocultural Connection by anthropologist-primatologist Meredith Small.
More recent than Ida and well into the Oligocene epoch, rich deposits of primate fossils have been found in Fayum, Egypt. These fossils include a diverse range of species including some of the earliest to possess a dental comb. But more relevant to human ancestry are the early anthropoid primate species from Fayum, identified through dental, cranial, and postcranial (the rest of the skeleton) remains. Some possess the ancestral dental formula (2-1-3-3) seen in New World monkeys and prosimians, while others have the derived dental formula shared by Old World monkeys and apes: two incisors, a canine, two premolars, and three molars on each side of the jaw. The eye orbits have a complete wall, the latter being a feature of anthropoid primates.11
Although there is still much to be learned about the Eocene primates, it is clear that they were abundant, diverse, and widespread. Among them were ancestors of today’s prosimians and anthropoids.82 With the end of the Eocene, substantial changes took place among the primates, as among other mammals. In North America, now well isolated from Eurasia, primates became extinct, and elsewhere their range seems to have been reduced considerably.
Climate change affected primate and mammalian evolution. Through the late Eocene, climates were becoming somewhat cooler and drier, but then temperatures took a sudden dive, triggering the formation of an ice cap over previously forested Antarctica. The result was a marked reduction in the range of suitable environments for primates. At the same time, cold climate led to lower sea levels through the formation of ice caps, perhaps changing opportunities for migration of primates.
During the Oligocene epoch, from about 23 to 34 mya, the anthropoid primates diversified and expanded their range. Fossil evidence from Egypt’s Fayum region has yielded sufficient fossils (more than 1,000) to reveal that by 33 mya, Old World anthropoid primates existed in considerable diversity. Moreover, the cast of characters is growing, as new fossils continue to be found in the Fayum, as well as in newly discovered localities in Algeria (North Africa) and Oman (Arabian Peninsula). At present, we have evidence of at least sixty genera included in two families. During the Oligocene, prosimian fossil forms became far less prominent than anthropoids. Only on the large island of Madagascar (off the coast of East Africa), which was devoid of anthropoids until humans arrived, is prosimian diversity still evident. In their isolation, they underwent a further adaptive radiation.
Fossil evidence indicates that these Old World anthropoids were quadrupeds who were diurnal, as evidenced by their smaller orbits (eyes). Many of these Oligocene species possess a mixture of monkey and ape features. Of particular interest is the genus Aegyptopithecus (pronounced “Egypt-o-pith-ee-kus”; Greek for “Egyptian ape”), an Oligocene anthropoid that has sometimes been called a monkey with an ape’s teeth. Aegyptopithecus possessed a mosaic of monkey and ape features as well as features shared by both groups. Its lower molars have the five cusps of an ape, and the upper canine and lower first premolar exhibit the sort of shearing surfaces found in monkeys and apes. Its skull has eye sockets that are in a forward position and completely protected by a bony wall, as is typical of modern monkeys and apes. The endocast of its skull indicates that it had a larger visual cortex than that found in prosimians. Relative to its body
12 Kay, R. F., Ross, C., & Williams, B. A. (1997). Anthropoid origins. Science 275, 803-804.
Biocultural Connection
Why “Ida” Inspires Navel-Gazing at Our Ancestry by Meredith f. smaii
Due to the fragility of Ida's remains, scientists have been using CT scan technology to study her. The radiographic image in Plate B reveals that parts of this image have been forged (compare it to the chapter opener photo). Only the true remains show up as white because the mineralized bones and teeth in fossils, as in living creatures, are opaque in an x-ray.
One long line of evidence that supports evolution is the ongoing discovery of “transitional” fossils that bridge the gap between one obvious kind of species and another. Nowhere are these transitional animals more interesting than when looking backwards through time at the human lineage.
This week, scientists from the University of Oslo announced the discovery (or re-discovery since the fossil was dug up in 1983) of a 47-million-year-old female primate known as Ida. This almost complete female animal appears to represent the transition between what are often called “primitive” primates, such as lemurs and lorises (known as prosimians), and the more “lofty” monkey, apes, and humans. And even more interesting, this transition was in place long before anyone realized.
We love this stuff because humans are a self-interested species, and some of that navel-gazing has been directed toward our ancient past. Luckily for us, we have living examples of our history still with us today.
Usually, the cycle of life involves repeated speciation, extinction, and survival of modified forms, so that what we see today is not anything like what came before. But as the human lineage went through a prosimian phase, then a monkey phase, then an ape phase over time, those branches didn't completely die off. Instead, representatives of every historical stage can still be found in forests, savannahs, and zoos around the world.
The ancient forms, of course, have been modified by natural selection during the millions of years they have survived, and their reign is not what it used to be. Prosimian primates were once found all across North America and Europe, and now they have retreated to specialized niches in Africa (especially Madagascar) and Asia. Monkeys ruled the Earth 34 million years ago during the Oligocene, but now they mostly rule forests that cling to the equator. And during the Miocene, about 23 million years ago, apes were all over the place until they fell from grace leaving only four endangered species.
That kind of ancestral mirroring is not so common. If, for example, we were modern horses, we wouldn't be able to find decent representatives of the various stages of horse evolution galloping across a field. We'd have to deduce everything about our horsy past from the fossils that happened to be preserved and unearthed.
But we humans have these living primate templates and so we know something about how the long line of our ancestors not only looked in the flesh, we also have an idea of how they behaved, ate, socialized, and mated.
And that's also why Ida is such a special find. She seems to be covering the entire history of primate anatomical evolution all on her own. She was the size and build of modern lemurs but lacked the “tooth comb” that prosimians use to clear their fur, which makes her more like a monkey. Ida also had the flat face of monkey, and, oddly, she had the heel bone of a human.
Ida seems to be cobbled together by evolution and looks like she could take off in any and all species directions.
The mishmash of Ida's features is a reminder that although we have living examples of our past, the story might be more complicated than we think. Sometimes it takes an animal that was buried long ago, had the unusual experience of becoming a fossil, was unearthed in Germany in 1983, sold off in parts, put back together, and then presented as the biological Rosetta stone for the Primate Order to make us take another look, and revaluate, our past.
BIOCULTURAL QUESTION
What cultural factors make the biology of the Ida specimen capture our collective imaginations?
Adapted from Small, M. F. (2009, May 15). Why "Ida” inspires navel-gazing at our ancestry. Live Science. http://www. livescience. com/ HistoryZ09o520-hn-ida. html
Size, the brain of Aegyptopithecus was smaller than that of more recent anthropoids. Still, this primate seems to have had a larger brain than any prosimian, past or present. Possessed of a monkeylike skull and body, and fingers and toes capable of powerful grasping, it evidently moved about in a quadrupedal, monkeylike manner.83
The teeth of Aegyptopithecus suggest that this species may be closely related to an ancestor of humans and modern apes. Although no bigger than a modern house cat, Aegyptopithecus was nonetheless one of the larger Oligo-cene primates. Differences between males and females include larger body size, more formidable canine teeth, and deeper mandibles (lower jaws) in the males. In modern anthropoids, such sexual dimorphism correlates with social systems in which competition among males is high.
The earliest evidence of primates in Central and South America dates from this time. These fossil primates are certainly anthropoid monkeys, with the eyes fully encased in bone and limb bones for quadrupedal locomotion. Scientists hypothesize that these primates came to South America from Africa, because the earliest fossil evidence of anthropoids is from the Old World.
Some of the African anthropoids arrived in South America, which at the time was not attached to any other landmass, probably by means of floating masses of vegetation of the sort that originate even today in the great rivers of West and Central Africa. In the Oligocene, the distance between the two continents was far less than it is today; favorable winds and currents could easily have carried “floating islands” of vegetation to South America within a period of time that New World monkey ancestors could have survived.84 Nearly all living and fossil New World primates possess the ancestral dental formula (2-1-3-3) of prosimians compared to the derived pattern (2-1-2-3) found in Old World anthropoids.
World History
