Introduction to The Emergence of Primates
Around 65 million years ago, at the end of a time called the Cretaceous Period, life on Earth was in the midst of a great transformation. This prehistoric world was warm and teeming with dense forests and lush wetlands filled with a rich variety of plants and animals, including the last of the dinosaurs. Conifers (cone-bearing plants) had been the dominant type of plant for more than 100 million years, but angiosperms (flowering, fruit-bearing plants) were beginning to spread. With angiosperms came new forms of life—new species of insects that lived off flower nectar and fruits and other kinds of animals that ate the flourishing insects and other foods in the lush forests. One of those animals may have been a hairy little shrewlike creature that scurried across the floor of the forests. This mysterious animal may have been the ancestor of all primates, a group of related animals that includes apes, monkeys, and human beings.
Exactly what sort of creatures the first primates were is uncertain, but they were probably small—less than 20 centimeters (8 inches) long—and they might have looked like miniature squirrels, perhaps with long snouts. They probably had some, but not all, of the characteristics of present-day primates. For over a century, paleontologists (scientists who specialize in ancient life) have studied fossils of early primates and their close relatives. They have generated many ideas about the early primates, but few conclusions.
What Are Primates?
Primates belong to a larger group of animals called mammals, animals that nurse their young with mother's milk. Present-day mammals vary widely, from elephants and rabbits to armadillos and kangaroos. Mammals first evolved around the same time that dinosaurs did, about 230 million years ago during a time known as the Triassic Period. Those mammals were small shrewlike creatures, but before dinosaurs died out around 65 million years ago, mammals began to diversify—to evolve into many different species. Some of the new mammalian species became the ancestors of present-day animals that may be close cousins of the primates, including bats, tree shrews (squirrellike mammals in Asia), and colugos (gliding cat-sized animals in Southeast Asia). And one of those early mammals became the ancestor of all primates.
The order (scientific grouping) of primates includes about 230 living species and more than 350 extinct species. All of the earliest primate species, which lived from about 65 million to 34 million years ago, are now extinct. Present-day primates range in size from 13 centimeters (5 inches) long to more than 2 meters (6 feet) long, and are divided into two major groups. One group, called the anthropoids, includes humans, apes, and monkeys. The other group, the prosimians, includes lemurs and lorises. Lemurs are long-tailed animals with long snouts and fluffy fur that live on islands off the east coast of Africa. Lorises are tree-dwelling animals found in Africa and southern Asia.
It is the versatility of primates, scientists believe, that has distinguished them from other mammals and enabled them to compete successfully for resources. Unlike most animals, which have features that make them ideally suited for specializing in procuring and eating one kind of food and occupying one particular habitat, primates are generalists. Many of their characteristics enable them to utilize a variety of resources and adapt to a variety of habitats. Humans, the most adaptable of the primates, live on every continent and in every climate zone. Nonhuman primates live mostly in tropical and subtropical regions, but they inhabit both forests and grasslands.
Another notable characteristic of primates is their brains, which are larger, relative to body size, than those of other mammals. This feature, in combination with other traits, has enhanced their adaptability. For example, the combination of large brains and large, forward-facing eyes enables primates to judge distances well. Paleontologists believe that this anatomy first evolved to aid in tree-climbing and sighting prey, but it also helps primates coordinate their eyes with the movements of their hands. And a larger brain means increased intelligence in general, useful for more than just getting from tree to tree.
Primates have other versatile features as well. A variety of teeth and a digestive system that handles a wide range of foods enable primates to eat a varied diet, including insects, fruits, small animals, eggs, leaves, nuts, and seeds—and in the case of humans, just about anything else on Earth. Primates have a bony bar that protects the outer sides of their eye sockets. They have flat nails instead of claws or hooves, and hands with five fingers—including a thumb—that enable them to grasp objects. These features make it possible for primates to carry food to their mouths. Because they do not have to root for food on the ground, primates typically have a short snout.
Drawing From the Fossil Evidence
Paleontologists have found the fossils of early primates in North America, Europe, Africa, and Asia. The fossils are usually found within layers of sedimentary rock, rock formed from sediments of mud, clay, silt, or sand deposited over long spans of time on the bottom of rivers, lakes, swamps, and oceans. In general, the lower a rock layer is in a sequence of layers, the older it is. Layers that are as old as the first primates could be quite deep, but they have often been exposed through natural processes, including uplift, in which one section of the Earth's crust runs into another and gradually pushes it up, and weathering by water and winds.
To extract fossils from ancient rock, paleontologists use many methods. Frequently, they stumble across partially exposed bones and remove the surrounding rock with a hammer and chisel or, for more delicate work, an icepick. When they come across layers of loose sand or mud, they brush, shovel, and sift their way through, looking for bones and teeth. In some cases paleontologists break open chunks of rock to see if there are bones inside. To better understand the fossils that they discover, researchers note the depth of the rock layer in which the fossils were found, the other fossils found in that layer, and the types of minerals and sediments in the layer. This helps them determine what kind of environments the primates inhabited when they were alive.
Discoveries of primate fossils have mostly been limited to teeth, jaw bones, pieces of skulls, and a few scattered fragments from other parts of the body. Teeth, which contain apatite, a hard and durable mineral that makes them tougher than other types of skeletal remains, account for the greatest number of early-primate fossils.
Analyzing the Fossil Record
Various factors make finding and analyzing early primate fossils difficult. One problem is their tiny size. For example, some early primate teeth are only a few millimeters in length and width. Another difficulty is that relatively few primate remains have survived to the present day. That may be because many areas where the early primates lived, such as the tropical forests of Southeast Asia, are still covered with dense vegetation, making excavation difficult. Most fossil finds today are on the sites of ancient forests that are now dry desert areas in places such as Wyoming, eastern Asia, and northern Africa.
Even though the fossil record of early primates is sketchy, paleontologists have been able to slowly piece together a picture of what many of these animals looked like, how big they were, what they ate, and where and how they lived. They have also gathered clues about how early primates probably interacted with other animals, what animals they were descended from, and what new species evolved from them. Much of this knowledge has been inferred from fossilized teeth.
Clues In Teeth
Primates have four different kinds of teeth—incisors (front biting teeth), canines (pointed teeth behind and next to the incisors), premolars (medium-sized grinding teeth behind the canines), and molars (larger grinding teeth in the back of the mouth). Knowing the number and kinds of teeth that a certain primate had enables scientists to determine the shape of the animal's mouth. For example, some of the earliest primates had four premolars, while lemurs have only three, and humans have two. The size of a single tooth can tell paleontologists how large a certain primate was—a tiny tooth implies a tiny primate. In addition, the pattern of bumps and crannies on each tooth can tell scientists what the animal ate. For example, insect-eating animals generally had molars with sharp cusps (bumps) that allowed them to puncture and slice their food, while animals that ate fruits generally had molars with low cusps that enabled them to crush the fruits.
Because mammals inherit the shapes of their teeth from their parents, paleontologists can trace the primate family tree by analyzing the patterns they discover in each tooth. For example, if an early species had sharp cusps on a particular molar, and several later species had progressively lower cusps on corresponding molars, paleontologists might infer that the sharp-cusped species was an ancestor of the successively lower-cusped species. Paleontologists piece together such clues to determine how different animals are related to one another. Sometimes, however, the information seems contradictory. For example, patterns on molars from a group of species might indicate a clear line of evolutionary descent, but evidence from incisors of the same group might seem conflicting and inconclusive.
To resolve such conflicts, scientists have devised rules that allow them to pay more attention to some characteristics than to others. For example, one rule states that the only characteristics that can be used to trace branching patterns in a family tree are those that are clearly shared by a group of animals and that are derived (originated and evolved) in that group. Scientists must be able to trace a progression of small changes in characteristics from an ancestral species through its various descendants. Thus, if two animals differ greatly in a certain characteristic, the two animals may nevertheless be related if evidence from the characteristics that they do share points to their kinship.
The Cloudy Origins of Primates
Although the evolutionary history of the primates is full of questions that have aroused heated debate among paleontologists, it is clear that two major evolutionary milestones occurred in early primate history. In these events, called adaptive radiations, many new species of primates or their close relatives evolved rapidly. Adaptive radiations can occur for all types of living things. They often coincide with changes in climate and the availability of food and habitats that provide an abundance of new opportunities to emerging species. By stumbling upon a new adaptive niche, or role in a community of wildlife, a species can use those resources to radiate—to proliferate and evolve. The first radiation coincided with the Paleocene Epoch, from about 65 million to 55 million years ago. The second major radiation occurred during the Eocene Epoch, from 55 million to 34 million years ago.
Some of the most common mammalian remains that scientists have been digging up since the 1800's at Paleocene excavation sites in North America are fossils of animals called plesiadapiforms (plee zee uh DAP uh formz). These primatelike creatures, which resembled small squirrels or tree shrews, proliferated into a major adaptive radiation during the Paleocene, a time when many new mammalian species found new niches in a cooling climate. Researchers have also discovered plesiadapiform fossils in Asia and Europe.
The earliest known plesiadapiform, a creature called Purgatorius that weighed about 60 to 90 grams (2 to 3 ounces), appeared about 65 million years ago and may have witnessed the end of the Age of Dinosaurs. Purgatorius has been identified only from teeth and part of a lower jaw bone, but scientists have found some skulls and bone fragments of other plesiadapiforms.
They were generally small creatures. Most weighed from 10 to 400 grams (0.4 to 14 ounces), but a few species weighed from 1 to 3 kilograms (2 to 7 pounds), placing them among the larger mammals of the time. Like primates, many plesiadapiforms had molars with low cusps, indicating that they ate fruit. Plesiadapiform teeth even had some characteristics that are present in no other mammals except primates. Nonetheless, some of the plesiadapiforms with the most primatelike molars had long, unprimatelike incisors, and—unlike primates—they had long, narrow snouts and unprotected eye sockets.
Debate Over the First Primates
Although many characteristics of plesiadapiforms differ from those of typical primates, most scientists until the late 1900's believed that the plesiadapiforms were the first primates. That belief was based on a few key observations: the similarities between the teeth of plesiadapiforms and those of known primates; the fact that plesiadapiforms were abundant throughout the Paleocene, while primates were abundant soon afterward in the Eocene; and the lack of fossils of other Paleocene mammals that could be candidates for primate ancestors.
A few scientists argued as long ago as the late 1960's that other Paleocene mammals could have been the first primates, but it was not until the 1990's that this view gained a sizeable following among paleontologists. Researchers began to change their minds after analyzing newly discovered plesiadapiform skulls excavated in Wyoming. They argued that even though plesiadapiform teeth had some primate characteristics, their skulls had no significant primate features. Thus, the skeptics contended that from the skull evidence, it seemed unlikely that plesiadapiforms were the ancestors of the primates.
But other scientists disagreed, arguing that the similarities between plesiadapiform and primate teeth were more important than differences between their skulls. They based that argument on the rule that fossil characteristics that are clearly shared in a line of progression are the traits most relevant to tracing a line of descent. Despite such objections, many primate researchers now argue, based on the differences in the skulls of the two groups of mammals and what the researchers view as inconsistent dental evidence, that the plesiadapiforms were not primates. Instead, those researchers argue, the creatures were probably just close cousins of the first primates.
And what about the animals from which bats, tree shrews, and colugos evolved? Might one of those creatures also have been the species that gave rise to the primates? Some paleontologists say no, because paleontologists have found few fossils from the Paleocene of early ancestors of bats, tree shrews, or colugos. However, many researchers have pointed out physical similarities that present-day primates share with these creatures. The visual systems of certain species of bats, for example, have specialized characteristics that are found in no other mammal but primates. Tree shrews share so many features with primates that scientists classified them as primates until the mid-1960's. And colugos are known as flying lemurs because they have some similarities with true lemurs. Such similarities provide strong evidence that the first primates might have evolved from an ancestor of one of those animals. Nonetheless, the evidence is inconclusive.
Primates of the Eocene
Whatever their ancestry, animals that were unquestionably primates emerged at the tail end of the Paleocene, and undoubted primates then multiplied in the Eocene in another major adaptive radiation in primate history. The Eocene primates were small, furry, tree-dwelling creatures, probably with the sort of acrobatic agility that characterizes many present-day primates. From the fossil evidence, paleontologists have determined that many primates of the Eocene had large eyes that enabled them to see better at night, so that they could look for food while predators were asleep. In addition, they shared the major characteristics of present-day primates, including five-fingered hands, fingernails, small snouts, a bony bar protecting their eye sockets, and relatively large skulls and brains.
The earliest creature that most paleontologists agree was indisputably a primate lived just over 55 million years ago at the very end of the Paleocene Epoch in what is now Morocco. Researchers have characterized this primate, which weighed 50 to 100 grams (2 to 4 ounces) and is called Altiatlasius (al tee ut LAY shus), from 10 teeth, which clearly display the characteristics of the teeth of primates that inhabited the Eocene and later eras. Some paleontologists argue that Altiatlasius teeth also display plesiadapiform characteristics, but no other evidence has been found that might help scientists decide whether there is a link between Altiatlasius and the plesiadapiforms.
All the other first primates with predominantly modern features emerged in the Eocene. Throughout the period, the climate warmed in comparison with the Paleocene, and primates thrived in Africa, Asia, Europe, and North America. The earliest-known primate of the Eocene was called Altanius. Many teeth from this animal, a tiny creature that weighed a mere 10 grams (0.4 ounces), have been discovered in Mongolia. Scientists think that Altanius, which lived slightly less than 55 million years ago, was the ancestor of all the later Eocene primates.
One of the best-studied primates from the early Eocene, called Notharctus, was a cat-sized creature that weighed up to 7 kilograms (15 pounds). It lived about 50 million years ago in the forests of ancient Wyoming and ate fruit, insects, and leaves. Scientists have found several complete skeletons of Notharctus. Those fossils show that the animal looked like present-day lemurs, but with a smaller brain. It had a long tail, long hind limbs, and a flexible torso, features that suited it for leaping from branch to branch through the trees.
New Groups of Primates Emerge
Although paleontologists disagree over the details of how present-day prosimians and anthropoids originated, they generally agree that some still-unidentified Eocene primate species probably split into these two groups sometime in the early Eocene. In the early 1990's, paleontologists discovered the earliest known candidate for an anthropoid ancestor—from the middle Eocene, about 45 million years ago—near the Yellow River in eastern China. By 2000, they had discovered and identified teeth, a bone from an inner ear, and ankle bones from the creature, which they named Eosimias. From the fossil evidence, researchers estimated that Eosimias probably weighed about 100 grams (4 ounces) and was the size of a chipmunk. Its fossils are very small--the teeth are about 1.5 millimeters (0.06 inches) wide—but researchers can discern important anthropoid characteristics in them. For example, the incisors are small and certain key molar cusps are distinctively broad.
At first, many paleontologists were skeptical that Eosimias was, in fact, an anthropoid. Previously, the earliest known anthropoid fossils were no more than 38 million years old and were found in Africa. But the case for Eosimias grew as more fossils were discovered. In Myanmar, southwest of the Chinese site, paleontologists found additional fossils of anthropoids that had lived later than Eosimias. Researchers believed those anthropoids might have been descendants of Eosimias.
In the 45 million years since Eosimias appeared, primates have evolved and diversified into many distinct groups. One group, the descendants of Notharctus and other North American early primates, seems to have died out. Another branch, which may have originated in Europe, evolved into present-day prosimians. One example of an early European prosimian was the Necrolemur, a creature that weighed about 300 grams (10 ounces) and lived about 40 million years ago. Necrolemur had a relatively short, narrow muzzle; limbs built for leaping; and large eyes suitable for seeing well at night.
The descendants of the Eocene prosimians dispersed through Africa and Asia, giving rise to present-day lemurs and lorises. Lemurs now live only on the islands of Madagascar and Comoros, off the southeastern coast of Africa. Lorises live in India, Sri Lanka, Indonesia, and Africa, but deforestation and civil strife threaten their habitat and survival.
Finally, most paleontologists believe that the anthropoid descendants of Eosimias populated Asia and Africa and diversified into monkeys, apes, and humans. Some ancient monkeys migrated to South America about 30 million years ago. Because South America was not attached to North America or any other continent at the time, paleontologists believe that the ancestors of South American monkeys may have crossed open ocean waters. Researchers have proposed that those monkeys drifted to South America on a kind of floating island—a mass of tangled vegetation, dirt, and wildlife large enough to provide food for the monkeys throughout their journey. Such an island raft may have broken off from a riverbank in the midst of heavy floods.
Links to Present-day Primates
Researchers believe that present-day monkeys in South America have probably retained many of the characteristics of their early ancestors. But as the anthropoid primates evolved in Africa, some of the most distinguishing primate characteristics generally became even more distinct—relatively large brains became larger, and grasping hands became more dexterous. Apes, which are similar to monkeys but are larger and smarter, began to appear less than 20 million years ago in Africa. Finally, many anthropologists believe that between 4 million and 2 million years ago, the ancestors of humans arose from apes in Africa.
Although early primates probably emerged during the Paleocene Epoch or perhaps even earlier, the evidence telling scientists what those animals were like is inconclusive. Primates such as Altiatlasius and Altanius, which were part of the transition from the elusive Paleocene species to the primates of the Eocene, could provide vital clues to the origins of primates by linking them with the plesiadapiforms or some other mammals of the Paleocene. But that is far from certain—they may not be linked to any known animals at all. Whatever new fossils are unearthed in coming years, scientists know that the primates of the Eocene gave rise to a remarkable group of animals, the anthropoids, from which the human species evolved. Human beings are the greatest generalists on Earth, climbing not just trees but also mountains, judging distances to the furthermost reaches of the universe, and manipulating tools to build civilizations and to solve the mystery of their own origin.