The Earliest Primates And Fossil Monkeys

The mammalian fossil record is full of fascinating extinct animals, a subset of which is comprised of primate fossils. The earliest primates are

Millions of years ago

Figure 3.4 Global temperature, as measured by ratios of oxygen isotopes in deep sea and ice cores, has undergone a steady average decrease in the last 5 million years marked by huge fluctuations.

Illustration by Jeff Dixon.

Millions of years ago

Figure 3.4 Global temperature, as measured by ratios of oxygen isotopes in deep sea and ice cores, has undergone a steady average decrease in the last 5 million years marked by huge fluctuations.

Illustration by Jeff Dixon.

found in the Paleocene and resemble squirrels. Primates split from the rest of the mammals very early along with rodents, so fossil primates are some of the oldest and most primitive fossil mammals on record.

The earliest animals to resemble primates are called plesiadapiforms (family Plesiadapiformidae) and are found in rocks in Europe and western North America (particularly Wyoming). The rock composition and the related fossil fauna indicate plesiadapiforms preferred warm wet forests that were dominant during this period in this region.

Like most early mammals, plesiadapiforms were nocturnal quadrupeds with good senses of smell—a trait inferred by large snouts and comparatively small eye sockets, which indicate relatively less reliance on vision. Like modern primates, plesiadapiforms had characteristic teeth and grasping hands and feet. But unlike modern primates they had claws, they lacked leaping traits of the skeleton (primitive for primates), and they did not have large, enclosed bony eye sockets.

Angiosperms, or plants that disperse seeds with flowers and fruits, evolved in the Cretaceous period creating a new food source for animals and a niche that the earliest primates capitalized on. They were arboreal, fruit, seed, and insect-eaters. There is a debate over which food item—animated insects (many of which were pollinating the an-giosperms) or colorful fruit—was the driving force in evolution of primate features like acute color and stereoscopic vision and dexterous grasping hands. Although it could have been both insects and fruit since all modern primates eat a variety of foods, and are not limited to one specialization.

By the Eocene, the fossils of the first fully recognized primates that display the primate features plesiadapiforms lacked, lived in North America and Europe which were connected by land at the time. The omomyids (from the family Omomyidae) were small insectivores resembling modern tarsiers. The adapids (from the family Adapidae) were larger fruit-and leaf-eaters resembling modern lemurs.

If we were lemurs, we would omit the following sections on fossil monkeys and apes and humans and we would follow the fossil trail of our ancestors from the adapids through to our present-day life on Madagascar. But we are humans, so we will follow the fossil trail of our ancestors from the earliest primates to monkeys, then to apes, and finally to humans (which includes those much larger, bipedal primates that also live on Madagascar).

All monkeys, apes, and humans (haplorhines) share a common ancestor from Asia in the early Eocene known as Eosimius or "dawn monkey."

Eosimius is remarkably tiny, as small as a person's thumb, but has features that distinguish it from lorises and lemurs and link it to monkeys.

Much of what we know about early haplorhine evolution comes from fossils preserved in the Fayum desert of Egypt—some 60 miles southwest of Cairo. Sites there were first excavated in the early part of the 20th century and continue to be excavated by teams led by Elwyn Simons. These sites on the edge of the Sahara desert are very dry today, but they are rich in fossil mammals from a time when the region was a forested swampland, 34 Mya. The primitive monkeys Apidium and Aegyptopithecus hail from the Fayum. Apidium has the dental formula of platyrrhines (New World monkeys), but Aegyptopithecus has that of catarrhines (Old World monkeys, apes, and humans). Both species have fused mandibles and totally enclosed bony eye sockets. The fossil record for Old World monkeys, once they split from the ape and human lineage, is very rich by the Pliocene and Pleistocene but earlier on in the lineage's evolution, fossils are rare.

Explaining how monkeys made it to South America is a big challenge. New World monkeys evolved in isolation from a common ancestor with Old World monkeys and since then they have become very diverse. Fossil New World monkeys first appear in South America at about 28 Mya. Some of the earliest fossils, found in Bolivia, are very similar to those discovered in the Fayum from the Oligocene. There are two possible routes to South America but both seem difficult. One path is from North America, but the fossil record tracking any sort of monkey evolution throughout North America at this time is very sparse and furthermore, the isthmus at Panama was not connected to South America yet, so they would have had to island-hop through the Caribbean. The second possible route would have been across the south Atlantic ocean from Africa where the early fossil precursors are found. New World monkeys may have floated over on rafts of clumped vegetation or "floating islands." The two continents were closer to one another because sea floor spreading at the mid-Atlantic ridge was in an earlier phase of pushing the Americas away from Europe and Africa (which is still happening at the same speed your fingernails grow).

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