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Primate Evolution

This week we will shift our focus from the present to the past.  We’re going to see how primates evolved from a primitive mammal form to the ape or hominoid forms that are ancestral to our lineage. This is going to cover a huge time span--about 65 million years ago (mya) to about 6 mya. Primate evolution will lead to our next major area which is human evolution. 

Why did primates evolve?

Remember in Chapter 6 we discussed the traits that distinguish primates from other mammals

What kinds of selective pressures might be responsible for these trends?  evolutionary forces may have shaped the evolution of the primate order.

Arboreal Theory
Traditionally, these traits have all been explained in terms of adaptation to an arboreal environment—to life in the trees. An increase in brain size relative to body size would allow better eye-hand coordination and facilitate feeding in a complex environment. Color vision is probably also helpful here, to help identify ripe fruit. Grasping hands and feet may have been better at moving around in the trees. 
Binocular vision would also be an advantage here for judging distances

The sense of smell may have declined because it is difficult to maintain a scent trail in the canopy of the trees. And having good vision may have been more beneficial than smell for finding food. 

Visual Predation Theory
However, there were challenges to this arboreal theory. An anthropologist named Matt Cartmill looked at other arboreal mammals to see if the primate traits were really necessary for life in the trees. 

The squirrel, for example, can scamper around the trees quite well, although it has claws, and doesn’t have binocular vision. In order to accept this arboreal hypothesis you have to be able to account for successful tree living mammals without primate traits. 

Cartmill looked at other animals that did have some key primate traits and found that they shared a common adaptation which was visual predation. For example, ocelots (like jaguars) and owls both have stereoscopic vision, which they use for hunting. 
Cartmill thought that the primate traits could have evolved as an adaptation to hunting insects in the dense underbrush of the forest. They needed good vision to locate and hunt their prey by sight. Also the grasping hands along with the good hand-eye coordination would serve them well in hunting. Nails would be better than pointy clawsfor snatching bugs.

squirrel loris

the loris, a sneaky bug-snatcher with good hand-eye coordination.

Angiosperm Radiation

Yet another theory for primate origins suggests that color vision and grasping hands developed to successfully feed on fruits and flowers at the ends of branches. This would have given primates an advantage over tree-dwellers like squirrels who had to carry food back to the main trunk of the tree in order to eat it. flower

The late Cretaceous period prior to the evolution of the primates was a time of diversification and spread of the angiosperms or flowering plants. This opened up new ecological niches for a host of animals. There was an explosion of insects, especially those that pollinate plants. This created opportunities for fruit eating animals such as the primates (before flowers, there was no fruit!).

You should keep in mind that these theories are not mutually exclusive—all three selective pressures could have at some point contributed to the origin of primate traits.

Primate Evolution
The Epochs of the Tertiary Period.  This period after 65 mya was when mammals including primates underwent great diversification.

Paleocene—possible primates

So when do we see the emergence of the first primates? It’s rather mysterious, because the early primate fossil record is so sparse. We do know that primates were one of many groups that originated from the great adaptive radiation of mammals at the beginning of the Paleocene Epoch about 65 million years ago. 

The best candidate so far for a primate ancestor was thought to be a group of small mammals called the plesiadapiformes. At first glance, they do not resemble primates at all. Their incisor teeth are long and pointy like rodents. Their eyes are on the sides of their skull and lacks the enclosed eye orbit of primates. The only suggested antomical link is that their posterior teeth (premolars and molars) look more like primate teeth. 

purgatorius skullpurgatorius
Purgatorius a Paleocene plesiadapiform.  What mammal do you think it most closely resembles?

For this reason, some paleontologists think that this does not constitute sufficient linkage to consider them primate ancestors. So in summary, the jury is still out on the ancestors of the primates.

Eocene: the first true primates
True primates don’t appear until the beginning of the Eocene epoch  (55-34 mya) when the fossil record shows an adaptive radiation of primate forms. These fall into two main forms. The Adapoids look like modern lemurs (with long snouts and larger body size) and the Omomyoids are more tarsier-like with shortened snouts and huge eyes. These primates also had long hind limbs and were probably vertical clingers and leapers, much like lemurs and tarsiers are today.
Adapoid skull.  Note the long snout Omomyoid skull. Note the huge eye sockets

Recent fossil finds suggest that the earliest Simiiform anthropoid primates may have their origins in the Eocene. One was a primate from China called Eosimias. Related to Eosimias was an incredible discovery of “thumb-sized primates."You can read about both discoveries here.  

In May 2009, a 47 million year old fossil find  was unveiled with much fanfare.  Nicknamed Ida, this well-preserved primate skeleton from Germany has her own website titled Revealing the Link. Check out the site, which has good information concerning the fossil's characteristics and primate evolution in general.  However, it seems to me that there's been a good deal of over-hype surrounding this discovery--particularly its characterization by the discoverers as the "missing link."  What do you think?  

Also, you might have wondered why you keep hearing about the "missing link" whenever a new fossil is found.  How many missing links are there anyway?  For a useful explanation, see this piece from Slate Magazine.

Oligocene anthropoids

Anthropoid primates gradually expanded into new niches during the next epoch, the Oligocene (34-24 mya).
The climate seems to have changed during the Oligocene, becoming cooler and drier, partly due to the effect of continental drift. This perhaps explains the rise of the anthropoid monkeys and the decline of the adapoid and omomyoids. Many species of Eocene primates become extinct and anthropoids become the dominant primate group.

Some of the best fossil evidence of late Eocene and early Oligocene primates comes from the Fayum site in Egypt. The Fayum is a desert region outside of Cairo. But 30-40 million years ago, it was much warmer and wetter, probably very swamplike. fayum

No complete skeletons were found in the Fayum deposits, just scattered bones. But paleontologists have been able to reconstruct these fragmentary fossils and have identified about 14 different genera of primates. 

One interesting Fayum primate is Aegyptopithecus.aegyptopithecus
Although Aegyptopithecus is definitely considered an anthropoid, it does retain some primitive features. For example, it may have relied more on smell than monkeys do today.

Aegyptopithecus (right) was probably one of the larger Fayum primates, about the size of a howler monkey. Aegyptopithecus also has a lot of variation in the size of its jaw and canine tooth. What we could be seeing here is size differences between males and females related to sexual dimorphism (sex-based differences in body size). This might indicate that Aegyptopithecus had a similar social structure to modern gorillas and baboons who have polygamous societies with competition over access to mates. 

Finally, the New World/Old World monkey split happened some time during the Oligocene. Remember, New World monkeys are classified differently from Old World monkeys because they have been evolving along separate lines since their separation. The earliest monkeys originated in the Old World, probably in Africa, and then they later migrated to the New World. But by 30 mya, the date of the earliest monkey fossils in the New World, continental drift had already separated South America from Africa by about 1000 miles. So how did they get there? 
oligocene map
The leading theory is that the monkeys may have been carried over to South America on floating rafts of vegetation. Actually, this isn’t as fanciful as it sounds! Sometimes storms rip up clumps of land with animals on them and pull them into the ocean. They’ve calculated that monkeys could probably have survived long enough to float across the Atlantic. To investigate further the monkey migration, click here

Miocene hominoids
At the end of the Oligocene, there is the appearance of the earliest apes or hominoids.

In fact the next epoch, the Miocene (23 - 6 mya) can be characterized as the age of apes. Like you the apes have an incredible diversification and move into a variety of arboreal and terrestrial habitats. Ape species are spread over a great distance, even to northern Asia and Western Europe. This is probably because tropical and temperate conditions existed over a large area at this time, because the climate was warmer and wetter than the preceding Oligocene.
Proconsul skeleton. The convention when depicting incomplete fossils is to distinguish between recovered and missing parts. 

One of the earliest hominoid forms is a genus found in Africa called Proconsul. Proconsulids tend to have relatively smaller brains than modern apes and lack the long forelimbs that would enable brachiation or knuckle walking. However, they lack a tail and have the flexible shoulder structure shared by modern hominoids.

The apes were very successful during the Miocene. They had a great variety of sizes and adaptations. They lived in temperate regions such as Northern Europe and Asia. In Europe during the Miocene, an ape called Dryopithecus lived in Oak forests.

gigantopithecusAn Asian ape named Sivapithecus bears many similarity to the modern orangutan. Another Asian ape called Gigantopithecus was the largest known primate that ever lived. Since only fossils of its jaws and teeth have been recovered, we can only estimate its body weight. But judging from the huge size of its jaws, it may have weighed as much as 600-1000 pounds and stood 10 feet tall on hind limbs. It had these huge grinding teeth that were clearly designed to process tough plant foods. In fact it may have had a diet like the giant panda, subsisting on tough grasses like bamboo. Read more about this extinct ape including its very unusual discovery here in this article by anthropologist Russell Ciochon.  Also, our local San Diego Museum of Man has a towering depiction of a living Gigantopithecus.  If you have a chance, visit the museum in Balboa Park and see it yourself!

Human origins
At the end of the Miocene, the climate became cooler and drier and many ape species became extinct. However, our own lineage, the hominins may have their roots in the  environmental change occurring at this time.

Is there a Miocene ape that is an obvious "missing link" between the apes and the bipedal homins?  So far, fossils from the late Miocene are very scarce.  However, there are some candidates for early hominin ancestors that we will examine next time.
Last Updated: 01/13/2015
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