Chapter 9. Primate Origins and Evolution

Transcription

1 Chapter 9 Primate Origins and Evolution

2 Overview The very first primate fossil was discovered in France described by a scientist we ve already met in a previous lecture: Georges Cuvier. Remember that Cuvier was one of the first to recognize that fossils were the remains of animals that were extinct, but that many of them were similar to animals that still lived today. Though Cuvier was quite brilliant, he actually got this primate fossil entirely wrong, identifying the small skull from France as an extinct, small, hoofed creature. But, before we criticize, keep in mind that Cuvier was studying the skull of a creature that no one had ever seen before. It was unlike anything in existence; therefore, classifying it was quite difficult. Fortunately, Cuvier provided a thorough anatomical description of the specimen, setting the foundation for future descriptions that allow researchers all over the world to understand the anatomy of these fossils. And since the discovery of Adapis parisiensis, literally tens of thousands of primate fossils have been unearthed in regions all over the world, giving us great insight into the evolutionary history of our own mammalian lineage. This is a perfect time to remind ourselves that the time of discovery does not mean that the find is more ancient. Based on the reality of the Law of Superposition, that at the top is the youngest. The means that what is usually found is new the top and is younger. Also, Cuvier s younger fossils are more likely to be intact; more time, more damage. find is Eocene, when true primates emerged, but proto-primates were earlier.

3 Why Did Primates Emerge? 1 Primates are very agile climbers, with fine grasping ability in both the hands and the feet; they have nails instead of claws, relatively large brains, and stereoscopic vision (forward-facing eyes). 1. For a long time, researchers suspected that primates evolved to fill the arboreal niche; they evolved adaptations to life in the trees. This traditional explanation was based on the observation that primates are arboreal and so this is called the arboreal hypothesis which is the idea that: Suggests primates adapted to living in the trees, while other mammals were ground dwellers. This explains the reliance on vision in the 3-D world of the trees, the omnivorous diet (eat leaves fruits, nuts, insects, small mammals) and depth perception. 2. Matt Cartmill argued in the 1970s that there are plenty of other tree-dwelling animals squirrels, for instance that are not primates. There must be something in addition to arboreality that sets primates apart. His idea, the visual predation hypothesis, was that the earliest primates were arboreal hunters, with forward-facing eyes (as predators often have) and fine grasping ability and nails to grab onto insects at the ends of branches. His evidence: The forward face of primates is found in all predators. Grasping hands and feet and the presence of nails may be an adaptation to shrubby forest undergrowth and lowest forest tiers where proto-primates say insects and small prey 3. But, almost all primates also eat fruit, and the forward-facing eyes may have benefitted the earliest primates navigating quickly through the trees, looking for ripe fruit. This hypothesis, formulated by Robert Sussman, is the angiosperm radiation hypothesis (aka mixed diet hypothesis) and adds fruit into the mix. Primates adapted to angiosperms (flowering plants) which appeared about 140 mya States that these same traits make for better eating of small food items such as fruits, berries and seeds located among branches and on stems.

4 Primate Family Tree If apes and monkeys share a common ancestor, then there should be fossils of creatures that are not quite apes and not quite monkeys but have important features of both in the fossil record. You already were introduced to one of these at the very start of this chapter: Aegyptopithecus, a primate with ape-like teeth but a monkey-like body. Notice that in the family tree presented here, we will have to go back 60 million to 65 million years to find fossils of the very first primates.

5 First True Primates 1 Primate origin should be thought of as a bush, rather than a tree. Three major radiations should be noted: 1. An adaptive radiation of primate-like mammals 60 mya led to true primates. Primate-like mammals showed evidence of initial adaptation to life in the trees. Most of these species died out; some evolved into primitive prosimians, then underwent another adaptive radiation. Many early prosimians became extinct; some survived to ultimately evolve into different lines of modern primates. 2. A second radiation of the strepsirrhines became the ancestral haplorrhines, including the anthropoids. 3. Third radiation evolved into the groups of NWM, OWM and primitive apes Scientists have been finding primate fossils of these animals in Europe and Western North America. During the Paleocene epoch ( mya) there is evidence of primate-like mammals. There were no monkeys, apes, or humans at that time. But the plesiadapiforms were very diverse as they were the result of an adaptive radiation -- There were 120 species that have been discovered. Usually the size of a cat or smaller (although there is great diversity in size); they were quadrupeds Body size and dental specializations distinguished one group from another. Some had large incisor that allows for heavy gnawing, some were slicers Others were nectar and insect eaters. There were very few primate features present: the eye orbit is not surrounded by a bony rim (called a postorbital bar); the eyes are on the side of the head and not as convergent, or forward facing, as one finds in all modern primates; and the hands and feet often have limited or no grasping ability and often have claws instead of nails, and Front teeth separated from rest of teeth (diastema). Plesiadapiforms also have a relatively small brain and quite specialized, sometimes rodent-like teeth.

6 First True Primates 2 Many researchers don t think they actually are primates, but that they are a sister group to primates, called proprimates. The reasoning is that there are some species of plesiadapiform that, while never possessing all of the features of modern primates, begin to evolve some of them. One such plesiadapiform is called Carpolestes. Carpolestes simpsoni is a very important fossil find discovered in Wyoming sediments dated to about 58 million years old. It is a plesiadapiform, but unlike many other proprimates, this one has some of the features we find in modern primates. It was a small, arboreal, fruit eater It has a grasping foot and a single nail on its big toe. But, there are claws on the other digits and the eyes of Carpolestes are on the sides of its head, not forward facing like in primates. This is a good example of what an animal that is transitioning between a proprimate and a true primate (euprimate) might look like. Why did an animal like Carpolestes evolve? Reconstructions of the environment at this time reveal that the planet was much, much warmer during the Paleocene and that Wyoming was covered with a lush tropical rainforest, with many fruit-bearing trees. Perhaps the transitional primates, like this one, originally filled the niche of fruit-eating on the ends of branches, but then some of them evolved their forward-facing eyes to hunt insects in this particular environment, making both the angiosperm radiation hypothesis and the visual predation hypothesis both relevant to the understanding of primate origins.

7 First True Primates 3 By the Eocene (34 56 mya), the fossil record is littered with more than 200 different species of actual, true primates called euprimates -- the timing of the first true primates was mya. They have grasping hands and feet with nails instead of claws, they have larger brains and more generalized teeth than the plesiadapiforms, and they have forward-facing eyes surrounded with bone (a postorbital bar). Some of these fossils are from animals as large as house cats, and others as small as chipmunks. They are found in fossil deposits in Western Europe, North America, Africa, and Asia, and generally fall into two categories. There are adapids, and there are omomyids. Adapids tend to be the larger of the two, and they were active during the daytime (diurnal). They display high levels of sexual dimorphism, which is a feature found in later anthropoids. They also lack the tooth comb found in later prosimians, though they strongly resemble lemurs in many other ways, leading some to suggest that they are in fact ancestral to lemurs and other strepsirrhines. Group was leaf and fruit eaters, similar to modern lemurs and lorises The omomyids are smaller and have large eyes, indicating they were most likely nocturnal. They had a relatively short snout, and resembled modern tarsiers in many ways. Because tarsiers are more closely related to anthropoids than they are to lemurs, there are some researchers who regard omomyids as ancestral to anthropoids (including humans). Fruit and insect eaters was more like tarsiers. A change from nocturnality to diurnality was the fundamental adaptive shift that occurred at the base of the tarsier-eosimiid-anthropoid clade. However, the relationship of these two primate groups to modern taxa is hotly debated.

8 Darwinius Darwinius, from the Messel site in Germany, was discovered in 2009 Dates to 47 mya (Eocene). Relationships to living primates are not confirmed. In part this is a problem because the specialists in this arena do not have access to the specimen. The more casual assessment is that it is not ancestral to the anthropoids (as claimed by the discovery team) and may not have a link to any living primate species (a dead end). Update: The infraorder is now determined to be adapiforms, early strepsirrhines. Interested in learning more? Visit this website or watch the video called The Link (in the library).

9 First True Primates 4 A very recent discovery has further complicated the primate fossil record. Archicebus is a remarkably well-preserved skeleton of a very small primate that lived in China 55 mya. It has an interesting combination of features found today in both tarsiers and anthropoids, suggesting that Archicebus may be one of the earliest haplorrhines. This tiny primate had small eyes, but sharp, cusped teeth, indicating that it hunted for insects during the day. It was arboreal, with monkey-like adaptations in the foot for climbing on the tops of branches. A fossil of this age, in this location, is evidence that the haplorhinae lineage evolved in Asia. By the end of the Eocene, some primates begin to look more and more like anthropoids. A very small primate from 42 million-year-old deposits in China, called Eosimias, the dawn monkey may be ancestral to all anthropoids Its teeth are quite anthropoid-like, especially the size and shape of the canine tooth. Eosimias has a short, anthropoid-like heel, which suggests that it is climbing through the trees in a monkey-like way, and not leaping as many strepsirrhines and tarsiers do. Asia is the likely origin of the anthropoids, but may have migrated: A set of 37 million-year-old teeth from Egypt, representing a basal anthropoid called Biretia, one of the very earliest to be found in Africa. New primate fossils from Myanmar and from Libya look so much alike that the hypothesis that anthropoids originated in Asia and then migrated to Africa is gaining support.

10 Early Anthropoids 1 Anthropoids diversified and flourished in a changing environment. The Oligocene (23 34 mya) was a period of global cooling, in which new habitats emerged. One of these places was the lush tropical forest environment of the Fayum Depression in modern-day Egypt. The Fayum was once a lush forest environment inhabited by ancestral crocodiles, primates, birds, and large mammals. Researchers working in the Fayum have found fossils of three different kinds of anthropoid primates: oligopithecids, parapithecids, and propliopithecids. 1. Oligopithecids are some of the oldest, absolute anthropoids in the fossil record. They date to about 35 mya, and include Catopithecus. 2. The slightly later parapithecids have a dental formula of 2/1/3/3, which is found in modern-day platyrrhines, or New World monkeys. This suggests that New World monkeys may actually have ancestry in the Old World. 3. Fossil anthropoids in Fayum sediments between mya are often attributed to the propliopithecids. These include the genera Propliopithecus and a primate you have already been introduced to, Aegyptopithecus. Many of these Fayum primates have low, round cusps on their cheek teeth, indicating that they lived on a diet of fruit and seeds.

11 Early Anthropoids 2 By 32 mya, the anthropoids had split into two groups: platyrrhines and catarrhines. The platyrrhines are currently represented only by the New World monkeys of South and Central America. The catarrhines are the Old World monkeys and the apes, including humans. Catarrhines can be identified by their 2/1/2/3 dental formula and some other features of their skull and skeleton. Aegyptopithecus has a 2/1/2/3 dental formula, but it does not have a suite of features that would align it with either monkeys or apes. Though the teeth are ape-like, the limb proportions are monkey-like. The legs and arms are about the same length, suggesting it moved on the tops of branches like a monkey does. Aegyptopithecus also had a tail, which apes do not have. Saadanius, a recent discovery in Saudi Arabia pushes the date of the last common ancestor between Old World monkeys and apes to about 28 mya. Saadanius is represented by a skull found by University of Michigan paleontologists. It is larger than Aegyptopithecus and has slightly more advanced catarrhine features, including a bony auditory tube, which is found in all catarrhines (including humans) but not in Aegyptopithecus. Soon after Saadanius, the ancestral catarrhines split into apes and Old World monkeys. New fossils from Tanzania indicate that the very earliest Old World monkeys and the very earliest apes had evolved by 25 mya.

12 Coming to America By the time of the NWM, continental drift had separated the continents. Platyrrhines live only in the Americas, and South and Central America are a long, long way from Africa. There are four hypotheses that explain the origins of the New World monkeys. 1. The first is that the platyrrhines evolved from a North American anthropoid. The problem with this hypothesis is that a North American anthropoid has never been found. 2. There is the strong possibility that the platyrrhines rafted across the Atlantic on mats of vegetation, perhaps from island chain to island chain, to the shores of South America. 3. A variant of this idea is more of a land migration from South Africa to the southern tip of South America via Antarctica, which was not nearly as cold in the early Oligocene as it is now. 4. The final hypothesis posits that the platyrrhines and catarrhines independently evolved from different prosimian lineages (once popular; called parallel evolution). An African origin is supported for 3 reasons: 1) No early anthropoids have been found in N. America; 2) there is evidence of rafting from Africa by rats; and 3) NW fossil evidence points to a close similarity to African anthropoids. Evidence: There is a fossil from Bolivia that is 26 million years old, from a platyrrhine primate called Branisella. It too has a 2/1/3/3 dental formula and is very similar in many ways to today s owl monkey, linking African fossils to modern South American primates. The link between Old World catarrhines and New World platyrrhines can be found simply by comparing their anatomy. Comparing their anatomy and their DNA supports the idea that these lineages are closely related, share recent ancestry, and the colonization of the New World was most likely from an African anthropoid migrating on mats of vegetation across the Atlantic (not deliberately, of course).

13 Apes Dominate the Miocene At the beginning of the Miocene epoch ( mya), the planet once again warmed, creating new and expanding forest habitats for the catarrhines. By about 23 mya, the catarrhines diverged into two lineages: the hominoids (or apes) and the cercopithecoids (or Old World monkeys). Proconsul The earliest apes are called proconsulids, and they were plentiful in a range of different habitats in Africa in the early Miocene and have been discovered at fossil sites mostly in Kenya and Uganda (see map at bottom). Interestingly, these early apes were a bit different from apes today. From the neck up, there were quite ape-like. They had large, flat molars adapted for fruit eating, and these molars had a Y-5 cusp pattern. They also had sexually dimorphic, honing (or shearing) canine teeth. However, their brains were rather small and more monkey-like relative to their body size. Like modern apes, though, they lacked a tail. But, other than the absence of the tail, from the neck down, the Proconsulids were very monkeylike. They walked on all fours in the trees and on the ground. They possessed less mobility at the wrist, shoulder, and elbow than what is found in the modern apes. Their hands were quite a bit smaller than ape hands, the arms were relatively shorter, and their rib cage was deep, like a monkey s rib cage. Modern apes have huge hands, long arms, and a shallow, but wide, rib cage. If Proconsul is an ancestral ape, then the modern ape body plan evolved later in the Miocene.

14 Reconstruction of Proconsul

15 Apes Leave Africa 1 The story of ape evolution up to this point is restricted to Africa. But, around 17 mya, the African hominoids expanded their territories into Eurasia. By 15 mya, African hominoids become very rare, and fossil hominoids instead become plentiful in Spain, France, Greece, Hungary, and Turkey. Of course, this change is being driven by the ecology. During the Middle Miocene, Southern Europe became covered in dense forests with fruit-bearing trees perfect habitat for early apes. The collection of different hominoids that inhabited Europe at this time are called dryopithecids, though this encompasses many different kinds of apes: Dryopithecus, Hispanopithecus, Pierolapithecus, Rudapithecus, Ouranopithecus, and, by the end of the Miocene, Oreopithecus. Like the proconsulids, the dryopithecids have large, honing canine teeth, and a Y-5 pattern on their molar teeth. However, unlike the proconsulids, the dryopithecids evolved more ape-like postcrania that allowed them to engage in more arm-swinging and suspensory behaviors in the trees. The dryopithecids had long arms and large hands. In addition, they evolved larger brains than the proconsulids, foreshadowing the intelligence we see in modern great apes. But, the great forests of Southern Europe receded by the end of the Miocene, and this climate shift led to the extinction of these European great apes. Pierolapithcus Dryopithecus

16 Apes Leave Africa 2 As the apes expanded out of Africa, many of them migrated to the west and evolved into the dryopithecids. Others migrated to the east and evolved into a different group of apes called the sivapithecids. The sivapithecids are found mostly in India and Pakistan, though one genus (Ankarapithecus) is from Turkey. Unlike many dryopithecids and unlike modern African apes, the sivapithecids had thick tooth enamel, which was useful for eating hard objects, like nuts and seeds. When first discovered, Sivapithecus was thought to be an orangutan ancestor. The profile of the skull is remarkably like that of a modern orangutan. However, fossils from below the neck indicate that Sivapithecus was not like an orangutan in terms of its locomotion, and was much more like a proconsulid. The alternative explanation is that Sivapithecus is not an ancestral orangutan and that the shared similarities in the skull evolved in parallel, perhaps because of similar diets. Recent discoveries of a more orangutan-like creature called Khoratpithecus may support this latter hypothesis. Regardless, by the Plio-Pleistocene, one of the more unusual and amazing primates ever discovered had evolved in Asia. It is called Gigantopithecus, and though rare and known only through jaws and teeth, it was enormous estimated to weigh upwards of 500 pounds and living on a diet of bamboo. It lived until quite recently, and probably overlapped in time with some of our own ancestors, Homo erectus, in Asia.

17 Wrap-up The very first hominoids, or apes, appear in the fossil record in the early Miocene, about 20 mya. These proconsulids are found exclusively in Africa, and, in particular, have been found in Kenya and Uganda. From the neck up, they are quite ape-like, but from the neck-down they are monkeylike, except that they did not have a tail. By the Middle to Late Miocene (9 14 mya), apes expanded out of Africa and inhabited the forests of Southern Europe and Asia. The European apes, called dryopithecids, are more ape-like in their limbs and hands, indicating that they had evolved suspensory, arm-swinging behaviors like those found in modern apes. Some of the latest European apes are the oreopithecids, which were highly capable of suspensory locomotion and had specialized teeth for eating leaves. The Asian apes, called sivapithecids, resembled orangutans from the neck up, but proconsulids from the neck down.

18 Catarrhine Evolution