Welcome to the second lecture in the session on primate evolution. In this one, we will look at the origin of primates, and review their evolution during the last 55 million years through the lens of prehistoric global climate change. However, the most important event paving the way for the origin of primates, took place 65 million years ago. As you may recall from a previous lecture this was the global mass extinction, which wiped out two thirds of the species on Earth, including the non-flying dinosaurs. Significantly, the disappearance of the dinosaurs allowed the mammals to diversify and evolve into new ecologies and forms. One of these mammal groups was the Plesiadapiformes, who developed an arboreal lifestyle. The first fossils of the group turn up in deposits just after the Cretaceous/Tertiary-boundary and the great mass extinction. Fossils indicate that the plesiadapiforms were a very diverse group and common in North America, Europe and Asia during the geological epochs of the Paleocene and Eocene. Systematical studies comparing their fossil anatomy indicate that the plesiadapiforms are the sister group to primates, and thus inform us about how the earliest primates lived and appeared. This figure shows the origin and radiation of plesiadapiforms and primates shortly after the K/T-mass extinction. The time of the extinction event is indicated by a horizontal red line; various lineages are indicated by vertical lines. The duration of these lineages in geological time are known from fossil finds of animals which belong to them. Circled in blue are the plesiadapiforms. As you will notice, they underwent a massive diversification with several lineages appearing during the first 10 million years after the mass extinction; the Paleocene epoch. The time of the origin of the primates has so far proven harder to nail down. Molecular clock studies, which try to discover when a group originated, by comparing and computing rates of DNA mutation, indicate that the first primates originated as far back as 85 million years ago. This would have been during the last part of the Cretaceous era and well before the great mass extinction. However, no fossils referable to primates have been found which are that old and molecular clock studies have been found to generally overestimate the age of origin of the lineages they research. The presence of the plesiadapiforms - the primates' sister group - indicate that they probably originated sometime within the Paleocene epoch, but the first certain fossil remains of primates are some 55 million years old. This is a global proxy temperature curve for the last 65 million years. It has been constructed on the basis of delta oxygen18-values from the shells of deep sea bottom-dwelling foraminifera from all over the planet. The delta oxygen18-values of the shells of these single-celled organisms record the ambient temperature of their environment, as you may recall from the previous lecture on climate change. The origin, evolution and diversity of primates throughout the last 55 million years were influenced by the effects of climate. We will not go into too many details of this curve, but you will notice that on the left-hand side of the curve that global temperatures were much higher 65 million years ago. This was at the end of the Cretaceous period and the beginning of the Tertiary. Continuing towards the right on the curve and towards recent times, you can observe that there has been an overall trend towards a steady drop in global temperature. The first event on the "curve" which we will visit, took place some 55 million years ago at the boundary between the geological epochs of the Paleocene and Eocene. Here, the records indicate that deep-sea temperatures may have increased between 3 and 4 degrees centigrade in as little as 10,000 years. This event is called the Paleocene-Eocene Thermal Maximum. The reason for the increase is most likely massive volcanic eruptions in the nascent North Atlantic between Greenland and Norway. The eruptions emitted large quantity of CO2 into the atmosphere, heating it, and may further have triggered the release of methane - another effective greenhouse gas - from deepwater methane hydrates ice on the seafloor. The cumulative effect of this on the Earth's environment was rampant global warming. Coincident with this global warming event, we find the first appearance of primates in the fossil record - the first time we find their fossilized remains. Other mammal groups, such as even-toed and odd-toed ungulates, also appeared at exactly this moment in time, almost simultaneously in the northern continents of North America, Europe and Asia. The mechanism under which global warming may have led to the origin of new lineages among mammals, is not understood at the moment. A few million years after the Paleocene-Eocene Thermal Maximum, temperatures remained high and subtropical and tropical evergreen forests were widespread. Fossil finds indicate that this was crucial to the primates, as these environments allowed them to diversify rapidly into several lineages, both living and extinct. It is in the beginning of the Eocene epoch that we find the roots of the two major divisions within the primates; the Strepsirrhini and the Haplorrhini. Respectively, the "wet noses" and the "dry noses" that you may recall from the previous lecture. The suborder Strepsirrhini consists among others of the living lemurs and the lorises, as well as the now extinct adapiforms. The other major radiation of primates in the early Eocene was the Haplorrhini or "dry noses". This very successful group includes recent Tarsiers, New World monkeys, and our own group, the Old World monkeys. Fossils referable to several haplorrhine lineages appear at the very beginning of the Eocene epoch, and the entire suborder was very diverse throughout most of the Eocene. However, following the Early Eocene Climatic Optimum, global temperatures fell gradually, and the climate became drier and cooler. This negatively affected the subtropical and tropical forests, and primate diversity went into a worldwide decline. This observation is supported by a decrease in the relative number of fossils towards the end of the Eocene. The plesiadapiforms - their sister-group suffered worse and became totally extinct. Some 33 million years ago, at the Eocene-Oligocene boundary, global temperatures plunged due to the formation of the first Antarctic ice caps. This event radically changed global marine circulation patterns, as well as resulting in a worldwide drop in sea level. This led to a minor mass extinction event. It also thrust the planet into the cool and dry "icehouse" climate, which has lasted until recent times. The following Oligocene epoch - lasting between approximately 33 and 23 million years ago - is marked by an all-time low in the fossil record of primates. Their natural habitats of warm, subtropical broadleaved forest were severely restrained in the cool and dry Oligocene epoch. However, it is also during the Oligocene epoch that we find the first evidence for the origin of the catarrhine primates - also called the Old World monkeys. Their colloquial name stems from the reason that all recent members live in Africa and Asia - "the Old World". And this is the group to which we humans belong. The fossil evidence for this is the skull of Saadanius, a fossil primate from Saudi Arabia which lived around 28 million years ago. Saadanius displays a number of features, which indicate it as intermediate between catharrhines and their nearest extinct relatives. We are now in the exhibitions of the Natural History Museum of Denmark. On the podium next to me are skeletons of various extinct and living members of the Hominidae - the great apes. The earliest fossils of this group derive from middle of the Miocene epoch some 12 million years ago. Living members of the group include the orang-outans, gorillas, chimpanzees and humans. This is an overview of the age and duration of the many extinct and extant species within the hominines for the last 7 million years. The hominines are the subfamily within the Great Apes to which humans belong. Each line on the figure represents the duration in time of a separate species. Note how many different species of human relatives that have been alive in the past - and how often near-similar species existed alongside each other in time. The pattern of species in time, indicate that hominine evolution was a bushy, directionless process with no overall end goal. Notice how our species - Homo sapiens - is just one short-lived branch. Today we are the only hominine species to survive. This is a far cry from the situation just 150,000 years ago, where several species of humans where alive at the same time in different parts of the world: neanderthals, erectuses, and the diminutive Flores hominid, amongst others. Going back in time and down at the bottom of the overview, we find the earliest representatives of the hominines: Sahelanthropus and Orrorin. This is a cast of the skull of Sahelanthropus. The original fossil was discovered in Chad in Central Africa and is around seven million years old. It has a brain size of a living chimpanzee. Interestingly, close investigation of the skull has revealed that Sahelanthropus may in fact have walked on two legs, not four. This has to do with the position of the foramen magnum on the skull. The foramen magnum is the hole at the base of the skull, where the spinal cord enters the cranium. In hominids walking on two legs, it is situated forward below the base of the skull. In hominids walking on all fours - such as the chimpanzee - it is situated towards the back of the skull. However, whether Sahelanthropus walked on two legs or all fours is still greatly debated amongst palaeontologists. Another slightly younger fossil, named Orrorin, was most likely bipedal - that is it walked on two legs. Orrorin was discovered in Kenya and is around 6 million years old. Among the fossil bones is a thighbone, whose shape indicates a bipedal or two-legged gait. This would indicate that bipedalism - walking on just the hind limbs - within the hominines, is a feature that is at least 6 million years old. Now this date is really interesting, because it could tell us something about the evolution of our closest living relative: the chimpanzee. Here is the skeleton of a chimpanzee. Humans share a 98.89 % overall similarity in our DNA with chimpanzees. Molecular clock studies in general indicate that humans and chimpanzees shared a common ancestor between 6 and 5 million years ago. This would be the time, when our predecessors were the same species, before branching off into two divergent evolutionary lineages. If this ancestral species was already walking bipedally like Orrorin, this in turn would mean that the ancestors of the chimpanzees returned back down to walking on all fours. So their four-legged gait does not represent the original primitive condition, but rather an evolutionary redevelopment from a two-legged stance. Back on the overview of hominine lineages, we find two extinct species belonging to the genus Ardipithecus, who lived after Orrorin. Both species derive from Ethiopia in east Africa, and are between 5.6 and 4.4 million years old. A well-preserved fossil skeleton shows that Ardipithecus was both able to walk upright, and had arms and hands well suited for grasping and climbing. Following Ardipithecus in time are the slender australopithecines. They were a very diverse and widespread group of hominines, who existed in east and south Africa between 4 and 3 million years ago, as fossils show us. It is from somewhere amongst the slender australopithecines that the genus Homo - the human genus - originated. This is a cast of the famous fossil find "Lucy", which was discovered in the Afar region of Ethiopia in 1974. This 3.2 million years old skeleton showed for the first time that australopithecines walked upright, despite having a brain size similar to a modern chimpanzee. Several later studies of Lucy and many more australopithecines have revealed that although they were bipedal, they also possessed highly mobile shoulder joints and fingers adapted for climbing. Additionally, australopithecines may have used primitive stone tools, but this is currently debated. Now, Lucy is an adult female; this is the skull of a male Australopithecus. There was a large degree of size difference between the sexes, with males being as much as 50% larger than females. As mentioned before, the human lineage originated from one of the species of Australopithecus. A number of events took place at this point, such as radical changes in diet; tool use and the taming of fire. This is something, which you will investigate on your own, in the next video assignment.
