Many absolute dating methods are based on the rate of decay of a radioactive isotope. A radioactive isotope is a chemical element that dissipates excess energy by spontaneously emitting radiation. These emissions happen at known and stable rates. Once the rate of decay of a radioactive isotope is established, the age of a specimen containing that isotope can be estimated within a range of possible error. C-14 The best-known method for determining the absolute age of fossils is carbon-14 or ¹⁴C (pronounced “C-14”) dating. All plants and animals contain the isotope carbon-14 (¹⁴C). Plants absorb ¹⁴C from the air, and animals ingest plants containing the isotope. Because plants only absorb ¹⁴C when they are alive and animals only consume plants when they are alive, scientists can determine how long ago an animal or plant died based on the amount of ¹⁴C that remains in their cells. Carbon-14 has a known half-life of 5,730 years. This means that approximately half of the original ¹⁴C in an organism will be eliminated in 5,730 years after its death. For example, if an organism had an original ¹⁴C value of 100, then after 5,730 years, only 50 units of ¹⁴C would be present. Thermoluminescence Another absolute dating technique that is frequently used by paleoanthropologists is thermoluminescence dating. Thermoluminescence dating requires that either the fossils to be dated or the sediments that the fossils are within have been exposed to a high-temperature event, such as a volcanic explosion. During such a high-temperature event, all the radioactive elements within the material are released. Consequently, the amount of radioactive elements that have accumulated in the artifact since the time of the high-temperature event can be used to calculate the artifact’s age. Primates of the Paleocene Epoch The Paleocene epoch began approximately 65 MYA and ended about 54 MYA. It is the most poorly understood epoch of the Cenozoic era, as it is the time period with the fewest fossils to represent it. However, this epoch is considered important to primate evolution because it offers the first unequivocal record of the earliest primates. Evidence of the most primitive primate yet identified was found in the U.S. state of Montana, in a geological deposit that was dated to the earliest part of the Paleocene. This creature is known as Purgatorius. Purgatorius is similar to extinct and living primates – and distinct from other mammals – in the presence of an elongated last lower molar and an enlarged upper central incisor (resulting in what one could think of as “Bugs Bunny teeth”). These two characteristics, which are shared by all living primates today, suggest that Purgatorius may be the common ancestor of later primates. Figure 4.28 Purgatorius unio may be the common ancestor of all later primate. Remains of Purgatorius unio have been found in deposits dated to be about 63 million years old. (attribution: Rice University, OpenStax, under CC BY 4.0 license) Primates of the Eocene Epoch The Eocene epoch, which began approximately 54 MYA and ended about 34 MYA, is marked by the disappearance of Purgatorius and the first appearance of primates that more closely resemble modern-day primates, especially in the fact that they possess postorbital bars composed entirely of bone. A postorbital bar is a bony ring surrounding the entirety of the eye orbit. This contrasts with other mammals whose postorbital bars are part bone and part cartilage. Some fossil specimens also possess a toothcomb and/or a grooming claw, characteristics that are exclusively found in strepsirrhine primates today. Other anatomical characteristics that are significant would be the ankle bones which researchers believe played a key role in the evolutionary success of primates. The evolution of primates during the Eocene was tremendous. It has been hypothesized that there were four times as many strepsirrhine primates during the Eocene than there are living primates today. Fossil primates in Eocene deposits are common in North America and Europe and are becoming known in Asia and Africa. However, there are currently no known fossil primates from the Eocene in South America or Antarctica. Primates of the Oligocene Epoch The Oligocene epoch, which began approximately 34 MYA and ended about 22 MYA, marks the appearance of the first fossil monkeys. The earliest unambiguous haplorrhine fossils were found at the Fayum, an archaeological site about 60 miles from Cairo, Egypt, that today represents part of the Sahara. The Fayum primates are divided into two main groups: Parapithecoidea and Propliopithecoidea. Based on their teeth, these primates are believed to be the earliest New World and Old World monkeys, respectively. Teeth are generally described according to a dental formula that indicates the number of each type of teeth in each quadrant of the jaw. An organism with a 2.1.2.3 dental formula has two incisors, one canine, two premolars, and three molars in each quadrant of their upper and lower jaws. Based on the presence of a third premolar, a trait found in all New World monkeys, it is probable that Propliopithecus represents the earliest New World monkeys, even though they first evolved in Africa. Likewise, it is probable the propliopithecoids represent the earliest catarrhine primate, as they are the first fossil monkeys that possess a dental formula of 2.1.2.3 found in catarrhine primates. Miocene Apes The Miocene epoch contains fossil evidence of some of the earliest apes such as Proconsul africanus africanus which lived in Africa from 23 to 14 mya. The earliest Miocene ape, found in Africa, is Proconsul. Unlike modern-day apes, the Proconsul lacked long, curved digits, suggesting that they were able to hang from branches but more often moved about on all four of their limbs. Proconsul also lacked a tail, which is why they are considered apes and not monkeys. Like all Old World monkeys and apes, including humans, their teeth show a pattern of 2.1.2.3. Another well-known ape from the Miocene is Sivapithecus. Sivapithecus fossils are very common throughout Asia, with a particularly large number having been found in Turkey. Like modern-day humans, they exhibit very thick dental enamel, suggesting that these apes regularly ate very hard foods. The most intriguing aspect of Sivapithecus morphology is that the skulls show a tremendous resemblance to the living orangutan in features such as its tall nasal openings and high eye sockets. Figure 4.29 Sivapithecus is one the earliest known ape species. Fossil remains exhibit the tall nasal openings and high eye sockets currently visible in orangutans. (credit: “Sivapithecus indicus (Fossil Ape) (Dhok Pathan Formation, Upper Miocene; Potwar Plateau, Pakistan)” by James St. John/flickr, CC BY 2.0) While it is known that orangutans probably evolved from a Sivapithecus-type ape, there are no clear candidates for the ancestors of modern African great apes. There have only been two fossils found that clearly and unequivocally belong to the ancestors of modern African apes. Samburupithecus is a large late Miocene ape found in northern Kenya. It is known to resemble modern African apes. It differs from other Miocene fossils in having molar teeth that are elongated in a direction from the front of the mouth toward the back, instead of from cheek to tongue. Another fossil from the late Miocene (9–10 MYA) that is sometimes identified as an ancestor of modern African apes is Ouranopithecus, found in Greece, which has facial morphology that links it to both African apes and humans.