Evolution of Simian Primates

Divergence of Simian Primates into New World and Old World Groups

Simian primates, encompassing monkeys, apes, and humans, underwent a pivotal evolutionary divergence into two distinct infraorders: Platyrrhini, the New World monkeys of South and Central America, and Catarrhini, which includes the Old World monkeys, apes, and humans of Africa and Asia. This split is associated with several adaptations to a diurnal lifestyle, such as the loss of the tapetum lucidum, an eye layer aiding nocturnal vision, and the development of color vision. New World monkeys are noted for their prehensile tails and a prevalence of color blindness in males due to a different visual system. The ancestors of New World monkeys are believed to have colonized South America through a dispersal event, possibly by rafting on mats of vegetation across the Atlantic or via a land bridge, although the exact mechanism remains debated. Old World simians, on the other hand, continued to evolve in Africa, with early representatives like Aegyptopithecus providing insight into their characteristics.

Evolutionary Split of Old World Monkeys and Apes

Within the Catarrhini, a further evolutionary split occurred, giving rise to the superfamilies Cercopithecoidea (Old World monkeys) and Hominoidea (apes, including humans). This division was marked by significant evolutionary developments such as the emergence of trichromatic color vision in some lineages and the reduction of the vomeronasal organ, which is involved in pheromone detection. The genus Proconsul, an early catarrhine, displayed a combination of monkey and ape traits, suggesting it may have been a common ancestor to both groups. Proconsul's dental structure, skeletal build, and arboreal lifestyle were monkey-like, while its lack of a tail and certain skeletal features were more ape-like. Proconsul africanus, in particular, is considered a potential ancestor to both the great and lesser apes, as well as humans.

Speciation of Great Apes and Early Hominids

The family Hominidae, which encompasses the great apes, diverged from the lesser apes (gibbons) around 20 to 15 million years ago. This lineage is characterized by several distinctive traits, including a flatter face with reduced snout and the loss of the ability to produce uricase, an enzyme involved in uric acid metabolism. The subfamily Homininae, which includes the human lineage, separated from the lineage leading to orangutans between 18 to 14 million years ago. Fossil species such as Pierolapithecus catalaunicus and Danuvius guggenmosi provide valuable insights into the locomotor abilities of our ancestors, with evidence of both arboreal climbing and bipedalism, suggesting that the last common ancestor of humans and other apes may have been adapted to a form of locomotion that combined these two modes.

The Emergence of Hominini and the Advent of Bipedalism

The tribe Hominini, which includes humans and their closest relatives, the chimpanzees, emerged between 10 to 5 million years ago. During this time, the larynx began to reposition lower in the throat, a feature that is important for complex vocalization and is present in both chimpanzees and humans. The earliest potential members of the Hominina or Homininae include genera such as Ouranopithecus and Sahelanthropus. Ardipithecus, a genus of early hominins, shows evidence of both arboreal and bipedal locomotion, indicating a transitional stage in the evolution of walking on two legs.

Australopithecus afarensis and the Progression of Australopithecines

Australopithecus afarensis, a significant early hominin species, is known for the Laetoli footprints, which provide clear evidence of bipedalism. Living between 3.9 and 2.9 million years ago, this species had a mix of primitive and advanced features, including smaller canines and molars and a prognathic face, with a brain size smaller than that of later hominins. Australopithecines inhabited savannah environments and had a varied diet that likely included meat. Adaptations in the spine of Australopithecus africanus suggest that bipedalism was a well-established mode of locomotion in these early hominins, even during pregnancy.

The Rise of the Genus Homo and the Onset of Tool Use

The genus Homo, which signifies a major evolutionary step with the advent of stone tool use, evolved from australopithecine ancestors in East Africa and marked the beginning of the Lower Paleolithic era. Homo habilis, an early Homo species, represents a transitional stage between Australopithecus afarensis and Homo erectus. The discovery of stone tools in China dating back to 2.12 million years ago suggests an early dispersal of hominins out of Africa. Homo erectus is notable for its association with the control of fire, increased brain size, and the spread throughout Eurasia. This period also saw the divergence of the Neanderthal and Denisovan lineages, with Homo heidelbergensis emerging as a potential common ancestor to both Neanderthals and modern humans.