The evolutionary history of the primates can be traced back 57-90 million years. One of the oldest known primate-like mammal species, Plesiadapis, came from North America; another, Archicebus, came from China. Other such early primates include Altiatlasius and Algeripithecus, which were found in Northern Africa. Other similar basal primates were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene. Purgatorius is the genus of the four extinct species believed to be among the earliest example of a primate or a proto-primate, a primatomorph precursor to the Plesiadapiformes, dating to as old as 66 million years ago.
The surviving tropical population of primates, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of the Faiyum depression southwest of Cairo, gave rise to all living species—lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and the anthropoids: platyrrhine or New World monkeys, catarrhines or Old World monkeys, and the apes, including Homo sapiens.
Early primate evolution
Primates are believed to have diverged from other mammals during the Late Cretaceous period of the Mesozoic Era. Fossils of Cretaceous primates are virtually non-existent, with estimates based on molecular data instead of morphological information. The oldest record of stem-primates comes from the order Plesiadapiformes in the earliest Paleocene epoch shortly following the Cretaceous-Paleogene (K/PG) extinction event. The plesiadapiform subfamily Purgatoriidae represents the earliest plesiadapiform lineage known to diversify, as fossils of the family are known from sediments across North America within ~140,000 years of the K/PG extinction. Though fossils of purgatoriids are rare and limited to fragmentary elements, they are considered to be stem-primates based on the morphology of their ankles which share several features with both Plesiadapiformes and wider members of Euarchonta. This placement has been supported in subsequent phylogenetic analyses. The morphology of purgatoriids suggests an arboreal lifestyle that has been hypothesized as ancestral to crown primates, while their tooth morphology, which possesses a mixture of pointed and rounded cusps, is theorized to have supported an insectivorous or omnivorous diet depending on the species. Other fossils in the group reveal long snouts with large upper incisors and laterally facing orbits lacking postorbital bars, also suggesting early primate traits.
Throughout the Paleocene and Eocene epochs, plesiadapiforms underwent a substantial radiation based in North America and Europe, though several species are known from Asia as well. In total, eleven different plesiadapiform families are recognized from the fossil record, encompassing over 25 million years of time and displaying a wide diversity of dietary behaviors. The oldest fossils of euprimates come from the Paleocene-Eocene transition, approximately 57 million years ago, and already demonstrate a split between haplorhines and strepsirrhines. This, in combination with molecular studies, suggests that the divergence of haplorhines and strepsirrhines came in the Paleocene, estimated to be sometime between 64 and 63 million years ago.
Evolution of strepsirrhines
A note on "prosimians" Strepsirrhines, the group of primates including lemurs, lorises, and galagos, is also part of the older categorization "prosimians". The "prosimian" category is a paraphyletic group which includes all strepsirrhines and tarsiers. Tarsiers are part of the monophyletic clade of Haplorhinii but share some traits with Strepsirrhines leading to the older mis-categorization creating the group "prosimians". A significant amount of fossil work still discusses Strepsirhini within the context of prosimians because of the shared morphological and locomotor traits between Strepsirhini and Tarsiformes, namely: presence of a Grooming claw, clinging and leaping locomotion, Unfused mandibular symphysis, high cusped molar teeth, and a bicornate uterus.
Adapoids: Potential early strepsirrhines
The earliest strepsirrhines are known as adapiforms, a diverse group that ranged throughout Eurasia and North America. Adapoids share several traits with modern Strepsirrhines including: long snouts, relatively small eyes, a long nasolacrimal canal indicating a rhinarium or "wet nose", a gap between small incisors to access their vomeronasal organ, canines larger than their incisors, and an inflated bony auditory bulla with a suspended tympanic ring. Adapoids originated in the Eocene, and though they survived till the late miocene, the earliest known strepsirhines appear in North African fossil beds from the Eocene. This indicates that, despite the longevity of adapoids, an early branch of this clade gave rise to lemuriform primates, including lemurs and their kin.
There are currently six adapiform clades: Notharctids, Cercamoniids or Protoadapids, Caenopithecids, Adapids, Sivaladapids, and Asiadapids.
Early European fauna is also exemplified by Darwinius, a basal strepsirrhine, or haplorhine dated to 47 million years ago (early Eocene)
Darwinius masillae Darwineus masilae is a primate fossil from an Eocene lagerstätte in Messel, Germany. Preserved in oil shale in an anoxic lake bed, the skeleton is laterally crushed but articulated, preserving the outline of the long bones and leaving irregular and short bones relatively intact. Darwinius (or "Ida," as the adolescent female has been nicknamed) has an estimated body mass of 650-1000g, and dentition and stomach contents indicating a herbivorous diet. Ida has a low intermembral index (long hindlimbs compared to forelimbs), indicating potential for clinging and leaping locomotion, and large orbits compared to skull length indicating nocturnality.
Since its discovery, Darwinius's mixture of primitive and derived traits has led to disagreements on its phylogenetic placement. Although this is fairly common within paleoprimatology, the Eocene time period and array of anthropoid and primitive traits specifically could influence the phylogenetic placement of adapoids and omomyoids depending on when the derived traits present in Darwinius evolved. It has a mixture of more basal and derived traits. Its short rostrum, fused mandibular symphysis, deep mandibular ramus, fused mandibular symphysis, spatulate incisors, quadrate lower molars, uncompressed mesocuneiform, and lack of tooth comb all align well with derived haplorhines.
Meanwhile, the lack of postorbital closure, a potentially sloping tibulo-fibular facet, and implacable grooming claw perpetuate the debate by indicating strepsirrhine phylogeny. Authors supporting this hypothesis state that many of the traits used to associate Darwinius with haplorhines are athropoid traits, while Darwinius lacks many basal haplorhine traits. They argue that spatulate incisors could be (and often are in primates) a symplesiomorphies for diet specialization rather than an indicator of haplorhine heritage. They also argue that a quadrate molar should not be counted as evidence for haplorhine relation, as many anthropoids do not have one.
Two points on which authors illustrate inter-observer variation are the condition of the tibulo-fibular facet and the nail or claw on digit II. Williams et al. argue that the tibulo-fibular facet is crushed and cannot be adequately observed for inference, while Gingerich et al. argue that it is not too crushed or obscured to infer it is sloping but also argue this is likely a primitive condition rather than a derived one. While both Williams and Gingerich say digit II has a nail, Williams argues it is symplesiomorphic with anthropoids while Gingrich claims it indicates early haplorhine traits.
Lorisiform evolution Lorisiforms are members of the superfamily Lorisoidea, within which are the families Galagidae (bushbabies and galagos) and Lorisidae (lorises and pottos). Loris and galago phylogeny is difficult to trace for a number of reasons, including the crypsis of both groups, arboreal lifetyles in densly forested environments, and frequent convergence on similar morphologies. Researchers continue to disagree on the phylogenetic organization of extant lorisiformes, primarily whether the subfamily Perodicticinae (angwantibos and pottos / African lorises) is truly a sister group to Lorisinae (Asian lorises) or more closely related to extant Galagidae (galagos) and subject to a surprising convergence with lorises. Galagos are characterized by leaping saltatorial locomotion aided by elongated tarsal bones and insectivory, while lorises and pottos are known for a form of slow arboreal quadrupedalism and secondarily shortened tarsal bones. However, lorises are subject to significant morphological plasticity based on diet; for example, slow lorises and pottos are primarily frugivorous and exudate eaters, requiring less stereoscopic vision, while slender lorises and angwantibos, which are nocturnal predators, have more convergent orbits and thereby more stereoscopic vision.
Based on mitochondrial cytochrome sampling of modern taxa, lorises and galagos split from lemurs has been estimated to the Late Paleocene. Continuing disagreement on the constitution of each clade makes classifying fragmentary fossil primates even more difficult. Karanisia clarki and Saharagalago, both dating to 37mya Fayum deposits in Egypt, are two of the oldest potential Lorisiformes. Saharagalago is a stem galagid from Fayum, Egypt, 37mya; although its dentition is very similar to modern galagos, it is still considered a potential stem member of Galagidae or Lorisoidae. Consisting of only a mandible fragment and a few isolated teeth, Karanisia clarki is the oldest fossil primate known to have a tooth comb. Karanisia's estimated body mass is 273grams. Dental morphology and microwear analysis of the molars indicate a potential for folivory. However, a completely folivorous diet is prohibited by the energy and metabolic requirements of its size, leading researchers to reconstruct Karanisia as an omnivore. Karanisia molars closely resembles those of the living lorisid Arctocebus, the golden potto. Although its position as the oldest tooth-combed primate fossil places it confidently within at least stem Strepsirrhini, many researchers debate if it is a crown strepsirhini, stem lorisoid, or stem lemuroid.
Wadilemur elgans is a later stem galagid from 35mya; in addition to similar dental features to modern galagos, it also has evidence of leaping traits.
Semicircular canals in primate locomotion Semicircular canals are often used by paleontologists and paleoanthropologists to inform locomotor estimations when there is limited postcranial evidence for a species. Semicircular canals are held within the petrous portion of the temporal bone, a very thick and often well-preserved portion of the skull. Semicircular canals with a larger radius of curvature and more intraspecific consistency can indicate faster movement, as more surface area is needed for more precise perception of balance. In contrast, animals that move more slowly tend to have more variation in general, as their canals are not under pressure to maintain a certain morphology. Galagos generally utilize a clinging and leaping locomotion, while lorises tend towards a slow arboreal quadrupedal form of locomotion. As a result, there is a noticeable and quantifiable difference in their semicircular canal volumes, which can be quantified and used to assign fossil crania or partial crania to a range of speed and by extension infer each locomotion.
Malagasy radiation Lemurs