|A catarrhine (common chimpanzee) and a platyrrhine (red-faced spider monkey)|
The simians or anthropoids are an infraorder (Simiiformes) of primates containing the parvorders Patyrrhini and Catarrhini, which consists of the superfamilies Cercopithecoidea and Hominoidea (including the genus Homo).
The simians are sister to the tarsiers, together forming the haplorhines. The radiation occurred about 60 million years ago (during the Cenozoic era). 40 million years ago, simians from Afro-Arabia colonized South America, giving rise to the New World monkeys. The remaining simians (catarrhines) split 25 million years ago into Old World monkeys and apes.
In earlier classification, New World and Old World monkeys, apes, and humans - collectively known as simians or anthropoids - were grouped under Anthropoidea (; Ancient Greek: , romanized: anthropos, lit. 'human'; also called anthropoids), while the strepsirrhines and tarsiers were grouped under the suborder "Prosimii". Under modern classification, the tarsiers and simians are grouped under the suborder Haplorhini while the strepsirrhines are placed in suborder Strepsirrhini. Strong genetic evidence for this is that five SINEs are common to all Haplorhines whilst absent in Strepsirrhines - even one being coincidental between tarsiers and simians would be quite unlikely. Despite this preferred taxonomic division, prosimian is still regularly found in textbooks and the academic literature because of familiarity, a condition likened to the use of the metric system in the sciences and the use of customary units elsewhere in the United States. In Anthropoidea, evidence indicates that the Old World and New World primates went through parallel evolution.
Primatology, paleoanthropology, and other related fields are split on their usage of the synonymous infraorder names, Simiiformes and Anthropoidea. According to Robert Hoffstetter (and supported by Colin Groves), the term Simiiformes has priority over Anthropoidea because of the taxonomic term Simii by van der Hoeven, from which it is constructed, dates to 1833. In contrast, Anthropoidea by Mivart dates to 1864, while Simiiformes by Haeckel dates to 1866, leading to counterclaims of priority. Hoffstetter also argued that Simiiformes is also constructed like a proper infraorder name (ending in -iformes), whereas Anthropoidea ends in -oidea, which is reserved for superfamilies. He also noted that Anthropoidea is too easily confused with "anthropoïdes", which translates to "apes" from several languages.
Extant simians are split into three distinct groups. The New World monkeys in parvorder Platyrrhini split from the rest of the simian line about , leaving the parvorder Catarrhini occupying the Old World. This group split about between the Cercopithecidae and the apes.
There are also some lines of extinct simian, either placed into Eosimiidae (to reflect their Eocene origin) and sometimes in Amphipithecidae, thought to originate in the Early Oligocene. Additionally, Phileosimias is sometimes placed in the Eosimiidae and sometimes categorised separately.
|Phylogeny of living (extant) primates|
|Cladogram. For each clade, it is indicated approximately how many million of years ago (Mya) newer extant clades radiated.|
Below is a cladogram with some of the extinct simian species with the more modern species emerging within the Eosimiidae. The Simians originated in Asia while the crown simians were in Afro-Arabia. It is indicated approximately how many million years ago (Mya) the clades diverged into newer clades.
In a section of their 2010 assessment of the evolution of anthropoids (simians) entitled 'What Is An Anthropoid', Williams, Kay and Kirk set out a list of biological features that are common to all or most anthropoids, including genetic similarities, similarities in eye location and the muscles close to the eyes, internal similarities between ears, dental similarities, and similarities on foot bone structure.