Catarrhines, the infraorder of primates that encompasses all Old World monkeys, apes, and humans, represent a major branch on the tree of life. Understanding what these primates evolved from provides a direct view into the deep history of our own lineage. The evolutionary transition that led to the Catarrhini group involved significant anatomical changes and is recorded primarily in the fossil beds of Afro-Arabia. These early ancestors laid the groundwork for the modern diversity of primates found across Africa and Asia today.
Defining Features of Catarrhines
The modern Catarrhine parvorder is defined by several distinct anatomical features that differentiate them from their New World counterparts. One of the most obvious traits is their narrow, downward-facing nostrils, a feature that gives the group its name, which translates to “down-nosed.” This nasal structure contrasts sharply with the wide, side-facing nostrils seen in New World monkeys.
The Catarrhine dental formula is consistently 2.1.2.3, meaning they possess two incisors, one canine, two premolars, and three molars in each quadrant of the jaw. This reduction to two premolars per quadrant is a shared derived trait separating them from most other anthropoids. Furthermore, all Catarrhines share a unique cranial feature: a bony ear tube, known as the tubular ectotympanic, which encloses the middle ear. These specific traits serve as diagnostic markers for identifying the ancestral Catarrhine lineage.
The Oligocene Ancestral Lineage
The immediate progenitors of Catarrhines are found in the Oligocene epoch, which spanned from approximately 33.9 to 23 million years ago. The most informative fossil record for this period comes from the Faiyum Depression in Egypt, a site that was once a lush, forested environment. This region has yielded several genera considered to be stem-Catarrhines, meaning they appeared after the split from New World monkeys but before the divergence of Old World monkeys and apes.
A major candidate for an ancestral Catarrhine is Aegyptopithecus zeuxis, an early Oligocene species dated to about 29 to 30 million years ago. Aegyptopithecus exhibits a mosaic of features, possessing the Catarrhine dental formula of 2.1.2.3 and fully enclosed eye sockets. However, it retained primitive postcranial traits suggesting it was a slow, arboreal quadruped. Its anatomical pattern places it as a common ancestor to both later Old World monkeys (Cercopithecoidea) and apes (Hominoidea).
Other Oligocene groups from Faiyum, such as the Propliopithecoidea and Parapithecoidea, also represent the earliest evidence of the Catarrhine grade of evolution. These fossils confirm Afro-Arabia as the geographic origin for the lineage. The transitional craniodental features found in these Oligocene primates, particularly the shift to the two-premolar condition, offer a direct record of the evolutionary steps leading to the crown Catarrhines. The density and diversity of these early anthropoid fossils in the Faiyum make it the primary source for understanding the first appearance of the Catarrhine body plan.
The Split from Other Early Anthropoids
The Catarrhine lineage first became distinct with its separation from the Platyrrhine parvorder, the ancestors of New World monkeys. This foundational divergence event is estimated by molecular clock studies to have occurred in the late Eocene, approximately 35 to 44 million years ago. This separation was primarily a geographical one, centered on the isolation of the Catarrhine ancestor in the Afro-Arabian landmass.
While the earliest Catarrhines remained in Africa, the ancestors of Platyrrhines successfully dispersed to the Americas. This trans-Atlantic journey, likely involving a ‘rafting’ event on floating mats of vegetation across a narrower, island-dotted Atlantic Ocean, resulted in the evolutionary isolation of the two groups. Following this split, the Catarrhines continued to evolve the unique set of anatomical characteristics that define the group today, such as the full bony enclosure of the ear and the reduced number of premolars. The sustained presence of the Catarrhine line in Afro-Arabia allowed for the subsequent diversification into the two major superfamilies, the Old World monkeys and the apes.