The order Primates is a diverse group of mammals characterized by a generalized body plan favoring mobility, specialized hands and feet for grasping, and a reliance on vision over smell. This lineage includes lemurs, monkeys, apes, and humans. Tracing the origin of primates requires looking back over 65 million years, following the extinction of the non-avian dinosaurs. Understanding this evolutionary history involves determining when and how the defining characteristics of modern primates first coalesced into a recognizable lineage.
The Archaic Ancestors
The earliest potential relatives of primates emerged during the Paleocene Epoch (66 to 56 million years ago). These archaic mammals are collectively known as Plesiadapiforms, representing a stem group close to the main primate line. Their fossil remains are found primarily across North America and Europe, indicating a wide distribution.
Plesiadapiforms, such as the genus Plesiadapis, were small, likely arboreal creatures living in tropical forests. They possessed skeletal features suggesting an affinity with later primates, including adaptations for climbing and a petrosal bulla, a bony casing protecting the inner ear. However, they lacked the full suite of traits that defines a true primate, or Euprimate.
Plesiadapiforms had eyes positioned on the side of the head, limiting depth perception, and lacked the full post-orbital bar (a bony ring enclosing the eye socket). They retained claws on their digits instead of the flattened nails seen in modern primates. Their teeth often included large, rodent-like incisors separated by a gap, suggesting a diet focused on seeds and plants rather than the insects and fruit favored by later primates.
The Eocene Radiation
True primates, or Euprimates, appeared at the boundary between the Paleocene and Eocene epochs, roughly 56 million years ago. This rapid diversification coincided with the Paleocene-Eocene Thermal Maximum (PETM), a global warming event where average temperatures rose by 5 to 8 degrees Celsius. The warmer climate expanded tropical forests into northern latitudes, creating new ecological opportunities for arboreal life.
This environmental shift allowed the first Euprimates to disperse across North America, Europe, and Asia. These early true primates are divided into two main groups: the Omomyids and the Adapoids. Omomyids were small, likely nocturnal, and insectivorous or frugivorous, while Adapoids were larger, diurnal, and herbivorous.
The Omomyids and Adapoids shared several evolutionary innovations. These traits included a full post-orbital bar protecting the eye socket and the forward rotation of the eye orbits, resulting in stereoscopic vision. They also developed grasping hands and feet with opposable digits and flattened nails instead of claws, providing a more effective grip on branches.
The Split of Modern Lineages
Following the Eocene, global temperatures began a cooling trend that culminated in the Eocene-Oligocene extinction event around 34 million years ago. This cooling led to the retreat of northern forests and the extinction of many Eocene primate species in North America and Europe. Surviving primate populations were largely restricted to Africa and Asia, where the major modern lineages began to diverge.
This period saw the split between the two modern primate suborders: the Strepsirrhines and the Haplorhines. Strepsirrhines, which include modern lemurs and lorises, are thought to have evolved from the Adapoid line, retaining ancestral traits. Haplorhines, encompassing tarsiers, monkeys, and apes, likely arose from an Omomyid-like ancestor and experienced a broad evolutionary radiation.
By the Oligocene Epoch (34 to 23 million years ago), the first Old World monkeys (Catarrhines) and New World monkeys (Platyrrhines) were present in Africa and South America, respectively. The Catarrhines, whose lineage includes the apes, continued to diversify into the Miocene Epoch (23 to 5.3 million years ago), when the first early hominoids began their radiation. Fossils like Rukwapithecus confirm that the split between the hominoid and Old World monkey lines was underway approximately 25 million years ago, establishing the foundation for the diverse primate family tree.