c. 55 million years ago
The first primates
In the early Eocene, in forests expanded by the PETM, large-eyed, grasping-handed primates emerged as a distinct mammalian lineage. The biological history of the human lineage begins here.
Around 55 million years ago, at the Palaeocene-Eocene boundary during the sudden warming event known as the PETM (Palaeocene-Eocene Thermal Maximum), which expanded forested habitats globally, primates appear distinctly for the first time in the fossil record as an independent lineage. These tiny mammals β large eyes set forward for stereoscopic vision, grasping hands and feet, nails rather than claws at their digit tips β carried a body plan shaped for arboreal life.
Archicebus achilles, recovered from China and dated to around 55 million years ago, is among the earliest candidate primate fossils; its precise relationship to tarsiers and other Euprimates is still debated. From North America, Teilhardina and Cantius are well-documented early Eocene primates. Whether the older Plesiadapiformes (c. 65 Ma) belong within Primates at all remains contested.
Why did primates move into the trees? The "visual predation hypothesis" argues that stereoscopic vision and grasping hands evolved for catching insects. The "angiosperm radiation hypothesis" suggests that the spread of flowering plants created a new food source β fruits and flower nectar. These hypotheses are not mutually exclusive; both pressures likely operated together.
Through the Eocene, primates spread across North America, Europe, Africa, and Asia. Two major groups took shape β Adapiformes and Omomyiformes β and today's monkeys, apes, and humans descend from these lineages. Notharctus, a well-known adapid from middle Eocene North America, is represented by complete skeletal specimens displayed in museums as one of humanity's most distant recognisable ancestors.
The enlarged brain-to-body ratio, social learning, and extended mother-offspring bonding that distinguish primates from other mammals began shaping themselves from the Eocene onward. Around 35 million years ago simians (true monkeys) emerged in Afro-Arabia; 25 million years ago the Hominoidea β ancestors of the apes β diverged. That chain leads back to a small, grasping, forest-dwelling mammal 55 million years ago.
Sources
- A new primate from the earliest Eocene of Europe inferred from its teeth β Nature
- An Eocene primate from China β Archicebus achilles β Nature
- Primate evolution β Smithsonian Human Origins β Smithsonian Human Origins Program