Primate Evolution

General Primate Traits

Most primate traits are found in haplorhines, including:

  • Binocular stereoscopic vision

  • Larger brains

  • Nails instead of claws

  • Reduced sense of smell

🤔 Hypotheses for Primate Trait Evolution

There are three primary hypotheses for why primates evolved these traits:

🌳 Arboreal Hypothesis

  • Primates adapted to living in the trees.

  • Consuming food in the trees led to a reduced sense of smell.

  • Moving within trees/branches and becoming more social led to binocular stereoscopic vision, increased intelligence, and prehensile hands/feet.

    Problem: Other tree-dwelling animals don't have these adaptations.

🐞 Visual Predation Hypothesis

  • Primate traits arose in response to preying on insects and small animals.

  • Hunting insects and animals were the pressures that led to primate adaptations.

    Problem: Many primates rely more heavily/exclusively on vegetation.

🌸 Angiosperm Radiation Hypothesis

  • Primate traits, particularly binocular stereoscopic vision, evolved as a response to fruits and flowers as food sources.

    Angiosperms: Fruit and flowering plants.

  • Little light in forests favored binocular stereoscopic vision.

  • Prehensile hands/feet aided in grasping fruit without moving back and forth across trees.

  • Fruit also evolved in the Cenozoic (age of mammals).

    Problem: What about senses beyond sight? Fruit is not in season year-round.

⏳ Primate Evolution Timeline

Paleocene Epoch (65-55mya)

  • Order: Plesiadapiforms

    • Small mammal, possible ancestor to primates.

    • Similar to tree shrews.

    • Found in Western North America, Western Europe, Asia, and possibly Africa.

    • Lacked post-orbital bar, opposability/prehensility in hands/feet, and nails.

    • Had specialized dentition and a tiny brain.

  • Plesiadapiforms went extinct at the end of the Paleocene with global warming and tropical conditions.

Eocene Epoch (55-34mya)

  • Basal anthropoids: Considered the first anthropoids.

    • Eosimias: Genus of small, monkey-like primate found in China; roughly 45mya. Tarsal bones similar to living anthropoids.

    • Biretia: Genus of anthropoids found in the Fayum Depression of Egypt; roughly 37mya. Two-cusped lower premolars.

  • Euprimates: True primates; found in many places across the globe.

    • Post-orbital bar, opposable digits, nails, generalized dentition, larger brain to body ratio.

    • Split into two groups: Adapoids and Omomyoids.

      • Adapoids: Likely ancestors to Strepsirhines.

        • Roughly lemur sized.

        • Sexually dimorphic in terms of body and canine size.

        • Northarctus

      • Omomyoids: Likely ancestors to Haplorhines.

        • Projecting lower incisors.

        • Small canines.

        • Narrow snout.

        • Large eye orbits - likely nocturnal.

Oligocene Epoch (34-23mya)

  • Rapid global cooling leads to new habitats.

  • Most Oligocene fossils found in the Fayum dated to 37-29mya.

  • Defined by the diversification of primates.

    • Multiple Strepsirhines

    • Ancestors of anthropoids: Oligopithecids, Parapithecids, Propliopithecids.

"New World" Expansion

  • Genus Perupithecus: fossil molars in Peru; ~36mya

  • Genus Branisella: 2-1-3-3 dental formula (Platyrrhine); dates to 26mya; found in Bolivia

Two hypotheses about primates arriving & evolving in the "New World":

  1. Platyrrhines evolved from an African anthropoid, then migrated across the Atlantic Ocean.

  2. Platyrrhines evolved from an anthropoid in Africa that migrated across Antarctica into South America.

Evidence for both:

  • Anthropoids in Africa predate Platyrrhines but have 2-1-3-3 dental formulae.

  • Eocene had a warm/dry climate so crossing into S. America possible.

  • DNA evidence shows similarities between Old World and New World primates.

  • Genetic divergence ~40mya.

🐒 Miocene Epoch (24-5mya)

  • Some gaps in fossil record, but Oligocene catarrhine fossils likely ancestors to Miocene proconsulids.

  • Warming trend again at the end of the Oligocene.

  • Defined by the radiation of apes, which outnumbered monkeys roughly 20:1.

  • Moved out of Africa, into Asia and Europe.

Key Groups and Expansions:

Group/Expansion

Characteristics

Proconsulids

Early Miocene apes found primarily in East Africa; date from 20-18mya

Parapithecids

Parapithecus & Apidium; 2-1-3-3 dental formula; potential link to Platyrrhines; leaper

Propliopithecids

Propliopithecus and Aegyptopithecus; 2-1-2-3; potential link to Catarrhines

Aegyptopithecus

Roughly howler monkey sized; sagittal crest; limbs of equal sizes – arboreal quadruped

European Expansion

Dryopithecus - genus of European apes (12-8mya) that likely ate fruit; noted for large canines and Y-5 molars; dental enamel studies shows that they grew more slowly than previous primates (getting us closer to hominoids); brain size close to modern chimp; skeletal structure shows they brachiated & hung from trees

Sivapithecus

12-8mya; thick enamel – indicates eating hard foods, like seeds and nuts (probably with leaves); likely ancestors to modern orangutans; ONLY direct link we can make from extinct to extant primates; Share concave faces, narrow nasals, oval eye orbits, broad front teeth, small lateral incisors, large canines

African

Proconsul – diverse genus with fossils found in a variety of environments; ancestor to Catarrhines; Skulls and teeth – ape-like:Y-5 cusp pattern with rounded cusps; 2-1-2-3; no tail; *large big toe; Postcrania – monkey-like: equal limb length (arboreal quadrupeds); carpals show limited arm mobility; slim tarsals

Gigantopithecus

8-0.3*mya; Known from tooth and mandible fragments only; Likely eating hard foods; Distant relative of Sivapithecus; Estimated to be roughly 10ft tall, though was likely a quadruped given its heft (~600-700lbs)

  • Dryopithecus is important because the Y-5 pattern is named after them.