2.2 Evolutionary History of Primates

Primates emerged around 65 million years ago, evolving from small nocturnal mammals. They developed key adaptations like grasping hands, forward-facing eyes, and larger brains. These changes helped them thrive in trees and eventually on the ground.

Climate shifts played a huge role in primate evolution. Warming periods allowed them to spread, while cooling events forced adaptations. As forests shrank, some primates moved to the ground, leading to new forms of movement like knuckle-walking and bipedalism.

Primate Origins and Early Evolution

Origins of primates

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• Emerged during Paleocene epoch ~65 million years ago evolved from small nocturnal insectivorous mammals adapted to arboreal environments
• Earliest primate-like mammals called plesiadapiforms lived during Paleocene and early Eocene shared some features with modern primates (grasping hands) but lacked others (forward-facing eyes)
• First true primates appeared in fossil record during early Eocene ~55 million years ago included two major groups:
  1. Adapiformes (adapids) early prosimian-like primates resembled modern lemurs (Notharctus, Darwinius)
  2. Omomyoidea small nocturnal primates possibly ancestral to tarsiers and anthropoids had large eyes for night vision

Transitions in primate evolution

• Strepsirrhine-haplorhine split occurred ~65-55 million years ago led to two distinct lineages:
  • Strepsirrhines wet-nosed primates (lemurs, lorises, galagos) retained more primitive features
  • Haplorhines dry-nosed primates (tarsiers, monkeys, apes, humans) developed more advanced traits
• Anthropoid evolution emerged ~40 million years ago in Africa or Asia split into:
  • New World monkeys (Platyrrhini) migrated to South America developed prehensile tails
  • Old World monkeys and apes (Catarrhini) remained in Africa and Asia evolved diverse forms
• Ape evolution diverged from Old World monkeys ~25-30 million years ago underwent adaptive radiation:
  • Gibbons (lesser apes) specialized for brachiation
  • Great apes (orangutans, gorillas, chimpanzees, humans) developed larger bodies and brains
• Hominin evolution split from other great apes ~7-8 million years ago marked by:
  • Emergence of bipedalism freed hands for tool use
  • Increased brain size led to complex cognitive abilities

Environmental Influences and Adaptations

Climate's role in primate evolution

• Paleocene-Eocene Thermal Maximum (PETM) ~55.8 million years ago rapid global warming facilitated primate dispersal and diversification across continents
• Eocene-Oligocene cooling event ~34 million years ago caused extinction of many primate species survivors adapted to cooler drier conditions
• Miocene climate changes ~23-5 million years ago cooling and drying trends expanded grasslands reduced forests influenced ape and early hominin evolution:
  • Prompted some primates to adopt more terrestrial lifestyles
  • Led to adaptations for ground dwelling (knuckle-walking, bipedalism)
• Pleistocene climate fluctuations ~2.6 million - 11,700 years ago alternating glacial and interglacial periods impacted hominin dispersal and adaptation:
  • Forced migrations out of Africa
  • Drove technological innovations for survival in varied environments

Key adaptations of primates

• Visual adaptations enhanced survival in arboreal environments:
  • Stereoscopic vision provided depth perception for accurate jumping
  • Color vision in some lineages improved foraging efficiency (ripe fruit detection)
• Grasping hands and feet with opposable thumbs and nails instead of claws enabled:
  • Efficient climbing and maneuvering in trees
  • Manipulation of objects and tools
• Increased brain size and complexity led to:
  • Enhanced cognitive abilities problem-solving and learning
  • Social intelligence complex social structures and communication
• Locomotor adaptations diversified primate niches:
  • Arboreal adaptations grasping limbs prehensile tails (spider monkeys)
  • Terrestrial adaptations knuckle-walking (gorillas chimpanzees) bipedalism (humans)
• Dietary adaptations allowed exploitation of various food sources:
  • Dental adaptations for different diets (leaf-eating molars, fruit-piercing canines)
  • Digestive specializations foregut fermentation in leaf-eating primates (howler monkeys)
• Reproductive strategies increased offspring survival:
  • Extended infant dependency allowed for prolonged learning periods
  • Reduced litter size enabled greater parental investment per offspring
  • Longer lifespans and slower maturation rates supported complex social learning