Primate Ancestry and Evolutionary History

Primate Ancestry: Tracing Our Evolutionary History

Introduction to Purgatorius

• Purgatorius:
  • An early mammal, a plesiadapiform, potentially among our earliest ancestors.
  • Lived around 65 million years ago in western North America.
  • Small, shrew-like creatures that climbed trees, eating insects and fruit.
  • Known from jaw and teeth fragments, resembling later primates more than other groups.
  • Mobile ankles suited for arboreal life.
  • Lacked classic primate traits like forward-facing eyes and nails.

Reconstructing Evolutionary History

• Scientists use traits and dates to reconstruct evolutionary history.

Traits and Synapomorphies

• Synapomorphies: Shared traits inherited from a common ancestor.
  • Closely related groups share more synapomorphies.
  • Emphasizes unique features and similarities with other primates.

Genome and Molecular Clock

• Genome reveals genetic similarities and provides a timeline using the molecular clock.

• Molecular Clock:

  • DNA accumulates mutations at a constant rate over time.
  • Uses non-coding DNA (not under selective pressure) for accuracy.

• Setting the Clock:

  • Quantify genetic differences between groups.
  • Divide by the age of a well-dated fossil with a known radiometric age.
  • Calculate the timing of splits between groups.

• Last Common Ancestor (LCA):

  • Appears at branching points on the primate family tree.

Taxonomy and Classification

• Taxonomy: Method for classifying and naming organisms.

• Taxonomic Ranks:

  • Genus (e.g., Homo) and Species (e.g., sapiens) are common ranks.
  • Nested hierarchy reflecting evolutionary relationships.

Human Classification Example

• Genus: Homo (immediate ancestors and close fossil relatives).
  • Earliest fossil: Partial mandible from Ledi-Geraru, Ethiopia, 2.8 to 2.75 million years ago (Pliocene Epoch).
• Tribe: Hominini (hominins).
  • Includes humans and extinct relatives since the last common ancestor with chimps and bonobos.
  • Lived between 4 and 8 million years ago (Pliocene or Miocene Epoch).
  • Key traits: Bipedalism and similar canine sizes in males and females.
  • Potential earliest hominins: Sahelanthropus tchadensis (7 million years old), Orrorin tugenensis (6 million years old), and Ardipithecus kadabba (5.8 to 5.2 million years old).
• Family: Hominidae (hominids or great apes).
  • Includes humans, chimps, bonobos, orangutans, and gorillas.
  • Traits: Large bodies, males often larger than females, lack ischial callosities (butt pads).

Hominid Evolution

• Orangutan lineage branched off 12 to 15 million years ago.
  • Early members: Sivapithecus (fossils found in Asia).
• Dryopithecines: Tribe of hominids in Europe around the same time.
  • Potentially gave rise to African apes (gorillas, chimps, bonobos).

Apes and Hominoidea

• Superfamily: Hominoidea (apes).

• Families:

  • Hominidae (great apes).
  • Hylobatidae (lesser apes: gibbons and siamangs).

• Split between great and lesser apes: 16 to 24 million years ago (Miocene).

• Shared Features (Hominoidea):

  • Related to upright posture and arboreal life.
  • Stiffer lower backs, fewer lumbar vertebrae.
  • Ability to rotate arms above the head.
  • Lack tails.
  • Slower maturation, longer lifespans, relatively larger brains.

Dental Characteristics

• Y-5 Molars:
  • Characteristic cusp pattern on lower molars.
  • Five cusps forming a Y shape.

Infraorder: Simiiformes (Monkeys and Apes)

• Includes New World monkeys and Old World monkeys.

• Traits:

  • Eye sockets completely enclosed by bone.
  • Dry noses (lack rhinarium).

• Nostril Orientation:

  • Old World monkeys (and humans): Nostrils open downward.
  • New World monkeys: Nostrils open sideways.

• Split from Old World monkeys: 24 to 38 million years ago (Oligocene), around 29 million.

• New World monkeys branched off: 33 to 44 million years ago (Eocene).

Suborder: Haplorhini

• Includes apes, monkeys, and tarsiers.
• Tarsiers:
  • Only primate that exclusively eats other animals.
• Haplorrhine Traits:
  • Dry noses.
  • Broad, flat incisors.
  • Eye orbits partially enclosed by bone.
• Split from tarsiers: Possibly 65 million years ago (early Paleocene).
• Ancestor of all haplorrhines: Small, arboreal, diurnal, ate insects and fruits (similar to Purgatorius).

Order: Primates

• Includes haplorhines plus lemurs and lorises (strepsirrhines).
• Primate Traits:
  • Forward-facing eyes (binocular vision and depth perception).
  • Eye orbits partially or completely enclosed by bone.
  • Opposable, grasping thumbs.
  • Fingernails instead of claws.
  • Relatively large brains.
  • Slower life histories.

Purgatorius Revisited and Early Primates

• Purgatorius: Earliest known potential primate (fossils ~65 million years ago).
• Molecular clock studies: Origin of primates may be 10 million years earlier (Cretaceous period).
• Debate: Whether Purgatorius and other plesiadapiforms are true primates.
• Earliest uncontroversial primates: Adapoids and omomyoids (55.8 million years ago, start of the Eocene).

Conclusion: Your Place on the Tree of Life

• Your classification: sapiens, genus Homo, hominin, hominid (great ape), hominoid (ape), simiiform (monkey/ape), haplorrhine, primate.
• Distant ancestors: Small, tree-dwelling creatures like Purgatorius.