The Primate Family Tree

Purgatorius and Early Primate Ancestry

• Introduction to Purgatorius:

  • Purgatorius is a plesiadapiform, a type of mammal considered a potential early primate ancestor.
  • Lived around 65 million years ago in western North America.
  • Resembled small, shrew-like creatures that lived in trees, eating insects and fruit.
  • Known from jaw and teeth fragments resembling true primates, and mobile ankles suited for arboreal life.

• The Primate Family Tree

  • Understanding the primate family tree helps trace the evolutionary path from creatures like Purgatorius to modern Homo sapiens.
  • Reconstructing evolutionary history relies on traits and dates.

Reconstructing Evolutionary History

• Traits and Synapomorphies:

  • Scientists identify synapomorphies, which are shared traits inherited from a common ancestor.
  • Closely related groups share more synapomorphies.
  • Evolutionary history is reflected in both visible traits and genetic similarities.

• Molecular Clock:

  • The molecular clock uses DNA mutation rates to estimate the timing of evolutionary splits.
  • It relies on non-coding DNA, where mutations occur at a constant rate without selective pressure.
  • Fossils with known radiometric ages help calibrate the molecular clock.
  • Process:
    • Quantify genetic difference between two groups of organisms.
    • rate = \frac{genetic\ difference}{age\ of\ relevant\ fossil}
    • Use the calculated rate to estimate the timing of splits between groups.

• Last Common Ancestor (LCA):

  • The relative that existed just before a split in the evolutionary tree.
  • LCAs appear at branching points in the primate family tree, indicating evolutionary relationships.

Taxonomy and Classification

• Taxonomy defined: A method for classifying and naming organisms.

• Taxonomic Ranks:

  • Species: sapiens (our species)
  • Genus: Homo (includes immediate ancestors and close fossil relatives)
  • Tribe: Hominini (hominins: humans and extinct relatives since the last common ancestor with chimps and bonobos, which lived 4-8 million years ago)

• Hominin Traits:

  • Bipedalism.
  • Canine teeth size is similar in males and females.
  • Three main contenders for the earliest known hominin:
    • Sahelanthropus tchadensis (around 7 million years old).
    • Orrorin tugenensis (about 6 million years old).
    • Ardipithecus kadabba (between 5.8 and 5.2 million years old).

• Family: Hominidae (hominids or great apes: humans, chimps, bonobos, orangutans, and gorillas).

  • Traits:
    • Large-bodied.
    • Males tend to be larger than females.
    • Lack ischial callosities (specialized fatty pads on buttocks).

• Evolution within Hominidae:

  • Orangutan lineage branched off 12-15 million years ago (e.g., Sivapithecus).
  • Dryopithecines lived in Europe around the same time and may have given rise to African apes (gorillas, chimps, and bonobos).

Apes and the Superfamily Hominoidea

• Superfamily: Hominoidea (includes apes).

  • Families:
    • Hominidae (great apes).
    • Hylobatidae (lesser apes: gibbons and siamangs).
    • Split occurred 16-24 million years ago during the Miocene.

• Shared traits of hominoids related to upright posture and arboreal life:

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

• Dental Characteristics:

  • Y-5 molars: Lower molar teeth with five cusps forming a Y shape.

Infraorder Simiiformes and Suborder Haplorhini

• Infraorder: Simiiformes (includes monkeys and apes).

  • New World monkeys and Old World monkeys.
  • Eye sockets completely enclosed by bone, dry noses (no rhinarium).
  • Old World monkeys share a more recent common ancestor, nostrils open downward.
  • New World monkeys' nostrils open sideways.

• Evolutionary Splits:

  • Hominoid lineage split from Old World monkeys 24-38 million years ago (around 29 million years ago in the Oligocene).
  • New World monkeys branched off earlier, 33-44 million years ago in the Eocene.

• Suborder: Haplorhini (includes apes, monkeys, and tarsiers).

  • Traits:
    • Dry noses.
    • Broad, flat incisors.
    • Eye orbits at least partially enclosed with bone at the back.
  • Tarsiers exclusively eat other animals (insects and small vertebrates).

• Origins of Haplorhines:

  • Tarsiers and Simiiformes may have diverged as far back as 65 million years ago in the early Paleocene.
  • The ancestor of all haplorrhines was likely small, arboreal, and ate insects and fruits, resembling Purgatorius.

Order Primates

• Order: Primates (includes Haplorrhini and Strepsirrhini: lemurs and lorises).

  • Defining Traits:
    • Forward-facing eyes for binocular vision (good depth perception).
    • Eye orbits partially or completely enclosed by bone.
    • Opposable, grasping thumbs.
    • Fingernails instead of claws.
    • Relatively large brains.
    • Slower life histories (slower maturation and longer lifespans).

• Dating the Origin of Primates:

  • Purgatorius dates from around 65 million years ago.
  • Molecular clock studies suggest primate origins may be 10 million years earlier, during the Cretaceous period.

• Ongoing Debate:

  • Debate about whether plesiadapiforms (like Purgatorius) are true primates because they lack enclosed bony orbits, nails, and forward-facing eyes.
  • Some researchers support their primate status based on teeth and ankle anatomy.

• Uncontroversial Primates:

  • Earliest uncontroversial primates with all primate features (adapoids and omomyoids) appeared about 55.8 million years ago at the start of the Eocene.