Cenozoic Era and the Tertiary Period Epochs

Geologic Ages: Cenozoic Era

• Origins of the primate order occurred approximately 65 million years ago (mya).
• Each major stage of primate evolution is linked to one of the five geological epochs of the Tertiary period.

Epochs of the Tertiary Period

• Paleocene (65-53 mya): Possibly the origins of primates; Plesiadapiformes existed.
• Eocene (53-37 mya): Abundant fossil evidence exists for early primates, including prosimians and possibly anthropoids.
• Oligocene (37-22.5 mya): Definite anthropoid proliferation occurred (with unambiguous remains dating back 36 mya).
• Miocene (22.5 – 5 mya): Abundance of fossil apes in Old World; hominin split from apes in Africa.
• Pliocene (5 – 1.8 mya): First hominin fossils and identifiable artifacts appeared.
• Pleistocene (1.8 mya – 10,000 ya): Hominin migrations outside of Africa occurred; appearance of modern humans and origins of socially complex societies began.
• Holocene (10,000 ya – present): Agriculture spread, and large-scale ‘civilizations’ rose.

Continental Drift

• Over the course of evolution, there were changes to the planet.
• Pangea existed.
• Movement of continental land masses had implications for climate and the evolution of living organisms.
• The end of Mesozoic and the beginning of Cenozoic (about 65 mya) marked the early Paleocene epoch.
• Separation of continents led to global cooling.
• During the Eocene epoch, primate fossils are recovered in North America and Europe, a time when those areas were located closer to the equator and were tropical. In the Eocene epoch, about 50 mya, Europe and North America split.
• In the Holocene, the continents exist as we know them today.

Primates of the Eocene

• The first Prosimians existed
• Closely resemble but are not identical to modern lemurs, lorises, and tarsiers.
• They have grasping hands and feet
• Nails (not claws) are present on some or all digits.
• Large, forward-facing eyes are also present.

Eocene Primates Superfamilies

• Omomyoidea (Eocene tarsiers)
• Adapoidea (Eocene lemurs)
• It is debated which superfamily is the anthropoid ancestor (lemurlike? Or tarsierlike?)

Omomyid

• Shoshonius is ancestral to modern tarsiers.

Oligocene Primates

• The first Anthropoids
• Temperatures cooled, and environments dried out (less tropical forest habitat).
• Anthropoids became prominent and competed with Prosimians (nocturnal adaptation).

Emergence of Anthropoids

• "Anthropoid Transition" involved a change in prosimian diet from fruit and leaves (soft) to seeds and hard-covered fruit (hard).
• Variants with larger, stronger teeth and jaws, and reinforced skulls capable of crushing, grinding, and strong biting existed.
• Loris (Prosimian): Face longer, No postorbital closure
• Squirrel Monkey (Anthropoid): Face shorter and fully enclosed eye orbit

Primate Classification

• Primates
  • Prosimians
  • Anthropoids
    • Platyrrhines (monkeys)
    • Catarrhines (monkeys, apes, humans)
      • Ceboids
      • Cercopithecoids (monkeys)
      • Hominoids (apes & humans)

Platyrrhines (monkeys in the Americas)

• Their ancestry is puzzling, it is unknown how they got there.
• Parallel evolution from an earlier prosimian ancestor?
• Differences too great for a common anthropoid ancestor?
• They have prehensile tails, live in the tree or ground, and distinctive nose shape.
• Molecular studies show that Platyrrhine monkeys are closely related to anthropoids than they are to prosimians.
• Platyrrhines and Catarrhines possibly share a common (anthropoid) ancestor, such as the genus Apidium, which appeared about 36 mya.
• One idea for their origin is that African anthropoids emerged earlier than fossil evidence suggests
• Rafted across the Atlantic when continental distances were much smaller.

Paleoanthropologists

• Still looking for the 'last common anthropoid ancestor' of monkeys, apes, humans
• When the platyrrhines and catarrhines diverged.
• When catarrhines branched into cercopithecoids (monkeys) and hominoids (apes and humans).
• Research begins by looking into the Oligocene.

Fayum Depression, Egypt

• Fossil layers were created in a warm, tropical rainforest area
• Home to diverse “primate communities,” including prosimians, but even more fossil anthropoids
• Aegyptopithecus (35 mya) was the largest of the Oligocene anthropoids and was initially thought to be a hominoid.

Dentition

• Aegyptopithecus was initially thought to be the earliest example of a hominoid due to its Y-5 molars.
• The Y-5 pattern was initially argued to be a shared derived trait possessed by hominoids.
• Bilophodont molars were thought to be an ancestral trait retained by Catarrhine monkeys.
• Y-5 is now argued to be the primitive trait of all anthropoids, while bilophodont is the shared derived trait of Catarrhine monkeys.

Miocene Epoch

• From 18 to 17 mya the African & Eurasian plates became joined by a land bridge
• Primates evolving in Africa could then cross into Eurasia
• Early Miocene saw emergence of fossil apes & great morphological variation
• By Late Miocene, monkeys were more (numerically) successful & many ape species became extinct (drying).
• Examples of species: Proconsul, Kenyapithecus, Sivapithecus, Gigantopithecus, and Dryopithecus.

Proconsul

• An Early Miocene African hominoid that had a cranial and postcranial skeleton that was a mixture of ape-like & monkey-like attributes
• Described as "evolutionary mosaic" because different traits respond to different selective pressures
• May be ancestral to later hominoids (apes & humans).
• They were relatively large (33-110 lbs), sexually dimorphic, and frugivorous
• Limb proportions were similar to modern-day quadrupedal monkeys
• Monkey-like: quadrupedal, arboreal. Most modern hominoids have anatomical features adapted for suspension locomotion
• Ape-like: no tail

Kenyapithecus

• The earliest definite hominoid fossils
• Existed from 16 to 10 mya
• A Middle Miocene Ape from Kenya.

Sivapithecus

• A Middle to Late Miocene fossil great ape
• Most evidence for hominoids is from Europe and Asia
• Found in India, Pakistan, Turkiye
• Ancestral to modern orangutan.
• The Orangutan lineage separated from lineage leading to African apes (chimpanzees, gorillas) and humans by 12 mya.
• Deep, concave face & narrow distance between the eye orbits.

Gigantopithecus

• The largest primate ever at approximately 10 feet tall & 600 pounds
• Went extinct about 250,000 years ago and coexisted with Homo erectus

Dryopithecus

• A Middle to Late Miocene ape located at European sites (e.g., Rudabanya, Hungary)
• Closely related to African apes and humans, more so than to Sivapithecus & orangutans
• Had the Y-5 arrangement of molar cusps typical of hominoids
• Capable of suspension locomotion
• Long arms & broad trunk = arboreal swinger which used suspensory gaits and postures; short hindlimb = feature shared specifically with the great apes
• Hominoids are connected with this evolutionary legacy
• Hominoids include apes and humans
• Hominoids share a lot of traits due to a shared ancestry

Early Hominins

• Trends in Hominin Evolution
• Australopithecines
• Early Species of Homo
• Early Hominin Cultures
• Out of the Miocene we enter into the Pliocene, and see the appearance of Hominins (further sharpening the taxonomic focus)

Trends in Hominin Evolution

• Bipedalism
  • Marks the appearance of hominins
  • Hominin = a bipedal hominoid, dating between 10 and 5 mya.

Hominin Characteristics

• Head held vertically over spine (foramen magnum)
• Short, wide pelvis
• Center of gravity in line with pelvis
• Broad, dish-shaped pelvis supports organs, and shorter blades stabilize weight transmission
• Longer femoral neck
• Foot is a platform, non-opposable big toe in line with body of foot
• Valgus Angle: Human legs have knock-kneed appearance
• Femur points inward toward the knee joint
• Allows humans to transfer center of gravity directly over foot when walking bipedally
• Ape femurs do not have this angle (hence the ‘ape waddle’)
• A. afarensis has the angle

Spinal Characteristics

• Two distinct curves among bipeds, a backward curve (thoracic) and a forward curve (lumbar)
• Keeps the trunk (and weight) centered above the pelvis

Why Bipedalism?

• Bipedal hominins emerged in Africa
• 16 to 11 mya = drying and cooling period began & continued into Pliocene (follows Miocene)
• Selection favored traits associated with greater success in ground-dwelling
• Adaptive for living in the tall grass environment of the savannah
• More efficient body heat dispersion – less surface area exposed to sun
• Freeing up of the hands - carrying food and tools from one place to another (food: perhaps away from predators)
• More efficient form of locomotion in savannah environment – good for sustained long-distance travel

Bipedalism: The Compromise

• "Evolutionary Compromise"
• Bipedal = working against gravity – harder to deliver sufficient blood supply to brain
• Body weight centered over pelvis and lower limbs stresses hips, lower back, knees, feet
• Females support extra weight in pregnancy

Evolutionary Trends in Hominin Evolution: Dentition

• Trend beginning 4-2 mya: Relatively smaller incisors & canines, larger molars.
• E.g., Australopithecines have huge cheek teeth (molars) and relatively smaller front teeth.

Evolutionary Trends in Hominin Evolution: Reduction of Face

• Australopithecines (like chimps) had large faces which project forward: prognathism.
• Evolutionary trend: reduced projection of face, jaws, cheek teeth (evident in Homo).
• Selection for strong chewing architecture relaxed.

Evolutionary Trends in Hominin Evolution: Expansion of the Brain

• Earliest (definitely bipedal) hominins — Australopithecines - had relatively small cranial capacities (380-530 cc), similar to chimps
• Beginning 2 mya - cranial capacity begins to enlarge, which defines trait for Homo
• First known member of genus Homo, Homo habilis, had a cranial capacity of 630-640 cc (modern human avg 1300 cc)
• Selection favored those who were better tool makers
• Larger, more bulbous cranium
• Development of the forehead

Evolutionary Trends in Hominin Evolution: Evidence of Culture

• Appears about 2 mya
• Greater reliance on learned behavior, tools appear and language is associated with Homo

Overview: Primate Evolution

• Chimpanzee
• Australopithecine
• Homo sapien