Cenozoic Era: Primate Evolution

Geologic Ages: Cenozoic Era

• The origins of the primate order are dated to 65 million years ago (mya).
• Each major phase of primate evolution connects to one of the Tertiary period's five geological epochs.

Cenozoic Era: Primate Evolution

• Paleocene epoch lasted from 65-53 mya, representing the time of primate origins, including Plesiadapiformes.
• Eocene epoch lasted from 53-37 mya, during which there is abundant fossil evidence for primates, including Prosimians and possibly Anthropoids.
• Oligocene epoch lasted from 37-22.5 mya, marking the period of definite Anthropoid proliferation.
• Unambiguous Anthropoid remains date to 36 mya.
• Miocene epoch lasted from 22.5-5 mya, which is a period of abundant fossil apes in the Old World.
• The hominin split from apes in Africa during the Miocene epoch.
• Pliocene epoch lasted from 5-1.8 mya, and represents the time of the first hominin fossils and identifiable artifacts.
• Pleistocene epoch lasted from 1.8 mya-10,000 ya, marking the time of hominin migrations out of Africa.
• The Pleistocene epoch also represents the appearance of modern humans and the origins of socially complex societies.
• Holocene epoch started 10,000 ya-present day, and includes the spread of agriculture, and the rise of large-scale civilizations.

Continents

• Over the course of evolution, environmental conditions changed on the planet.
• Pangea's movement of continental land masses had implications for climate and the evolution of living organisms.
• The end of the Mesozoic Era and the beginning of the Cenozoic Era occurred approximately 65 mya, during the early Paleocene epoch.
• The separation of continents caused global cooling.
• During the Eocene epoch, primate fossils were recovered in North America and Europe, which were located closer to the equator and had a tropical climate at that time.
• Approximately 50 mya, during the Eocene epoch, Europe and North America split.
• The continents look as they do today in the Holocene epoch.

Primates of the Eocene

• The first Prosimians closely resembled modern lemurs, lorises, and tarsiers.
• The first Prosimians had grasping hands and feet.
• The first Prosimians had nails (not claws) on some/all digits.
• The first Prosimians had large, forward-facing eyes.

Eocene Primates

• There were two main superfamilies during the Eocene epoch: Omomyoidea (Eocene tarsiers) and Adapoidea (Eocene lemurs)
• There is debate about which superfamily is the anthropoid ancestor (lemurlike or tarsierlike).

Omomyid

• Shoshonius is a specimen of Omomyid.

Oligocene Primates

• The first Anthropoids appeared during this time.
• Temperatures cooled and environments dried out creating less tropical forest habitat.
• Anthropoids became prominent and competed with Prosimians, and showed nocturnal adaptation.

Emergence of Anthropoids

• "Anthropoid Transition" involved a change in Prosimian diet.
• The new diet went from Fruit and Leaves (soft) to Seeds and Hard-covered fruit (hard).
• Variants featured larger, stronger teeth and jaws, and reinforced skulls capable of crushing, grinding, and strong biting.

Loris (Prosimian) vs Squirrel Monkey (Anthropoid)

• Loris (Prosimian) have a longer face, and no postorbital closure.
• Squirrel Monkeys (Anthropoids) have a shorter face, and a fully enclosed eye orbit.

Primate Classification

• Primates can be divided into Prosimians and Anthropoids (Suborder).
• Anthropoids can be further divided into Platyrrhines (monkeys) and Catarrhines (monkeys, apes, humans) (Infraorder).
• Catarrhines includes Ceboids, Cercopithecoids (monkeys) and Hominoids (apes and humans) (Superfamily).

Platyrrhines

• The ancestry of Platyrrhine monkeys is puzzling because it's unclear how they got to the Americas.
• One theory is that they evolved in parallel from an earlier prosimian ancestor, citing differences too great for a common anthropoid ancestor.

Platyrrhines (monkeys in the Americas)

• Molecular studies show that Platyrrhine monkeys are more closely related to anthropoids than they are to prosimians.
• Platyrrhines and Catarrhines may share a common (anthropoid) ancestor.
• The genus Apidium, abt 2 lbs, arboreal quadruped, appeared abt 36 mya, like a modern squirrel monkey.
• African anthropoids may have emerged earlier than fossil evidence suggests.
• Early African anthropoids rafted across the Atlantic when continental distances were much smaller.

Palaeoanthropologists

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

Fayum Depression, Egypt

• Fayum Depression, Egypt, has fossil layers which were created when the area was a warm, tropical rain forest.
• Home to diverse "primate communities," including prosimians, and fossil anthropoids.
• Aegyptopithecus (35 mya) is the largest of the Oligocene anthropoids.
• At first, it was thought to be a hominoid.

Dentition

• The Y-5 pattern was first thought to be a shared derived trait possessed by hominoids.
• Bilophodont molars were thought to be an ancestral trait retained by Catarrhine monkeys.
• Now, some argue that the reverse is true.
• In the new model Y-5 molars are a primitive trait of all anthropoids.
• Bilophodont are a shared derived trait of Catarrhine monkeys.

Miocene: Emergence of Hominoids

• 18 to 17 mya, the African & Eurasian plates joined by a land bridge.
• This allowed primates evolving in Africa to cross into Eurasia.
• The Early Miocene is characterized by the emergence of fossil apes and great morphological variation.
• By the Late Miocene, monkeys were more (numerically) successful and many ape species became extinct due to drying.

Proconsul

• Proconsul is an Early Miocene African hominoid.
• The Cranial and postcranial skeleton features a mixture of ape-like and monkey-like attributes
• Exhibit an "evolutionary mosaic" meaning different traits respond to different selective pressures.
• Proconsul may be ancestral to later hominoids.
• Proconsul was relatively large (33-110 lbs), sexually dimorphic, and frugivorous
• Limb proportions are similar to modern-day quadrupedal monkeys.
• Monkey-like features: quadrupedal, arboreal.
• Most modern hominoids have anatomical features adapted for suspensory locomotion.
• Proconsul is considered Ape-like because of its lack of a tail.

Kenyapithecus

• The Earliest definitive hominoid fossils appeared in Kenyapithecus
• Kenyapithecus lived from 16 to 10 mya.
• Kenyapithecus was a Middle Miocene Ape
• Fossils found in Kenya

Sivapithecus

• Sivapithecus lived during the Middle to Late Miocene.
• Sivapithecus fossils are from Europe and Asia.
• Specimens found in India, Pakistan, and Turkiye.
• Ancestral to modern orangutan.
• The Orangutan lineage separated from the lineage leading to African apes (chimpanzees, gorillas) and humans by 12 mya.
• Orangutan and Sivapithecus share similarities, including a deep, concave face and narrow distance between the eye orbits.

Gigantopithecus

• Gigantopithecus was the largest primate ever.
• They were some 10 feet tall and 600 pounds.
• They became extinct about 250,000 years ago.
• They coexisted with Homo erectus.

Dryopithecus

• Fossil Great Ape whose fossils are from Middle to Late Miocene European sites (e.g., Rudabanya, Hungary)
• is more closely related to African apes and humans than to Sivapithecus and orangutans.
• Has the Y-5 arrangement of molar cusps typical of hominoids.
• It was capable of suspension locomotion.
• Has long arms and a broad trunk, for arboreal swinging.
• It used suspensory gaits and postures, and it had a short hindlimb
• This feature is shared specifically with the great apes

Hominoid Traits

• Hominoids today are connected with their evolutionary legacy.
• Hominoids today include apes and humans.
• Hominoids share a lot of traits because of shared ancestry.

Early Hominins

• The evolution of hominin traits is trending into the Pliocene.

Bipedalism as a trend in Hominin Evolution

• Marks the appearance of hominins
• Hominin = a bipedal hominoid
• Bipedalism is dated to between 10 and 5 mya.
• Hominins Characteristics:
• Head held vertically over spine with the foramen magnum.
• A short, wide pelvis and the center of gravity are in line with the pelvis.
• Broad, dish-shaped pelvis supports organs, and shorter blades stabilize weight transmission
• They have a longer femoral neck, with a foot as a platform, non-opposable big toe in line with the body of foot.

Valgus Angle

• Human legs show a knock-kneed appearance as a Valgus Angle.
• This involves the femur pointing inward toward the knee joint.
• The Valgus Angle allows humans to transfer the center of gravity directly over the foot when walking bipedally.
• Ape femurs do not have this angle (hence the 'ape waddle').
• A. afarensis has the Valgus Angle.

Spine Curvature

• In bipeds, the spine has two distinct curves.
• A backward curve (thoracic) and a forward curve (lumbar).
• This keeps the trunk (and weight) centered above the pelvis.

Why Bipedalism

• Bipedal hominins emerged in Africa.
• 16 to 11 mya marked the beginning of a drying and cooling period, continuing into the Pliocene.
• The Environment was changing from extensive tropical forest to mixed forest and open country called savannah.
• Selection favored traits associated with greater success in ground-dwelling.
• Bipedalism offers an adaptive advantage for living in the tall grass environment of the savannah.
• Bipedalism gives more efficient body heat dispersion because there is less surface area exposed to the sun.
• Bipedalism frees up the hands for carrying food and tools from one place to another, possibly away from predators.
• Bipedalism is a more efficient form of locomotion in savannah environment, which is good for sustained long-distance travel.

Downside to Bipedalism

• Bipedalism exhibits an "evolutionary compromise".
• Bipedalism involves working against gravity, which makes it harder to deliver sufficient blood supply to the brain.
• Body weight is centered over pelvis and lower limbs, which stresses hips, lower back, knees, and feet.
• Pregnancy places extra weight is supported by females

Evolutionary Trends in Hominin Evolution

• Distinctive dentition evolved beginning 4-2 mya
• There was relatively smaller incisors and canines, and larger molars.
• Australopithecines had huge cheek teeth (molars) and relatively smaller front teeth.

Reduction of Face / Prognathism

• Australopithecines (like chimps) have large faces which project forward.
• Human evolution shows a trend toward reduced projection of face, jaws, and cheek teeth.
• There was selection for strong chewing, but that architecture relaxed over time.

Trends in Hominin Evolution

• Earliest bipedal hominins, the Australopithecines, had relatively small cranial capacities (380-530 cc), similar to chimps.
• Beginning 2 mya, cranial capacity began to enlarge, helping define the Homo genus.
• The first known member of genus Homo, Homo habilis, had a cranial capacity = 630-640 cc, compared to modern human avg 1300 cc.
• Selection favored those who were better tool makers.
• Cranial architecture changes with brain expansion include larger, more bulbous craniums, as well as development of the forehead.

Evidence of Culture

• Evidence of Culture is dated to about 2 mya, which includes greater reliance on learned behaviour, the appearance of tools and language associated with Homo.

Overview

• Evolutionary changes can be observed when comparing chimpanzees, Australopithecines, and Homo sapiens.