Evolution of Humans and Other Primates

When did primate evolution occur?

Throughout the Cenozoic Era

What type of habitat were early primates adapted for?

Arboreal (tree-dwelling)

What are the key adaptations of primates for arboreal life?

Opposable thumb and toe for grasping

- Nails instead of claws

- Fleshy, sensitive pads on digits

- Front-facing eyes with overlapping view for 3D vision and depth perception

- Better color vision (3 cones)

- Larger brain to process extra visual information

What are the two major clades of extant primates?

Strepsirrhini (wet-nosed) and Haplorhini (dry-nosed)

What characterizes Strepsirrhini?*

A:

- Wet-nosed primates

- Include lemurs and lorises

- Most are nocturnal and small

- Eyes adapted for low-light environments

- Long snout with damp, glandular nose (good sense of smell)

What characterizes Haplorhini?

A:

- Dry-nosed primates

- Include tarsiers and Anthropoidea (monkeys, apes, humans)

- Most are diurnal

- Larger brain size

- Short snout, rounded nostrils, dry noses

- Often frugivorous

What are the two groups within Anthropoidea?

Platyrrhines (New World Monkeys) and Catarrhines (Old World Monkeys, Apes, and Humans)

What does "platyrrhine" mean and what are their key features?

A:

- Flat or broad-nosed (wide nose with nostrils far apart)

- Small body size

- Arboreal, some with prehensile tails

- Many are frugivorous

- Examples: tamarins, marmosets, capuchins, spider and howler monkeys

How did New World Monkeys likely get to South America?

Rafted from Africa to South America during the mid-Cenozoic

What are the distinguishing features of Catarrhines?

- Narrow-nosed (nostrils close together)

- Large size

- 2 premolars instead of 3 (Platyrrhines have 3)

What characterizes Old World Monkeys?

- Found in Africa and Asia

- Examples: baboons, macaques, mandrills

- Males often have brightly colored face skin

- Have tails, but often short and not prehensile

What are the two groups of Catarrhines?

Old World Monkeys and Hominoids (apes and humans)

What are the key features that distinguish hominoids from monkeys?

- No external tail

- Chest flatter and broader than monkeys

- Shoulder blade shifts to back

- Aids balance in upright bipedal posture

- All able to walk bipedally

- Earliest hominoids were mainly arboreal

What are the five groups of hominoids and their characteristics?

1. Gibbons: smallest, arboreal, specialized brachiators (arm swinging), monogamous

2. Orangutans: large, arboreal, slow climbers, solitary (Asia)

3. Gorillas: large, more terrestrial, knuckle-walk, social, folivores (Africa)

4. Chimpanzees: medium size, somewhat arboreal, omnivore, knuckle-walk, social (Africa)

5. Hominins: humans and relatives - large, terrestrial, social, mainly bipedal (originally Africa)

What skeletal changes occurred in hominins for bipedalism?

- Longer, more slender femur (upper leg bone)

- Foot forms flat platform, opposable big toe lost (apes have curved toes for grasping)

- Pelvis short, bowl-shaped to support guts

- Spine S-shaped, shortened

- Arms shortened

- Foramen magnum (hole where spinal cord enters skull) directly under brain case rather than at rear, balancing skull on top of vertebrae

Why did bipedalism evolve?

- Changes in habitat from closed to open (forested habitats)

- Early bipedal hominins (6-5 mya) from forested habitats

- Frees hands for other functions like food gathering, carrying tools

How did hominin jaws and teeth change from apes?

- Canines became smaller, more blunt and more similar between sexes (indicates more monogamous relationships)

- Reduction in projection of face (shorter jaw)

- Gap between canines and incisors disappears

- Teeth and jaws bowed, rather than rectangular

- Palate becomes arched

What facial features distinguish hominins?

1. Nose becomes more prominent feature of face with distinct nose bridge and tip

2. Brow ridges reduced

3. Braincase enlarged due to enlarging of forebrain

4. Development of a vertical forehead rather than a sloping forehead

What intermediate traits did Ardipithecus show?

Hominin-like traits:

- Foramen magnum below braincase

- Pelvis bowl-shaped (bipedal)

- No evidence of knuckle-walking

- Small canines, less sexual dimorphism in dentition (monogamous?)

Ape-like traits:

- Longer digits and arms (arboreal)

- Foot had highly opposable big toe (arboreal)

- Brain relatively smaller than in later hominins

- Face sloped less than chimps, but more than later hominins

What distinguishes genus Homo?

Much larger brain size compared to body size

- Australopithecus afarensis ("Lucy"): 380-522 cm³

- Homo habilis: ~500-750 cm³

- Homo sapiens: ~1350-1500 cm³

What limits brain size in humans?

Size of baby head that can pass through birth canal

How do humans circumvent the birth canal limitation?

Rapid brain growth after birth (born early in development)

Why Humans are Altricial

- Wider hips/birth canal increases energetic costs of bipedalism in females

- Mother's body can only handle 9 months of development

- Being born early may allow for more complex thinking later

- Having altricial young corresponded to:

- Shift from polygamous to monogamous mating system

- Longer post-reproductive life (grandparents help raise young)

- Move out of arboreal habitats

What developments came with larger brain sizes?

- First signs of complex tool making (hand axes with H. erectus)

- Trend towards larger body size

- A. afarensis: 1-1.5 m tall, ~30-45 kg

- Homo erectus: up to 1.85 m tall, at least 65 kg

When and where did Homo neanderthalensis arise?

~130,000 years ago in Eurasia

What are the key features of Neandertals?

• Similar brain size to H. sapiens, but larger on average (~1410 vs ~1350 cm³)

  - Stockier, more muscular (adaptation to colder climate?)

  - 30 year lifespan

  - Stronger brow ridge

  - Small or no chin

  - Evidence of culture (bury dead, wore pendants, artistic ability)

When did Homo sapiens arise?

~200,000 years ago

What features distinguish Homo sapiens from Neandertals?

- Big chin, weak brow ridge

- Larynx shifts to front of neck, lower down in trachea, which helps enable speech

- Neandertal larynx may have been at intermediate state between ape and human

- Evidence of culture that differs from Neanderthals

What evidence supports African origin of H. sapiens?

- Fossils and DNA support African origin

- Greater genetic variation in Africa than in rest of world

When did Neandertals go extinct?

Last Neandertal fossils from 28,000 years ago

What are hypotheses for Neandertal extinction?

1. Suffered when glaciers retreated and climate changed

2. H. sapiens drove Neandertals to extinction directly or indirectly:

- Evidence H. sapiens had more efficient means of gathering food

- H. sapiens had longer range weapons than Neandertals

3. Hybridization:

- Old skeletons show features intermediate between Neandertals and H. sapiens

- Some modern human cultures contain small amounts of DNA from other Homo species, including Neandertals

What were major developments in hominin evolution besides bipedalism?

1. Reduction in body hair and increase in skin pigmentation (~1 mya): Both protect skin from UV radiation

2. Controlled use of fire (~750,000 ya, before Neandertals and H. sapiens):

- Increases food digestibility (teeth decreased in size)

- Reduces bacterial infection

- Promotes warmth and social gathering

3. First use of clothing (~170,000 ya):

- Evidence from presence of body lice with hominin specimens

- May have enabled survival of ice ages (~180,000 ya) and migration out of Africa (~100,000 ya)

How many mass extinctions have occurred during Earth's history?

5 mass extinctions (over 50% of all living species going extinct)

What pattern do mass extinctions show?

- Survivors not necessarily better adapted to local environment

- Loss of many types of vertebrates could pave the way for diversification of new types

- Permian extinction gave rise to Mesozoic reptiles

- Cretaceous extinction gave rise to extant mammals

What characterized the Pleistocene epoch?

- Series of warming and cooling cycles

- Extensive glaciation in Northern hemisphere

- Much drier climate due to water locked up in ice

What happened during the Pleistocene extinction (12,000-10,000 ya)?

- Massive die-off of large vertebrates (megafauna)

- Small mammals increase in diversity

- Extinctions geographically concentrated (North America, South America, Australia)

What are the two hypotheses for Pleistocene extinctions?

1. Overkill: Excessive hunting by early humans

- Extinctions affected only large animals

- Humans only predator that kills large, healthy adults

- Occurred about time humans moved onto these continents

- BUT: Europe (also N. hemisphere) suffered few extinctions

2. Climate change: Loss of glaciers, warming of Earth

- BUT: Humans first moved to NA, then SA, but extinctions occurred from SA to NA

- May not have been enough humans at the time

Are we causing a mass extinction?

Yes, we are causing a 6th mass extinction

What is the current vertebrate extinction rate?

- 800 vertebrate extinctions over last 500 years

- 1,000 times higher than natural baseline extinction rate based on fossil record

- ~40% amphibian, ~50% of turtles, 26% mammal, and 14% bird species endangered

What drives modern extinctions?

Hunting, habitat destruction, pollution, introduction of invasive species, etc.

How can we help stop extinctions?

Our knowledge of behavior can help stop extinctions!