15.3: Human Evolution

Age Dating

• Scientists use isotopes (a version of an element with a

different number of neutrons) to age date rocks and fossils

• Typical isotopes used are 14C and 40K– 14C best when used to age date organic fossils that

eat and/or photosynthesize (age dates between 5,730 to 70,000 years old)

– 40K best to age date rocks b/c once rocks harden, the amount of 40K present is locked in and then degradation begins (age dates 100,000 to 4.6 byo)

Half-life• Half-life – the amount of time it takes for half the

radioactive isotope in a substance to decay

What happens to 14C?

# of half-lives

# of years passed

% of original isotope remaining

1 5,730 50

2 11,460 25

3 17,190 12.5

4 22,920 6.25

• Curve decreases exponentially with time• Theoretically never reaches 0

Who are our closest ancestors?

Describe major anatomical features that

define humans as primates

– Opposable thumbs = grasping abilities– Long, thin, straight fingers = fine motor skills– Finger pads, finger prints, fingernails– Shoulder (ball and socket) joint = increased range

of movement– Forward-facing eyes = allows for stereoscopic

vision (seeing in 3 dimensions/depth perception)

Trends in Hominid Fossils(hominid = bipedal and direct ancestor of humans)

Anatomical Feature

Ape-like form Human-like form

Position of foramen magnum*

Towards the back of the skull

Towards the center of the base of the skull

*hole in the skull where spinal cord emerges; humans’ are more centered b/c that helps with bipedal, upright walking

Trends in Hominid Fossils(hominid = bipedal and direct ancestor of humans)

Anatomical Feature

Ape-like form Human-like form

Canine teeth Long and sharp Short and dull

Molars Long and narrow Short and wide

Jaw Tall and thick Small and thin

Brow ridge Protruding Flat

Face below brow Protruding Flat

Cranial capacity Small Large

http://humanorigins.si.edu/evidence/human-family-tree

Ardipithecus ramidus

• Lived 5.8 to 4.4 mya in Ethiopia

• Bridge between primates and humans; closely related to chimpanzee– Fossils of teeth show ape-like

molars and hominid-like canines– Skull fragments indicate

bipedalism was possible (maybe?)

Australopithecus afarensis

• Lived 4.0 to 2.5 mya in eastern Africa

• Famous fossil of Lucy (~3.5 myo)

• Had tall lower jaw, fairly large molar teeth, and a protruding face

• Cranial capacity 380-430 cm3

Australopithecus africanus• Lived 3 to 2.5 mya in southern Africa

• Tall, thick lower jaw

• Larger molars, smaller canines

• Protruding face

• Cranial capacity 435-530 cm3

•Femur indicates bipedal, but shoulder and hand bones indicate it was still good at climbing

Homo habilis• Lived 2.4 to 1.6 mya in

various parts of Africa– Ethiopia, South Africa– Kenya, Tanzania

• Flatter face and large molars

• Cranial capacity ~600 cm3 (large enough brain to use simple stone tools and fire)

• New evidence suggest they co-existed with Homo erectus

Homo erectus • Lived 1.8 mya to 100,000 years ago in Africa, Europe, and Asia (unsure if first species to migrate to those

areas)

• Smaller jaw, receding forehead, large brow ridges, and smaller molars

• Cranial capacity 1000 cm3

• More modern human size and shape

• Use of stone tools and fire

Homo neanderthalensis• Lived 200,000-30,000 years ago in Europe and

western Asia

• Species survived several ice ages

• Smaller jaw, brow ridges, and molars compared to previous species

• Had larger brains than modern humans with cranial capacity ~ 1600 cm3

Homo sapiens• Date back ~ 200,000 years

ago in Africa, Europe, and Asia

• Skulls have high forehead, no brow ridges, flat face, small jaw, and small molars

• Developed art and technology (cave paintings, tools, weapons)

• Cranial capacity ~ 1300 cm3

Coexistence of Hominid speciesCoexistence of Hominid species

• Some hominid species existed at the same time

• These may or may not have lived in the same region or “bumped into” one another– Ex: A. afarensis and A. africanus both lived ~ 3 mya– Ex: H. erectus, H. neanderthalensis, H. sapiens

~100,000 ya

• These most likely interacted with one another but did not produce fertile offspring– H. neanderthalensis and H. sapiens both lived

~ 100,000 years ago

?

Uncertainties about Human EvolutionUncertainties about Human Evolution

• Data based on limited fossils (skull/bone/teeth fragments)

• Cranial capacity varies per male, female, juvenile

• Fossils must be accessible (both geographically and politically) and well-preserved and expeditions must be properly funded $$$

• Old fossils (5-6 myo) may be destroyed by rock cycle

Result = incomplete data means multiple hypotheses

Changes in Brain SizeChanges in Brain Size

• To get bigger brains you must:– More food in diet– Foods richer in energy and protein

• Hominids did this by increasing the amount of meat in their diet and then cooking that meat

• Proof is animal bones found at fossil sites• This is a cyclical occurrence, foods richer in energy and

protein increase brain size which in turn allowed hominids to build better tools/weapons, which in turn allowed hominids to hunt more efficiently…this continued for millions of years

Genetic vs. Cultural EvolutionGenetic vs. Cultural Evolution

• Humans have evolved genetically and culturally

• Genetic evolution = deals with inherited characteristics like body morphology (cranial size, skull shape, height), number of chromosomes

• Cultural evolution = deals with acquired knowledge like language, customs and rituals, art, technology

Recent Human EvolutionRecent Human Evolution

• Genetically humans have not evolved too much in the past 30,000 years

• Most evolution is cultural:– Languages developed and evolved– Rituals (religion)– Art– Technology (architecture, weapons, agriculture)

Ape vs. modern human teeth

canine

molars

Ape vs. Human Skull

Jaw bone

Brow ridge

Face below

brow

Cranial Capacity