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