Understanding Human Evolution: Dating, Ancestry, and Key Anatomical Features

1. 15.3: Human Evolution

2. 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)

3. Half-life • Half-life – the amount of time it takes for half the radioactive isotope in a substance to decay

4. Curve decreases exponentially with time • Theoretically never reaches 0

5. Who are our closest ancestors?

6. 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)

7. Trends in Hominid Fossils(hominid = bipedal and direct ancestor of humans) *hole in the skull where spinal cord emerges; humans’ are more centered b/c that helps with bipedal, upright walking

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

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

10. 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?)

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. Coexistence 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 ?

18. Uncertainties 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

19. Changes 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

20. Genetic 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

21. Recent 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)

22. Ape vs. modern human teeth canine molars

23. Ape vs. Human Skull Brow ridge Face below brow Jaw bone

24. Cranial Capacity