1. Human Evolution and PREHISTORY
Chapter Six:
THE FIRST BIPEDS
Link to the Canadian Association for Physical Anthropology

2. Chapter Preview
What is the Anatomy of Bipedalism and How is it Preserved in the Fossil Record? Who Were the Australopithecines and What Were They Like? Why Had Australopithecus Become A Bipedal Walker?

3. THE ANATOMY OF BIPEDALISM
Shared derived characteristics distinguishing hominins from the other African apes
Position of foramen magnum is more forward
S-shaped spinal column
Wide and foreshortened pelvis
“kneeing-in” of thigh bones (femora)
Stable arched foot and absent opposable big toe
Shorter toes

4. THE ANATOMY OF BIPEDALISM

5. THE ANATOMY OF BIPEDALISM

6. Australopithecus
a well-known hominin that lived between 4.2 and 1 mya in East and South Africa
Bipedal when on the ground
Apelike brain

7. Australopithecus

8. Early hominins

9. Gracile Australopithecines
3.9 to 2 mya in East Africa
3.5 to 2.3 mya in South Africa
Erect bipeds, about m. in stature
Apelike skull morphology
Teeth for chewing food in a hominin fashion

10. Gracile Australopithecines
Earlier fossils show dental features similar to some late Miocene apes; later South African fossils do not
Sexually dimorphic, in body size and canine tooth size

11. Gracile Australopithecines
Foramen magnum is forward and downward-looking, as in genus Homo
Brain is about a third of a modern human brain in size, and three times larger than Miocene apes
Endocasts suggest that the human cerebral reorganization has not yet occurred
Blood drainage system for the brain is significantly different from genus Homo

12. Australopithecus Afarensis…“Lucy”
“Lucy”…Discovered by Don Johanson in Kenya in 1974.
Lucy was 40% complete. A great deal can be known of an animal when you are in possession of 40% of its original skeleton.
Video...
Animation of Australopithecines

13. Australopithecus Africanus…”The Taung Child”
Discovered by Raymond Dart, 1924, South Africa.
Not accepted at the time of discovery as representative of human ancestry.

14. Australopithecines Two indisputable facts:
Retention of some form of adaptation to arboreal life
2. Evolution of erect bipedal position long before acquiring highly enlarged brain

15. Robust Australopithecines South Africa
1.8 to 1 million years ago
Thick bones for their size, with prominent muscle markings
Sagittal crest, for huge temporal muscles (more evident in males) – an example of convergent evolution in gorillas and hominins

16. Robust Australopithecines East Africa
2.5 to 1.3 million years ago
More massive skull and larger body size than South African relatives
Enormous molars, premolars, mandible and palate

17. Gracile-Robust Relationship
East African robust forms likely evolved from earlier gracile forms in east Africa
In South Africa the robusts were either an offshoot of the East African lineage, or convergent evolution from a South African ancestor

18. Robust-Homo Relationship
Robust australopithecines had evolved into highly efficient and specialized consumers of plant food
Many anthropologists believe this allowed early Homo and robust australopithecines to co-exist for 1.5 million years because they avoided competition for the same ecological niche
** law of competitive exclusion

19. AUSTRALOPITHECINE PREDECESSORS
Sahelanthropus tchadensis (Chad)
7-6 mya
Small canines and humanlike face
Probably bipedal
Recent 3D reconstruction confirms it is more closely related to hominins
Likely close to common ancestor

20. Sahelanthropus tchadensis

21. AUSTRALOPITHECINE PREDECESSORS
Orrorin tugenensis (east Africa)
6 mya
13 fragments of lower jaw, teeth, thigh bones
Molars are thickly enameled like Australopithecines, but smaller
Suggestion of bipedalism
Uncertain evolutionary relationship

22. Orrorin tugenensis

23. AUSTRALOPITHECINE PREDECESSORS
Ardipithecus (east Africa)
5.8 to 4.4 mya
Sp. kadabba and ramidus
Likely bipedal when on the ground
Mixture of ape and hominin dental features
Forested environment

24. Ardipithecus

25. AUSTRALOPITHECINE PREDECESSORS
Kenyanthropus platyops (east Africa)
Contemporary with early east African australopithecines
Maeve Leakey sees her fossil as ancestral to the genus Homo

26. Kenyanthropus platyops

27. AUSTRALOPITHECINE PREDECESSORS
Relationship to Hominins
Current evidence indicates that hominins evolved from late Miocene apes, becoming distinct about 5 mya
More than one bipedal model emerged from this new primate niche; one of them was Australopithecus
Are any of these predecessors ancestral to the australopithecines or to chimpanzees, or did they become extinct?
Pattern in early hominin evolution has been short periods of change, marked by prolonged periods of stasis

28. ENVIRONMENT, DIET, AND AUSTRALOPITHECINE ORIGINS
Major climatic changes in the late Miocene:
Drying up of the Mediterranean Sea
Breaking up of forests
Creation of a mosaic environment with more open areas, interspersed with forest patches

29. Effect of Climatic Change
Change in diet
- less “tree” food
- more open ground foraging, e.g. seeds, grasses, roots
2. Change in dentition
- smaller canine teeth
- male canines became as small as those of females

30. Australopithecines and Tools
No evidence of toolmaking clearly associated with Australopithecines, although hands of later Australopithecines were suitable
Could Australopithecus have been a tool-user and maker of simple tools such as bonobos and chimpanzees today?
They could have used wooden tools, convenient stones, animal bones

31. HUMAN BIPEDALISM Disadvantages More visible to predators
Exposes soft underbelly
Interferes with ability to change direction instantly while running
Not a fast locomotion method
Frequent lower back problems and circulatory problems
Serious impediment if one leg is injured
** all of these disadvantages placed our early hominin ancestors at risk
Becoming Human Site

32. HUMAN BIPEDALISM How did bipedalism become a viable adaptation?
Possible selective pressures:
1. Males gather and transport food to females, who were restricted by dependency of offspring
2. Nonterritorial, far-ranging scavenging, because a biped is able to travel long distances without tiring
3. To cope with heat stress out in the open

33. The Naked and the Bipedal
ORIGINAL STUDY
The Naked and the Bipedal
Upright stance greatly reduces the amount of the body’s surface area that is directly exposed to the sun and increases the amount exposed to the cooler breezes a few feet above the ground
Hair loss helps to lose more body heat
The “naked biped” becomes an adaptation to the heat of the savanna by keeping the brain cool and allowing for its expansion

34. HUMAN BIPEDALISM
Bipedalism is far more economical than quadrupedal locomotion at walking speed
The causes of bipedalism are likely to be multiple
e.g. food transport AND carrying infants AND reaching for food AND seeing predators AND using hands as protection

35. Homo habilis and Cultural Origins
NEXT TIME:
Homo habilis and Cultural Origins