COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Human Evolution and PREHISTORY Link to the Canadian Association for Physical Anthropology Link

COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED.

Human Evolution Human Evolution andand PREHISTORYPREHISTORY

Link to the Canadian Association for Physical Anthropology

Chapter Six:Chapter Six:THE FIRST BIPEDSTHE FIRST BIPEDS


Chapter PreviewChapter PreviewChapter PreviewChapter Preview

What is the Anatomy of Bipedalism and How is it What is the Anatomy of Bipedalism and How is it Preserved in the Fossil Record?Preserved in the Fossil Record?

Who Were the Australopithecines and What Were Who Were the Australopithecines and What Were They Like?They Like?

Why Had Why Had AustralopithecusAustralopithecus Become A Bipedal Become A Bipedal Walker?Walker?


THE ANATOMY OF THE ANATOMY OF BIPEDALISMBIPEDALISM

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






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


Australopithecus


Early homininsEarly hominins


Gracile AustralopithecinesGracile Australopithecines

3.9 to 2 mya in East Africa 3.5 to 2.3 mya in South Africa Erect bipeds, about 1-1.5 m. in stature Apelike skull morphology Teeth for chewing food in a hominin fashion



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



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


Australopithecines

Two indisputable facts:

1. Retention of some form of adaptation to arboreal life

2. Evolution of erect bipedal position long before acquiring highly enlarged brain


Robust Australopithecines Robust Australopithecines South AfricaSouth 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


Robust Australopithecines Robust Australopithecines East AfricaEast 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


Gracile-Robust RelationshipGracile-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


Robust-Homo RelationshipRobust-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


AUSTRALOPITHECINE AUSTRALOPITHECINE PREDECESSORSPREDECESSORS

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



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



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



Kenyanthropus platyops (east Africa)

Contemporary with early east African australopithecines

Maeve Leakey sees her fossil as ancestral to the genus Homo



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


ENVIRONMENT, DIET, AND ENVIRONMENT, DIET, AND AUSTRALOPITHECINE ORIGINSAUSTRALOPITHECINE 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


Effect of Climatic ChangeEffect of Climatic Change

1. 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


Australopithecines and ToolsAustralopithecines 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


HUMAN BIPEDALISMHUMAN 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



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


ORIGINAL STUDY

The Naked and the BipedalThe 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



4. 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


NEXT TIME:

Homo habilis and Cultural Homo habilis and Cultural OriginsOrigins

Documents

COPYRIGHT © 2008 Nelson Education Ltd. ALL RIGHTS RESERVED. Human Evolution and PREHISTORY Link to the Canadian Association for Physical Anthropology Link