Chapter 6

The First Bipeds

Chapter Outline

What Is the anatomy of bipedalism and how is it preserved in the fossil record?

Who were the Australopithecines and what were they like?

What is the role of bipedalism in human evolutionary history?

Anatomy of Bipedalism

Forward position of the large opening in the base of the skull

Series of curves in the spinal column Basin-shaped structure of the pelvis Angle of the lower limbs from the hip

joint to the knees Shape of the foot bones

Australopithecus: The Earliest Hominine Appeared in Africa 4 million years ago. From the waist down, fully adapted for

bipedalism. From the waist up, still apelike, with

intellectual abilities comparable to those of a modern-day African ape.

Foramen Magnum

Bipedal Gait

Forms of Australopithecus

Earliest forms preserve a number of features that indicate an apelike ancestor.

By 2.5 m.y.a. new form with larger chewing apparatus and more massive head, while brain size remained stable.

Gracile Australopithecus

The size of a modern human pygmy. Chewed food like humans. General appearance resembled an

apelike human. Size and outward appearance of brain

suggest a intelligence similar to a modern bonobo, chimpanzee, or gorilla.

Robust Australopithecus

Shared most of the traits of the Gracile Australopithecus.

Bones were thick with prominent markings where muscles attached.

Skull was thicker and larger with a slightly larger cranial capacity.

Skull possessed a simianlike sagittal crest.

Species of AustralopithecusGracile Species - Location A. afarensis - Ethiopia and Tanzania A. africanus - South Africa A. anamensis - Kenya

Species of Australopithecus Robust Species - Location A. aethiopicus - Kenya A. boisei - Kenya A. gahri - Ethiopia A. robustus - South Africa

Sexual Dimorphism in Canine Teeth

Bipedalism: Drawbacks

Makes an animal more visible to predators.

Exposes the soft underbelly. Interferes with the ability to change

direction instantly while running.

Bipedalism: Drawbacks

Quadrupedal chimpanzees and baboons are 30 to 34% faster than bipeds.

Frequent lower back problems, hernias, hemorrhoids, and other circulatory problems.

Consequences of a serious leg or foot injury seriously hinders a biped and they are an easy meal for some carnivore.

Fossil Sites in South Africa Dirt, bones, and other

matter fell down the shaft becoming fossilized. In the Pliocene, the earth next to the shaft’s opening provided a shelter for trees that may have been used by predators for eating without being bothered by scavengers.

Foot Bones

A 3- to 3.5-million-year old Australopithecus

Note how long and flexible the first toe (at right) is.

Vegetation Zones of Africa

Humans: Blood Carries Heat From the Brain

Reasons for Bipedalism

A way to cope with heat stress. Allowed them to gather food and transport it to

a place of safety for consumption. Mothers were able to carry their infants safely. They could reach food on trees too flimsy to

climb. Allowed them to travel far without tiring.

Reasons for Bipedalism

Food and water were easier to spot. More likely to spot predators before

they got too close for safety. Hands freed from locomotion provided

protection by allowing them to brandish and throw objects at attackers.