Walking Upright

Chapter 1 Major Themes of Anatomy and Physiology 11

Table 1.1

Brain Volumes of the Hominidae

Time of Origin

Brain Volume

Genus or Species

(millions of years ago)

(milliliters)

Australopithecus

3.9-4.2

400

Homo habilis

2.5

650

Homo erectus

1.1

1,100

Homo sapiens

0.3

1,350

About 4 to 5 million years ago, much of the African forest was replaced by savanna (grassland). Some primates adapted to living on the savanna, but this was a dangerous place with more predators and less protection. Just as squirrels and monkeys stand briefly on their hind legs to look around for danger, so would these early ground-dwellers. Being able to stand up not only helps an animal stay alert but also frees the forelimbs for purposes other than walking. Chimpanzees sometimes walk upright to carry food or weapons (sticks and rocks), and it is reasonable to suppose that our early ancestors did so too. They could also carry their infants.

These advantages are so great that they favored skeletal modifications that made bipedalism11—stand-ing and walking on two legs—easier. The anatomy of the human pelvis, femur, knee, great toe, foot arches, spinal column, skull, arms, and many muscles became adapted for bipedal locomotion, as did many aspects of human family life and society. As the skeleton and muscles became adapted for bipedalism, brain volume increased dramatically (table 1.1). It must have become increasingly difficult for a fully developed, large-brained infant to pass through the mother's pelvic outlet at birth. This may explain why humans are born in a relatively immature, helpless state compared to other mammals, before their nervous systems have matured and the bones of the skull have fused.

The oldest bipedal primates (family Hominidae) are classified in the genus Australopithecus (aus-TRAL-oh-PITH-eh-cus). About 2.5 million years ago, Australopithecus gave rise to Homo habilis, the earliest member of our own genus. Homo habilis differed from Australopithecus in height, brain volume, some details of skull anatomy, and tool-making ability. It was probably the first primate able to speak. Homo habilis gave rise to Homo erectus about 1.1 million years ago, which in turn led to our own species, Homo sapiens, about 300,000 years ago (fig. 1.8). Homo sapiens includes the extinct Neanderthal and Cro-Magnon people as well as modern humans.

This brief account barely begins to explain how human anatomy, physiology, and behavior have been shaped by

10stereo = solid + scop = vision

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Physiology: The Unity of Anatomy and Physiology Companies, 2003 Form and Function, Third Edition

12 Part One Organization of the Body

12 Part One Organization of the Body

Family Tree Showing One Branch

Figure 1.8 The Place of Humans in Primate Evolution. Figures at the right show some representative primates. The branch points in this "family tree" show the approximate times that different lines diverged from a common ancestor. Note that the time scale is not uniform; recent events are expanded for clarity.

Which is more closely related to humans, a gorilla or a monkey? How long ago did the last common ancestor of chimpanzees and humans exist?

Figure 1.8 The Place of Humans in Primate Evolution. Figures at the right show some representative primates. The branch points in this "family tree" show the approximate times that different lines diverged from a common ancestor. Note that the time scale is not uniform; recent events are expanded for clarity.

Which is more closely related to humans, a gorilla or a monkey? How long ago did the last common ancestor of chimpanzees and humans exist?

ancient selection pressures. Later chapters further demonstrate that the evolutionary perspective provides a meaningful understanding of why humans are the way we are. Evolution is the basis for comparative anatomy and physiology, which have been so fruitful for the understanding of human biology. If we were not related to any other species, those sciences would be pointless. The emerging science of evolutionary (darwinian) medicine traces some of our diseases and imperfections to our evolutionary past.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Define adaptation and selection pressure. Why are these concepts important in understanding human anatomy and physiology?
  2. Select any two human characteristics and explain how they may have originated in primate adaptations to an arboreal habitat.
  3. Select two other human characteristics and explain how they may have resulted from adaptation to a grassland habitat.

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Responses

  • layla mckay
    What is the anatomy of upright walkers?
    6 years ago
  • tove kovanen
    Who was the first homo Habilis to walk upright?
    6 years ago

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