Human Origins and Adaptations

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Objectives

When you have completed this section, you should be able to

• define evolution and natural selection;

• describe some human characteristics that can be attributed to the tree-dwelling habits of earlier primates;

• describe some human characteristics that evolved later in connection with upright walking; and

• explain why evolution is relevant to understanding human form and function.

If any two theories have the broadest implications for understanding the human body, they are probably the cell theory and the theory of natural selection. Natural selection, an explanation of how species originate and change through time, was the brainchild of Charles Darwin (1809-82)—probably the most influential biologist who ever lived. His book, On the Origin of Species by Means of Natural Selection (1859), has been called "the book that shook the world." In presenting the first well-supported theory of evolution, On the Origin of Species not only caused the restructuring of all of biology but also profoundly changed the prevailing view of our origin, nature, and place in the universe.

On the Origin of Species scarcely touched upon human biology, but its unmistakable implications for humans created an intense storm of controversy that continues even today. In The Descent of Man (1871), Darwin directly addressed the issue of human evolution and emphasized features of anatomy and behavior that reveal our relationship to other animals. No understanding of human form and function is complete without an understanding of our evolutionary history.

Evolution, Selection, and Adaptation

Evolution simply means change in the genetic composition of a population of organisms. Examples include the evolution of bacterial resistance to antibiotics, the appearance of new strains of the AIDS virus, and the emergence of new species of organisms. The theory of natural selection is essentially this: Some individuals within a species have hereditary advantages over their competi-tors—for example, better camouflage, disease resistance, or ability to attract mates—that enable them to produce more offspring. They pass these advantages on to their offspring, and such characteristics therefore become more and more common in successive generations. This brings about the genetic change in a population that constitutes evolution.

Natural forces that promote the reproductive success of some individuals more than others are called selection pressures. They include such things as climate, predators, disease, competition, and the availability of

Saladin: Anatomy & I 1. Major Themes of I Text

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

10 Part One Organization of the Body food. Adaptations are features of an organism's anatomy, physiology, and behavior that have evolved in response to these selection pressures and enable the organism to cope with the challenges of its environment. We will consider shortly some selection pressures and adaptations that were important to human evolution.

Darwin could scarcely have predicted the overwhelming mass of genetic, molecular, fossil, and other evidence of human evolution that would accumulate in the twentieth century and further substantiate his theory. A technique called DNA hybridization, for example, suggests a difference of only 1.6% in DNA structure between humans and chimpanzees. Chimpanzees and gorillas differ by 2.3%. DNA structure suggests that a chimpanzee's closest living relative is not the gorilla or any other ape— it is us.

Several aspects of our anatomy make little sense without an awareness that the human body has a history (see insight 1.1). Our evolutionary relationship to other species is also important in choosing animals for biomedical research. If there were no issues of cost, availability, or ethics, we might test drugs on our nearest living relatives, the chimpanzees, before approving them for human use. Their genetics, anatomy, and physiology are most similar to ours, and their reactions to drugs therefore afford the best prediction of how the human body would react. On the other hand, if we had no kinship with any other species, the selection of a test species would be arbitrary; we might as well use frogs or snails. In reality, we compromise. Rats and mice are used extensively for research because they are fellow mammals with a physiology similar to ours, but they present fewer of the aforementioned issues than chimpanzees or other mammals do. An animal species or strain selected for research on a particular problem is called a model—for example, a mouse model for leukemia.

Insight 1.1 Evolutionary Medicine

Vestiges of Human Evolution

One of the classic lines of evidence for evolution, debated even before Darwin was born, is vestigial organs. These structures are the remnants of organs that apparently were better developed and more functional in the ancestors of a species. They now serve little or no purpose or, in some cases, have been converted to new functions.

Our bodies, for example, are covered with millions of hairs, each equipped with a useless little piloerector muscle. In other mammals, these muscles fluff the hair and conserve heat. In humans, they merely produce goose bumps. Above each ear, we have three auricu-laris muscles. In other mammals, they move the ears to receive sounds better, but most people cannot contract them at all. As Darwin said, it makes no sense that humans would have such structures were it not for the fact that we came from ancestors in which they were functional.

Primate Adaptations

We belong to an order of mammals called the Primates, which also includes the monkeys and apes. Some of our anatomical and physiological features can be traced to the earliest primates, descended from certain squirrel-sized, insect-eating, African mammals (insectivores) that took up life in the trees 55 to 60 million years ago. This arboreal8 (treetop) habitat probably afforded greater safety from predators, less competition, and a rich food supply of leaves, fruit, insects, and lizards. But the forest canopy is a challenging world, with dim and dappled sunlight, swaying branches, and prey darting about in the dense foliage. Any new feature that enabled arboreal animals to move about more easily in the treetops would have been strongly favored by natural selection. Thus, the shoulder became more mobile and enabled primates to reach out in any direction (even overhead, which few other mammals can do). The thumbs became opposable—they could cross the palm to touch the fingertips—and enabled primates to hold small objects and manipulate them more precisely than other mammals can. Opposable thumbs made the hands prehensile9—able to grasp branches by encircling them with the thumb and fingers (fig. 1.6). The thumb is so important that it receives highest priority in the repair of hand injuries. If the thumb can be saved, the hand can be reasonably functional; if it is lost, hand functions are severely diminished.

8 arbor = tree + eal = pertaining to aprehens = to seize

8 arbor = tree + eal = pertaining to aprehens = to seize

Human Hand Grasp
Figure 1.6 Primate Hands. The opposable thumb makes the primate hand prehensile, able to encircle and grasp objects.

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

Chapter 1 Major Themes of Anatomy and Physiology 11

Primate Bone Tool Use
Figure 1.7 Primitive Tool Use in a Primate. Chimpanzees exhibit the prehensile hands and forward-facing eyes typical of primates. Such traits endow primates with stereoscopic vision (depth perception) and good hand-eye coordination, two supremely important factors in human evolution.

The eyes of primates moved to a more forward-facing position (fig. 1.7), which allowed for stereoscopic10 vision (depth perception). This adaptation provided better hand-eye coordination in catching and manipulating prey, with the added advantage of making it easier to judge distances accurately in leaping from tree to tree. Color vision, rare among mammals, is also a primate hallmark. Primates eat mainly fruit and leaves. The ability to distinguish subtle shades of orange and red enables them to distinguish ripe, sugary fruits from unripe ones. Distinguishing subtle shades of green helps them to differentiate between tender young leaves and tough, more toxic older foliage.

Various fruits ripen at different times and in widely separated places in the tropical forest. This requires a good memory of what will be available, when, and how to get there. Larger brains may have evolved in response to the challenge of efficient food finding and, in turn, laid the foundation for more sophisticated social organization.

None of this is meant to imply that humans evolved from monkeys or apes—a common misconception about evolution that no biologist believes. Observations of monkeys and apes, however, provide insight into how primates adapt to the arboreal habitat and how certain human adaptations probably originated.

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

Saladin: Anatomy & I 1. Major Themes of I Text I I © The McGraw-Hill

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

Human Adaptation Tree

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:

7. Define adaptation and selection pressure. Why are these concepts important in understanding human anatomy and physiology?

8. Select any two human characteristics and explain how they may have originated in primate adaptations to an arboreal habitat.

9. Select two other human characteristics and explain how they may have resulted from adaptation to a grassland habitat.

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  • HADDAS YONATAN
    Why evolution is relevant to understanding human form and function?
    9 years ago
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    Why is adaptation and selection pressure important in anatomy and physiology?
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