The scientific view is that “modern humans” evolved in much the same way as other forms of life did. But from there, much remains to be explained about why and how humans developed their proclivity for walking upright, thinking abstractly, using language, manipulating objects to make increasingly sophisticated tools, and — jumping to the present — developing technology that until fairly recently would have defied anyone's imagination. In addition, the story often changes with new “finds” by paleontologists and physical anthropologists who study fossil records and by biogeneticists who attempt to trace the evolution of human DNA. Following is a look at factors in evolution with emphasis on humans.

Darwin's Survival of the Fittest

Darwin explained evolution in terms of natural selection, in which the fittest have the best chance to survive and reproduce. This is called direct fitness. But as profound as the concept was, Darwin had no biological basis for how this might happen — an understanding of the operation of genes was still decades away.

In the modern and more complete view of Darwin's survival of the fittest, hominids across the millennia either were subjected to changing environments or they migrated and subjected themselves to change. Change is the key. It often meant living in different environments such as jungles versus open savannahs and dealing with radically different climates. It meant finding different plant and animal food sources — plus catching or scavenging the latter. And it meant coping with different predators, of which there were many.

Thus, those of our ancestors who were best able to adapt and survive long enough to reproduce were more likely to pass along their genes to future generations. Those who were least able were less likely to reproduce their genes. Across generations, then, the gene pool would gradually change to favor adaptive characteristics in a given population, in accord with the environment in which they had to live.

Why would some individuals be more fit than others? As noted earlier, children aren't exact genetic blends of their parents. Chapter 12 will discuss several ways that this happens when it looks at the reproductive cycle, but one important way is mutation. At times and seemingly at random, the DNA that makes up genes changes in subtle ways, some of which are adaptive. Chance is again the key. If a person happens to inherit a mutation that is highly adaptive for a given environment, the odds are that the individual will have more children and likewise more successful ones — thus having a major impact on the gene pool down the line.

Indirect Fitness and Inclusive Fitness

Lacking information about the role of genes, Darwin couldn't see another important way that some characteristics get passed along and others don't. In addition to direct fitness, there is the concept of indirect fitness, and the two taken together constitute inclusive fitness. As elaborated by modern geneticists and evolutionary psychologists, indirect fitness works as follows. Anything you do that increases the survival chances of your offspring, or of relatives such as your sisters and brothers and their offspring, passes along at least part of your genes — the ones that you and they have in common. To a lesser but still significant sense, this also applies to members of your tribe or clan, who — at least in prehistoric times — would also share some of your genes.

Thus, for example, whatever genes might be involved in a tendency to help and cooperate with others can also be passed along. Indirect fitness therefore explains how a tendency toward altruism — helping others without regard to yourself, perhaps to the point of self-sacrifice — could have evolved in humans. More generally, it helps explain how humans have evolved to be the highly social creatures that they are.