This answer comes from Michael J. Dougherty, assistant director and senior staff biologist at Biological Sciences Curriculum Study in Colorado Springs, Col.
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Anyone who has ever visited a home built around the time of the Revolutionary War along the back alleys of Philadelphia or Boston has been struck, metaphorically if not literally, by the characteristically low ceilings and small door frames. Even houses built in the early 1800s can make a person of average height by today's standards wonder how the orignal occupants managed to stay conscious long enough to participate in an industrial revolution and a civil war.
For most people, contemporary buildings do not prompt similar claustrophobic concerns. The reason for this difference, as many people have correctly guessed, is that modern humans are taller than those from the eighteenth and nineteenth centuries. In fact, over the last 150 years the average height of people in industrialized nations has increased approximately 10 centimeters (about four inches).
Why this relatively sudden growth? Are we evolving to greater heights, vertically speaking? Before answering these questions, we need to remember that evolution requires two things: variation in physical and/or behavioral traits among the individuals in a population; and a way of selecting some of those traits as adaptations, or advantages to reproduction.
For example, finches that have large, powerful beaks also have an advantage cracking large, tough seeds during periods when small, soft seeds are scarce. As a consequence, large-beaked birds are more likely to eat better, survive longer and reproduce than small-beaked birds. Because beak shape is an inherited trait (one that is substantially influenced by genes), more successful reproduction by large beaked birds means that the genes predisposing finches to large beaks are transmitted to the next generation in relatively larger numbers than those genes encoding small beaks. Thus, the population of finches in the next generation will tend to have larger beaks than finches in their parent's generation.
Let's use this basic operating principle of evolution to predict, retrospectively, the direction of change in human height if evolution were the cause of the change. We know from studies conducted in industrial England that children born into lower socioeconomic classes were shorter, on average, than children born into wealthy families. We also know that poorer families had larger numbers of children.
Given those initial conditions, what would evolution predict? The average population should have become shorter because the shorter individuals in the population were, from an evolutionary fitness perspective, more successful in passing on their genes. But this did not happen. Instead, all segments of the population--rich and poor, from small and large families--increased in height. Thus, natural selection, the process whereby differences in reproductive success account for changes in the traits of a population, does not explain why we are taller.
If evolution doesn't explain height increases, what does? Most geneticists believe that the improvement in childhood nutrition has been the most important factor in allowing humans to increase so dramatically in stature. The evidence for this argument is threefold:
First, the observed increase in height has not been continuous since the dawn of man; it began sometime around the middle of the nineteenth century. In fact, examinations of skeletons show no significant differences in height from the stone age through the early 1800s. Also, during World Wars I and II, when hunger was a frequent companion of the German civilian population, the heights of the children actually declined. They only recovered during the post-war years.