Why have humans evolved to be taller over the last three hundred years?
Category: Biology Published: August 19, 2015
Humans have not evolved to be taller in the last three hundred years. While the average adult height has indeed increased in many countries over the last few hundred years, this increase was not caused by evolution. Additionally, the average height gain over the last few hundred years has not been very large. If you have ever taken a tour of a commoner's house that has been preserved in its original state since the 1700's or 1800's, you may have been struck by the uncomfortably low ceilings. You may have concluded, or have been told by a tour guide, that people were generally much shorter hundreds of years ago and didn't need high ceilings. However, this conclusion is misleading. Let's look at this subject more closely.
First of all, evolution is not the cause of the overall gain in height seen in many countries over the last few hundred years. Biological evolution takes many generations to occur. Humans have a life span of about a century, meaning that human evolution requires thousands to millions of years. A few hundred years is simply not enough time for significant evolutionary changes to come about. Therefore, the small gain in average human height experienced in many countries over the last few hundred years was not caused evolution. The most likely cause is improved nutrition and health. While this subject of study is too complex for scientists to currently draw definite conclusions, the most reasonable explanation is that the overall increase in average height is a reflection of the overall improvement in health. As western civilizations entered the modern age, improved technology made it possible for more and more people to have consistent access to a nutritious diet and a healthy environment. While a person's genetic code may specify the potential height that he can reach once mature, he will fall short of that height if his body does not have adequate health and nutrition. The textbook Height, Health, and History by Roderick Floud, Annabel Gregory, and Kenneth Wachter states, "it seems safe to say only that the upward trend in European heights reflects the economic development of European nations and the increasing living standards of their populations."
Now let's look at some numbers. As reported in the book Height, Health, and History, the change in average adult male height in the United States over the last three hundred years did not follow a simple trend. From around 1710 to 1830, the average height of adult American men remained effectively at around 173 cm. Over the next sixty years (1830 to 1890), the average height actually dropped from 173 cm down to 169 cm. Then over the next forty years (1890 to 1930), the average height climbed back up, reaching the value of 176 cm. The average height of adult American men today is still 176 cm.
We can take a few things away from these numbers. First, there has not been a smooth trend in average height change over the last 300 years in the United States. For most of this period, the average height has either stagnated or dropped. Only during the brief forty years leading up to the Great Depression did the average height see rapid increase. Therefore, in the United States, an increase in average height is neither steady nor inevitable.
Secondly, Americans today are not really that much taller than Americans during the 1700's. Three centimeters is less than the diameter of a golf ball. Therefore, a house from the 1700's that has its ceiling three feet too low according to today's standards is not that way because people were three feet shorter back then. Such a house must have been built that way for another reason (perhaps the inhabitants were too poor to afford the extra material for high ceilings).
The last point we can draw from the U.S. height data is that the variation in height among a population at any point in time is much greater than the variation of the average height itself over time. Gather a group of adult males and line them up by height. You will find a variation in height between the shortest one and the tallest of about 20 or 30 cm (if your group is a reasonable statistical sample). Compare this to the 3 cm that Americans have on average gained over the last three hundred years. Simply put, the difference in height between you and your neighbor will likely be several times greater than the difference in height between the average of your neighborhood now and that of your neighborhood three hundred years ago (assuming, for the sake of the argument, that you are all adult American males). In other words, the variation in height from family to family right now is far larger than the variation in average height from century to century. If you time-traveled to the 1700's, you would not notice that the people are shorter on average.
Now, let's look at Europe. Unlike in the United States, the numbers reported in the book Height, Health, and History indicate that in European nations, the average male height has been changing more steadily. In the Netherlands, Denmark, Sweden, and Norway, the average male height upon conscription has steadily increased from about 167 cm in 1870 to about 181 cm today. Similarly, in Italy and France, the average male height upon conscription has steadily increased from about 164 cm in 1870 to about 176 cm today. Thus, these European nations have gained about 13 cm in average height over the last 150 years. To give you an idea of what this means, 13 cm is about the width of two tennis balls. As we can see, these European nations did indeed see a more significant gain in height over the last 150 years—much larger than that seen in the U.S.—but it is still no where near the three foot height gain that some people may have in mind.
We could continue on and look at other countries, but I think we have looked at enough data now to allow us to summarize the main points:
- Height changes over the last few hundred years are not caused by evolution.
- The changes in average height over the last few hundred years are different from one nation to the next.
- In general, humans have indeed been getting taller on average in the U.S. and in many European nations over the last few hundred years, but the overall amount of change has been fairly small (from a few centimeters to a dozen or so centimeters).
- The variation in height from one individual to the next is much larger than the change in average height over the last few hundred years.