Our Fast-Changing Genes

There was indeed widespread evidence of malnutrition at the beginning of farming. Paleopathologists found evidence of poor oral health, iron deficiency anemia, and weak bones in the skeletons of early farmers. The farmers from 5,000 years ago were about 6 inches shorter than their pre-agricultural ancestors. The beginning of farming seems to have been a scourge on human health (not to mention the beginning of inequality, as Jean-Jacques Rousseau claimed in Discourse on the Origin and the Foundation of Inequality Among Mankind).

However, that’s only half of the story. The skeletons from 4,000 years ago showed people’s height had returned to their pre-agricultural levels and had far fewer signs of malnutrition. Instead of proving grains are bad food, old bones seem to tell us humans adapt to grain pretty quickly. Nonetheless, just as we didn’t want to rely only on fossil records to prove the climate history of the Earth, we need more evidence than some ancient skeletons. We find that in our genes.

An unexpected advocate for the opinion that we didn’t evolve to eat grains is people who sell starches for a living. The True Neapolitan Pizza Association (Associazione Verace Pizza Napoletana, AVPN) publishes strict rules for making and fermenting dough to be used in authentic Neapolitan pizzas. It says following those processes “will result in less stress on our digestive system because the starches will be broken down into simple sugars. Our bodies are not able to assimilate these long chains.” Actually, our bodies are relatively good at digesting starch. Alpha-amylase is an enzyme that helps the digestion of polysaccharides (starch is a polysaccharide). It’s encoded by the AMY1 gene. Individuals from hunter-gatherer populations with high-starch diets are found to have more AMY1 copies than those with low-starch diets. The idea that diet changes genes is further confirmed by the discovery that humans have more AMY1 genes than chimpanzees, who eat a very low-starch diet.

The LCT gene provides instructions for making the enzyme lactase, which is needed to digest lactose, the sugar in milk. All mammals, by definition, are nursed with milk as babies. All nonhuman mammals, and 65% of people, lose the ability to digest milk sometime after weaning. The other 35% of people have the LCT gene. An interesting hypothesis for the spread of the gene is that soldiers who can drink milk bring with them a walking food source. Those armies have an advantage over their enemies who rely on more sedentary food sources.

That the lactose persistence gene did not spread by chance is further supported by the genetic patterns in the vicinity of the LCT gene. Random chemical changes could happen to a few people’s chromosomes. They had more offspring, and their offspring had more offspring, etc. If that had been the case, the genes near the LCT gene should also have changed randomly. With the advancement of gene sequencing technology in the 21st century, scientists concluded that blocks near the lactase persistence gene show statistically significant similarity. They were dragged along with the LCT gene. This kind of selective sweep is an indication of natural selection at work.

This is a good place to stop and clarify two terms that often get mixed up: evolution and natural selection. Evolution means a change in the frequency of a particular gene or genes in a population. Natural selection is about how some individuals have a better shot at surviving and having offspring because of their genes. Evolution is the result, natural selection is one of the mechanisms to reach the result, but not the only one. For instance, the prevalence of Anglo-Saxon heritage in America and the decline of the native Indian population are not the result of natural selection.

Quantitatively, how fast could a small genetic advantage translate to a large population shift? Anthropologists have calculated that as little as a 3 percent increase in the reproductive fitness of those with lactase persistence would result in the widespread distribution of such a gene after only 300-350 generations. That means Homo sapiens have had enough time to evolve after the Paleolithic ended.

An example of that rapid evolution is the population living on the Tibetan Plateau. There is less oxygen in the air at high altitude. Normally, the body reacts by increasing the number of red blood cells, which transport oxygen to different organs. It doesn’t work in Tibet because the problem is not the transportation of oxygen, but the sourcing of it. The Tibetans exhibit unique genetic adaptations to living at high altitude. They don’t have elevated red blood cell counts, so they don’t suffer from various health issues known as altitude sickness due to the high concentration of hemoglobin, the protein that binds to oxygen inside the red blood cells. Instead, they breathe at a higher rate at rest. This discovery was reported in The New York Times under the headline “Scientists Cite Fastest Case of Human Evolution.”

Genetic Shifts are Faster and More Effective in Larger Populations

Time is not the only factor that determines the speed and quality of genetic shifts in a population. When flies are subjected to low doses of pesticide in the lab, small populations tend to acquire very complicated patterns of resistance, often with serious side effects. It’s an imperfect adaptation. But if pesticides are sprayed over a large area, sometimes flies appear very quickly with a single mutation that confers complete resistance. The important thing to realize is this: the best mutation is incredibly rare. It might never happen in a small population.

The hypothesis that there has been too little time for the human race to evolve since the beginning of agriculture misses the population dimension. Instead of just counting years, we should calculate how many man-years the human race has had for evolving during the Paleolithic. But first, we should define what we mean by the “human race.”

Homo sapiens is the scientific name for the modern human race. Biological taxonomy classifies living organisms in a hierarchical manner. Species is the smallest and most fundamental unit of taxonomy. A species is a group of organisms that can interbreed and produce fertile offspring under natural conditions. Species are grouped into genera (singular: genus). Genera are further grouped into families, and so on and so forth. Humans belong to the sapiens species in the Homo genus. Here is a partial view of the taxonomy tree.

Below is the timeline:

• 300,000 BP: Anatomically modern humans emerged in Africa.
• 100,000 BP: Homo sapiens appeared.
• 60,000 BP: They moved out of Africa.
• 50,000 BP: All other Homo species became extinct, including the Neanderthals.

If we don’t want to take into account evolutionary dead ends, we should start the clock at 50,000 BP. But let’s be generous and start at 300,000 BP. The highest estimate I could find of the population of Homo species around 300,000 years ago was 300,000. At the end of the Paleolithic, 10,000 years ago, the estimated human population reached 1 million. Assuming a linear increase in population (I could assume exponential increase, but the math would not work out in our Paleolithic ancestors’ favor. It has to do with the integral of the exponential function), there were around 180 billion man-years over that period. In 1970, the population was 3.7 billion. There are roughly 8 billion people on Earth now. We actually do know the population has been increasing linearly since the end of WWII. That comes out to about 310 billion man-years between 1970 and now. In other words, the probability of genetic mutation is almost twice as high since the Unix epoch as in all of the Paleolithic. Anthropologist John Hawks and his colleagues calculated that in the last 50,000 years, nearly 3,000 new adaptive mutations arose in Europe alone. It’s been suggested that the explosion in population has led to a 100-fold acceleration in our evolution. Not to mention these may be high-quality mutations because they happened in a larger population.

In comparison, our ancestors were evolving more slowly because 1) there were fewer of them, and 2) they were exposed to slower environmental changes of fewer varieties. Since the Paleolithic ended, the environment has been subject to more frequent, capable, and audacious human meddling. Coupled with the exploding population, our genes may well have been changing at an unprecedented speed.