Change in Human Brain Size, Natural Selection, and Evolution

A few years ago Kathleen McAuliffe wrote an interesting article for Discover Magazine: If Modern Humans Are So Smart, Why Are Our Brains Shrinking? She was startled to learn that over the past 20,000 years or so the human brain had actually decreased in size. 

"Over the past 20,000 years, the average volume of the human male brain has decreased from 1,500 cubic centimeters to 1,350 cc, losing a chunk the size of a tennis ball. The female brain has shrunk by about the same proportion."

Like most of us, she assumed that big brains were the indicator of our lofty intellects. Prior to that, the data demonstrated that the human brain had been growing ever larger. McAuliffe was surprised to learn that this remarkable conclusion was rather difficult to interpret, and most discussion was confined to a small group of paleontologists who were working in this area.

Brain size is known to be a function of an animal’s body mass. This follows from the assumption that the greater the body size, the more brain needed to control it. This ratio of brain volume to body mass is referred to as the encephalization quotient (EQ). Humans do have a significantly larger EQ than the other apes.

Human body mass has shrunk since the Stone Age, so some decrease in brain size might be expected. However, the data on humans since that period indicates that brain size has shrunk considerably faster than body mass. In fact, if EQ were a fundamental constant, then our bodies would have had to shrink to perhaps half our current size. The issue becomes more intriguing when one considers that this phenomenon has occurred all over the globe. John Hawks, one of the experts McAuliffe relied on for her article, provides this insight:

"The decline in body mass in human populations during the last 10,000 years has been estimated as less than 5 kg, or less than a 10 percent reduction in mass from a Late Upper Paleolithic mean of some 63 kg….A decline of 5 kg would predict a decrease in endocranial volume only around 22 ml. The observed decline in several regions (including Europe, China, Southern Africa, and Australia) is between 100 and 150 ml during the past 10,000 years. Therefore, the reduction in body mass would be expected to have decreased brain size by only one-fifth to one-seventh the observed decline."

Note that areas like Australia had little communication with other peoples over that period in history. It is not likely that mutations that could have affected brain size would have spread through Europe, China, Southern Africa, and Australia in such a uniform fashion. It seems something more fundamental is going on.

McAuliffe received several possible explanations for why the brain might be shrinking from experts she consulted. One is referred to glibly as the "idiocracy theory." The decline in brain size has been associated with increased population density and, presumably, increased social interdependence.

"The observation led the researchers to a radical conclusion: As complex societies emerged, the brain became smaller because people did not have to be as smart to stay alive….individuals who would not have been able to survive by their wits alone could scrape by with the help of others—supported, as it were, by the first social safety nets."

While natural selection in a "complex" society will likely enhance different characteristics than those in favor in a "simple" society, it is a bit of a stretch to assume complexity leads to diminished cognitive capabilities.

Another hypothesis is based on the fact that the brain consumes an enormous amount of energy given its small mass. About 20% of the energy our food provides is devoted to keeping the brain working. It could be that a smaller more efficient brain has been selected by nature. It would be a mistake to assume that the brain has only been changing in size while keeping all other parameters constant. There is no reason to assume that a new smaller brain is less capable than an old larger one.

The most intriguing suggestion is based on the fact that all of the animal species humans have domesticated developed a smaller brain than their wild counterparts.

"Some 30 animals have been domesticated….and in the process every one of them has lost brain volume—typically a 10 to 15 percent reduction compared with their wild progenitors. Domesticated animals also have more gracile builds, smaller teeth, flatter faces, a more striking range of coloration and hair types—and, in many breeds, floppy ears and curly tails. Except for those last two traits, the domesticated breeds sound a lot like us."

One of the explanations for this phenomenon is based the notion that domestication, or taming of animals, involves systematically breeding animals in which aggression is minimized. To support this view, an experimental result is described.

"In 1958 the Russian geneticist Dmitri Belyaev started raising silver foxes in captivity, initially selecting to breed only the animals that were the slowest to snarl when a human approached their cage. After about 12 generations, the animals evidenced the first appearance of physical traits associated with domestication, notably a white patch on the forehead. Their tameness increased over time, and a few generations later they were much more like domesticated dogs. They had developed smaller skeletons, white spots on their fur, floppy ears, and curlier tails; their craniums had also changed shape, resulting in less sexual dimorphism, and they had lower levels of aggression overall."

There is no superior species controlling the breeding of humans and "domesticating" us, so how is this relevant? It can be claimed that by imposing rules against aggression we are "domesticating" ourselves. The death penalty for those who commit crimes of aggression is one way of promoting a tamer society. The genes of an overly aggressive human need only be removed from the gene pool by making it unlikely they will be successfully reproduced. Death is unnecessary.

While circumstantial evidence supports this hypothesis, it fails to explicitly explain the connection between aggression and brain size. A possible explanation seems to depend on the fact that it is impossible for complex animals to be bred in such a way as to only affect a single property. Natural selection works on all properties at the same time. The experiment with the silver foxes arrived at a more docile animal, but one that seems to be more like a new species than a new version. And the change in shape of the cranium suggests changes in brain size as well.

What is striking about McAuliffe’s article and the issue of brain size reduction is the indication that animals, even humans—perhaps especially humans—are continuing to evolve; and the evolution can be quite rapid.

Perhaps the best example of quick adaption of a species to a changing environment involves a type of finch in the Galapagos Islands. This particular bird had evolved to feed off of a large sized nut and had a beak appropriate for that size. At some point a larger sized species of finch was introduced into the environment. This bigger bird was able to control access to the preferred nuts, leaving only smaller ones for the original finch to feed on. Unfortunately, their beaks were inefficient at opening these smaller nuts. What was observed to happen?

"Researchers from New Jersey's Princeton University have observed a species of finch in Ecuador's Galápagos Islands that evolved to have a smaller beak within a mere two decades."

"Surprisingly, most of the shift happened within just one generation, the scientists say."

Many animals can exist for very long times because they adapt to their environments. As long as their environments are stable, they are stable.

Humans, however, no longer adapt to their environment; rather they attempt to make their environment adapt to them. Humans alter their food supply, introduce unnatural chemicals into their bodies, pollute the air they breathe, shine intense forms of radiation on themselves, create ever changing social environments, and even change their mating habits. All this is done without a care for what the ramifications might be. Meanwhile, new diseases and types of malfunction emerge at near epidemic levels (autism, food allergies, diabetes, obesity, asthma, mental illness….).

Every variation we make to our environment and to our society alters the results of natural selection. There will be change, but we have no way of knowing where it will take us.

We are not who we were, and who we are, is not who we will be.