What Makes Humans Unique?

This is the eighth of a series of posts I will be doing on human evolution. The previous one can be found here. As always, please leave any questions you’d like to have answered.

What makes our species unique? We are not the only species with culture, nor are we the only ones to teach ideas to our young. We are not the only species to make tools, nor the only one to make war. We are not alone in caring for our offspring or in forming life-long bonds. It seems that a few species of primates even have a few “words” that have a universally understood meaning. Humans share about 98% of our genes with our closest relative, the chimpanzee. In fact, we even know that chimpanzees can understand what another individual knows. That seems to exhaust the list of traits we usually think of as uniquely human.

However, there are four cognitive abilities present in humans that are not present in any other species (we think). These four traits have been identified by Marc Hauser, the Harvard psychologist/evolutionary psychologist, and constitute what he has called “humaniqueness,” or the traits that are unique to humanity.

The first of these traits has been termed “generative computation.” This is our ability to create an almost infinite number of combinations of words, musical notes, combinations of actions, or concepts. This is why every author can write a different book (even if they sometimes do seem to follow the same plotline) or why every composer can write a new piece of music. There are two types of generative computation: recursive and combinatorial, both of which are involved in writing books or music. Recursion is “the repeated use of a rule to create new expressions.” In other words, the standard declarative sentence follows a simple pattern: subject then verb then object. With recursion, a child can learn one sentence (“I am four years old.”) and then apply the rule to form a completely different concept (“I want my [insert toy name].”). The other type, combinatorial operation, “is the mixing of discrete elements to engender new ideas.” For example, in his “1812 Overture” Tchaikovsky combined Russian church hymns, the Tsarist national anthem, and “La Marseillaise,” all distinct pieces of music.

The second trait is what Hauser refers to as the “promiscuous combination of ideas.” This is the ability to combine ideas from different areas of knowledge to create “new laws, social relationships and technologies.” For example, if the EPA wants to regulate carbon dioxide emissions, it needs to draw on scientific findings concerning the affects of greenhouse gases (physics, earth sciences, engineering, etc) while also staying within the bounds of the law (the Clean Air Act, the Supreme Court’s decision in Massachusetts v. EPA in 2007, etc). These ideas belong to completely different domains of knowledge, but must be synthesized if the EPA is to do its job.

The third trait is our ability to create “mental symbols.” This is our ability to encode memories of real and imagined sensory experiences. This allows us to have an intricate and complex system of communication, in which we can mention a symbol (the Lincoln Memorial) that evokes a response from the listener (patriotism, reverence, etc). Such symbols can be pictures, letters, movies – basically anything that we can associate memories or emotions with without specifically mentioning those memories or emotions; you do not need to suggest to your listener that he should feel reverent when you mention Abraham Lincoln.

The final traits identified by Hauser is our capacity for “abstract thought.” This is our ability to think about things that are not capable of being experienced through our senses. In other words, things that we cannot smell, see, hear, taste or touch. We can think about the direction we want our country to go in. We can contemplate theories of justice. We can react to Supreme Court decisions (and the Supreme Court can write them).

This is not all that “defines” humanity, but it is a fairly comprehensive list when you are considering behavior. There are 78 genes found in humans that were not present in Neandertals, our closest (extinct) relatives. That’s 78 out of about 30,000 genes. Not much uniqueness there. The four cognitive capacities identified by Hauser are the behaviors that are unique in humans, much as every other species has their own unique abilities. These define “humaniqueness.”
Hauser M. 2009. “The Origin of the Mind.” Scientific American 301(3): 44-51.