What Makes Humans Different?

Paul Chadwick
December 29, 2009

Virtually all scientific evidence, fossil and DNA alike, points to the evolutionary relatedness of humans with other living organisms on Earth, both past and present, and suggests common ancestors with other primates, mammals, vertebrates, and indeed even plants and single-celled eukaryotes if we look back far enough. We have a sense, however, that human beings are somehow unique and different from all other species. That sense is reflected in the very word "animal" -- which we hesitate to apply to human beings in spite of our evolutionary relationship.

What is it that makes us different?

Aside from various physical details, we generally recognize that it is our brains and intellect. But what is unique about that? All vertebrate species have complex brains and exhibit complex behavior. Among other primates, as well as mammals, birds, and fish, in addition to innate and apparently intuitive behaviors -- very elaborate in some species -- we also observe reactions to environmental conditions that show evidence of learning, even planning on occasion. We see group activities and coordination with peers. We occasionally see primitive use of available objects as tools. We see the expected aggressive, self-preservative behavior in most species, but in many we also see affection for other individuals, and apparent emotions such as happiness and sadness. In some traits human beings actually appear to be relatively deficient. Young birds can fly and navigate with little or no training or trial-and-error when they are ready to leave the nest, and young quadrupeds like horses and cattle stand and walk the day they are born, whereas it takes new humans the better part of a year to learn to move about reliably on their own. But these are all capabilities and conditioned behaviors that occur in the immediate present.

Do other species think? Do they have abstract thoughts separate from immediate circumstances? Do they apply logic? Two possible examples come to mind. One is the fact that dogs, at least, and I suspect many other species appear to dream when they are asleep. This would seem to indicate that they have mental images in their brains that are related to real world situations, but separate and to some extent abstract. The brain, at least, appears to go through the motions even in the absence of environmental stimulus or physical activity. Another is a story told to me about a dog trying to retrieve a ball that was thrown into a moving stream: he had the presence of mind to run along the bank downstream, ahead of the object, then stand on the bank and wait for the current to bring it to him. Was this a logical plan, or just built-in intuitive behavior relating, perhaps, to pursuit of prey?

Do other species have language? In many instances, various species appear to communicate via sounds as well as gestures. Some even learn words and replicate sounds from others or their environment -- parrots come to mind. Likely these are not so much words with meanings as mere sounds associated with certain circumstances. But, what are words, really? Certainly many words are just sounds associated with certain objects or situations in the environment. However, one must admit that in terms of vocabulary, humans are far advanced, with the ability to learn and associate hundreds or thousands of distinct sound patterns, while other species have at most just a few. So likely the capacity to create. learn, and associate a large number of special sounds -- which we call words -- is a unique feature of human intelligence and represents some advanced chemical or structural feature of the human brain in comparison to all others. We use the word "language" to represent this ability to communicate with a large number of unique sound patterns -- words -- associated with specific things, as opposed to the ability to communicate with only a very limited set of special noises.

In addition, humans have the ability to associate words not only with specific physical objects and sensations, but also with abstract concepts such as history, causation, space, a theory, an electron. Albeit, other species may be able to do this as well, associating certain sounds with, for example, danger, or desire for a mate. But, again, such sounds are generally made up or learned sound patterns not so much as they are innate built-ins characteristic of the species. In the human case, our words are not innate, genetically endowed features, but inventions and learned patterns, as evidenced by the variety of languages that exist with different words for the same objects or concepts. Even more amazing is our ability to learn multiple languages and to keep them more or less separated from each other in our brain -- although not totally separated, as when we have difficulty remembering a word in one language we may readily substitute the corresponding word in another. Furthermore, as for words associated with things and sensations, humans have a very large repertoire and can describe a vastly greater variety of abstracts than any other species -- again likely a consequence of some complex feature of our larger brain -- an extension of, but not necessarily physiologically different from the capabilities of other animal species.

Do other species have memory? Do they learn? Clearly the answer is yes. Otherwise, there would be no animal training. Pets would not recognize their owners. Wild animals would not be able to find their way home to their lairs. They would not have dreams.

Spoken language is one thing, and although perhaps not totally unique, the extent of it in humans is clearly superior to any other species. But what about written language? Does any other species write, i.e., use symbols in sequence to represent sounds or spoken words? Not as far as I am aware. Thus, this would be a unique characteristic of human beings. In addition to the capacity to associate a large vocabulary of words with objects, feelings, and abstract concepts, we also have an astounding ability to associate a large number of visual images, generally two-dimensional, with individual words in that vocabulary, either as distinct symbols in logographic writing systems, Chinese characters for example, or as combinations of symbols representing sounds in alphabetic writing systems. We tend to take this ability for granted, but if our ability to develop large spoken vocabularies is amazing, our further ability to completely represent those vocabularies by mapping to two-dimensional visual symbols can only be thought of as amazing squared.

The existence of written language allows human beings to communicate over space and time, without the necessity for the communicator to be physically present. While no longer unique in this regard -- humans having now developed technologies for audio and video transmission, recording, and playback -- writing was the first mechanism we developed for communicating over space and time and remained our only means of doing so for thousands of years. The ability to write greatly enhanced our ability to pass and preserve knowledge -- history, stories, technology -- from one generation to the next and indeed to subsequent generations virtually ad infinitum, without relying on the inherent imprecisions of personal presentation and storytelling. Does any other species have the ability to do that? Apparently not. So language, especially written language that allows human beings to pass knowledge between generations -- and thereby greatly increase the total store of knowledge by not requiring each generation to acquire it all on its own -- appears to be a uniquely human trait.

What about telling stories? That is probably something that preceded the written word for communicating between generations and over distances in human history. Do other species tell stories? Not as far as we can tell.

What is a story? It is an abstract description of a sequence of events which may or may not have actually occurred. Which brings up another point: it is one thing to recount events that actually occurred, as a history. But what about the fact that human beings can create fictional stories that describe sequences of events that never occurred in reality. This would be called "creativity". Do any other species exhibit creativity?

There is, I think, a similarity between creativity and dreams. Often, in dreams, we seem to experience a sequence of events that are plausible, but that never actually occurred in the real world. Often we cannot remember our dreams for very long after they have occurred. But perhaps creativity consists of the ability to remember dreams, or is an extension of dreaming consisting of the ability to consciously fabricate dreams and record them. This would certainly be a uniquely human trait -- especially the part about recording the dreams. Such creativity need not be restricted to storytelling or written language. We also see it in the visual arts and music, which are probably diverse exhibitions of the same unique capability.

Storytelling and the ability to fictionalize -- i.e., to invent stories by combining in our minds sequences consisting of representations of events that were never in our direct experience combined in reality -- are probably closely related to two another uniquely human traits: the ability to think logically and technological development.

Thinking logically: the ability to visualize and to think in terms of causation, of relations, the role of events of the past in determining events of the future, the ability to formulate theories, to perceive an order in the environment and a logical progression in the way events occur and the relation between them, rules and laws of nature. Does any other species do this? I think not.

Science is a branch of logical thought. It consists of the ability to observe and perceive relations between objects and events in our environment, to deduce from observation and invent theories to explain and predict relations between objects and events, to further observe and test our theories, and to communicate those theories to others who may consequently invent even more accurate and insightful theories of natural phenomena.

Technology -- the ability to draw from nature, modify and combine natural things to create new tools, the ability to invent machines, the ability to record the designs of these inventions and pass them along so that future generations can build on the technology and create new and more powerful tools -- also appears to be a uniquely human characteristic. It is directly related to the human propensity for creativity, invention, and abstract thought -- the ability to visualize things and combinations of things not simply as they are presented to us in our environment, but as they might be, as if in a dream -- to implement that dream physically, and to make a record that can be passed to others, including future generations, empowering them to replicate the invention and to build upon it with further innovations. Technology and science are related in that science educates technology in understanding what is and what may be possible.

Although the use of tools in most often cited in describing technological developments in early human history, language and written communication can also be seen as among the earliest examples of technological development. The combination of tools and language has enabled us to actively modify our environment to improve our lives and suit our convenience with increasing impact and capacity through each succeeding generation.

Hypothetically, creativity, invention, logical thought, science and technology are likely all consequences of one fundamental capability of the human brain that is greatly expanded in relation to that found in any other species: that capability is memory. Memory may be seen as the capacity of the brain to model real-world events in some form of neural encoding that is not only constant in its ability to replay those events in our consciousness, but also plastic in its ability to recombine various models in perhaps random and novel ways to produce dreams, create fictional stories, and generate inventions.

If memory is the key to understanding the human brain and what differentiates from all other species, it is also the most elusive. The physiology and biochemistry of memory has fascinated scientists for many decades. When I was contemplating graduate schools in the late 1960s there were laboratories engaged in the study of memory that I though might be interesting to join. Fortunately, the course of events took me elsewhere, into molecular biology, which had just emerged from the discovery of the structure of DNA and the elucidation of how information contained in that structure directs the construction of living cells --a potent enabling discovery that ushered in a very rapid and interesting half century in our understanding of the basic functions of biological systems. In contrast to our current detailed understanding of genetics, biochemistry, and increasingly cellular control systems, differentiation and development, very little has been learned during the past 50 years about memory, its biochemistry, and the neural structures that empower and extend this feature in humans to produce our uniqueness. This understanding appears to be awaiting some additional enabling discovery that can produce an explosion in our knowledge of neuroscience. Perhaps that enabling development will result from the sequencing of genomes and our significantly advanced understanding of genomics, proteomics, and cellular control mechanisms.

It is worthy of note that chimpanzees are shown by DNA evidence to be very closely related to humans genetically. Although chimpanzees are not our ancestors, we must have in common an ancestor more recently than with any other species. Therefore, the genetic differences between humans and chimpanzees should be highly informative with regard to human uniqueness.

With reference to those differences, the following appears on page 211 of Why Evolution Is True by Jerry A. Coyne -- a very readable book highly recommended for anyone interested in the current state of evolutionary theory. "Now that we've finally sequenced the genomes of both chimp and human, we can see directly that more than 80 percent of all the proteins shared by the two species differ in at least one amino acid. Since our genomes have about 25,000 protein-making genes, that translates to a difference in the sequence of more the 20,000 of them. That's not a trivial divergence." Also: "... humans and chimps differ not only in the sequence of genes, but also in the presence of genes. More than 6 percent of genes found in humans simply aren't found in any form in chimpanzees. There are over fourteen hundred novel genes expressed in humans but not in chimps."

My guess would be that the key to understanding human uniqueness, and the key to understanding the biochemistry and physiology of memory, lies in those fourteen hundred novel genes. I would be very surprised if many or most of them are not eventually found to be involved in brain function. If there is a miracle in the creation of humanity, it would exist in the creation of these 1,400 additional genes.

Have I missed anything?

To summarize:

What characteristics are unique to humans among all species on Earth?

  1. Abstract thought.
  2. Language: the extensive use of structured sounds generated by invention, not determined by simple genetics, to represent objects, sensations, and abstract concepts.
  3. Writing: the use of graphic images and symbols to represent words and to pass information from among individuals and between generations.
  4. Storytelling.
  5. Creativity.
  6. Logical thought: The ability to see relations between events and objects. Science. The ability to invent and test theories.
  7. Technology.

All of which I hypothesize to be related to a unique and greatly expanded memory function in the human brain -- perhaps not so different from functions that exist in the brains of other species, because other species do learn, remember, and dream -- but perhaps biochemically much more developed, varied and diverse, resulting from all or part of 1,400 additional genes not found in chimpanzees, leading to a greatly enhanced ability to communicate, create, invent, and manipulate our environment.