There are multiple theories about what makes us human, some related and interconnected. We've been pondering the topic for thousands of years — the ancient Greek philosophers Socrates, Plato, and Aristotle all theorized about the nature of human existence as have countless philosophers since. With the discovery of fossils and scientific evidence, scientists have developed theories as well. While there may be no single conclusion, there is no doubt that humans are, indeed, unique. In fact, the very act of contemplating what makes us human is unique among other animal species.
Most species that have existed on planet earth are extinct. That includes a number of early human species. Evolutionary biology and scientific evidence tell us that all humans originated from and evolved from ape-like ancestors over 6 million years ago in Africa. From knowledge gained from the discovery of early human fossils and archaeological remains, it appears that there were probably 15-20 different species of early humans that existed, some beginning as early as several million years ago. These species of humans, called "hominins," migrated into Asia about 2 million years ago, then into Europe, and the rest of the world much later. While different branches of humans died out, the branch leading to the modern human, Homo sapiens, continued to evolve.
Humans have much in common with other mammals on earth in terms of make-up and physiology but are most like two other living primates in terms of genetics and morphology: the chimpanzee and bonobo, with whom we spent the most time on the phylogenetic tree. However, as much like the chimpanzee and bonobo as we are, the differences are still vast.
Apart from our obvious intellectual capabilities that distinguish us as a species, humans have several unique physical, social, biological, and emotional traits. While we can't know precisely what is in the minds of another being, such as an animal, and may, in fact, be limited by our own minds, scientists can make inferences through studies of animal behaviour that inform our understanding.
Thomas Suddendorf, Professor of Psychology at the University of Queensland, Australia, and author of the fascinating book, "The Gap: The Science of What Separates Us From Other Animals," says that "by establishing the presence and absence of mental traits in various animals, we can create a better understanding of the evolution of mind. The distribution of a trait across related species can shed light on when and on what branch or branches of the family tree the trait is most likely to have evolved."
Following are some traits thought to be unique to humans, and theories from different fields of study, including theology, biology, psychology, and paleoanthropology (human anthropology), those postulate theories about what makes us human. This list is far from comprehensive, though, for it is nearly impossible to name all the distinct human traits or reach an absolute definition of "what makes us human" for a species as complex as ours.
The Larynx (Voice Box)
Dr. Philip Lieberman of Brown University explains on NPR's "The Human Edge" that after humans diverged from an early ape ancestor more than 100,000 years ago, the shape of our mouth and vocal tract changed, with the tongue and larynx, or voice box, moving further down the track. The tongue became more flexible and independent, and able to be controlled more precisely. The tongue is attached to the hyoid bone, which is not attached to any other bones in the body. Meanwhile, the human neck grew longer to accommodate the tongue and larynx, and the human mouth grew smaller.
The larynx is lower in the throat of humans than it is in chimpanzees, which, along with the increased flexibility in the mouth, tongue, and lips, is what enables us to not only speak but also to change pitch and sing. The ability to speak and develop language was an enormous advantage. The disadvantage of this evolutionary development is that this flexibility comes with an increased risk of food going down the wrong track and causing choking.
Our shoulders have evolved in such a way that "the whole joint angles out horizontally from the neck, like a coat hanger." This is in contrast to the ape shoulder which is pointed more vertically. The ape shoulder is better for hanging for trees, whereas the human shoulder is better suited for throwing and, thereby, hunting, giving us invaluable survival skills. The human shoulder joint has a wide range of motion and is very mobile, giving humans the potential for great leverage and accuracy in throwing.
The Hand and Opposable Thumbs
While other primates also have opposable thumbs, meaning they can be moved around to touch the other fingers, imparting the ability to grasp things, the human thumb differs from that of other primates in terms of exact location and size. Humans have "a relatively longer and more distally placed thumb" and "larger thumb muscles." The human hand has also evolved to be smaller and the fingers straighter. This has given us better fine motor skills and the ability to engage in detailed precision work, such as required by technology.
Although there are other mammals who are hairless — the whale, elephant, and rhinoceros, to name a few — we are the only primates to have mostly naked skin. We evolved that way because of changes in climate 200,000 years ago that demanded that we travel long distances for food and water. Humans have an abundance of sweat glands, called eccrine glands. In order to make these glands more efficient, bodies had to lose their hair in order to better dissipate heat. By doing so, humans were able to obtain the food they needed to nourish their bodies and brains while keeping them at the right temperature and allowing them to grow.
Standing Upright and Bipedal
Probably one of the most significant thing that makes humans unique, that preceded and possibly led to the development of the aforementioned characteristics, is being bipedal — that is, using only two legs for walking. This trait developed in humans early on in our evolutionary development, millions of years ago, and gave us the advantage of being able to hold, carry, pick up, throw, touch, and see from a higher vantage point, with vision as our dominant sense, giving us a feeling of agency in the world. As our legs evolved to became longer about 1.6 million years ago and we became more upright, we were able to travel great distances as well, expending relatively little energy in the process.
In his book, "The Expression of Emotions in Man and Animals," Charles Darwin said that "blushing is the most peculiar and the most human of all expressions." It is part of the "fight or flight response" of our sympathetic nervous system that causes the capillaries in our cheeks to dilate involuntarily in response to feeling embarrassment. No other mammal has this trait, and psychologists theorize that it has a social benefit, given that "people are more likely to forgive and view favourably" someone who is visibly blushing. Since it is involuntary, blushing is considered to be more authentic than a verbal apology, which may or may not be sincere.
The human feature that is most extraordinary is the human brain. The relative size, scale, and capacity of our brains are greater than that of any other species. The size of the human brain relative to the total weight of the average human is 1 to 50. Most other mammals have a ratio of only 1 to 180. The human brain is three times the size of a gorilla brain. It is the same size as a chimpanzee brain at birth, but the human brain grows more during the lifespan of a human to become three times the size of the chimpanzee brain. In particular, the prefrontal cortex grows to become 33 percent of the human brain compared to 17 percent of the chimpanzee brain. The adult human brain has about 86 billion neurons, of which the cerebral cortex comprises 16 billion. In comparison, the chimpanzee cerebral cortex has 6.2 billion neurons. At adulthood, the human brain weighs 3 lbs.
It is theorized that childhood is much longer for humans, with children remaining with their parents for a longer period of time, because it takes much longer for the larger, more complex human brain to fully develop. In fact, recent studies suggest that the brain is not fully developed until the ages of 25-30, and changes continue to occur beyond then.
Our Mind: Imagination, Creativity, and Forethought: A Blessing and a Curse
The human brain and the activity of its countless neurons and synaptic possibilities contribute to the human mind. The human mind is different from the brain: the brain is the tangible, visible part of the physical body; the mind consists of the intangible realm of thoughts, feelings, beliefs, and consciousness.
Thomas Suddendorf says in his book, "The Gap":
"Mind is a tricky concept. I think I know what a mind is because I have one — or because I am one. You might feel the same. But the minds of others are not directly observable. We assume that others have minds somewhat like ours — filled with beliefs and desires — but we can only infer those mental states. We cannot see, feel, or touch them. We largely rely on language to inform each other about what is on our minds." (p. 39)
As far as we know, humans have the unique power of forethought: the ability to imagine the future in many possible iterations, and then to actually create the future we imagine, to make visible the invisible. This is both a blessing and a curse for humans, causing many of us endless worry and anxiety, expressed eloquently by the poet Wendell Berry in "The Peace of Wild Things":
When despair for the world grows in me/ and I wake in the night at the least sound/ in fear of what my life and my children's lives may be,/ I go and lie down where the wood drake/ rests in his beauty on the water, and the great heron feeds./ I come into the peace of wild things/ who do not tax their life with forethought/ of grief. I come into the presence of still water./ And I feel above me the day-blind stars/ waiting with their light. For a time/ I rest in the grace of the world, and am free.
But forethought also gives us generative and creative abilities, unlike any other species, spawning magnificent creative arts and poetry, scientific discoveries, medical breakthroughs, and all the attributes of culture that keep many of us progressing as a species and attempting to constructively address the problems of the world.
Religion and Awareness of Death
One of the things that forethought also gives us is the awareness of the fact that we are mortal. Unitarian Universalist minister Forrest Church (1948-2009) explained his understanding of religion as "our human response to the dual reality of being alive and having to die. Knowing we are going to die not only places an acknowledged limit upon our lives, it also gives a special intensity and poignancy to the time we are given to live and love."
Regardless of one's religious beliefs and thoughts about what happens to us after we die, the truth is that, unlike other species who live blissfully unaware of their impending demise, as humans, we are all conscious of the fact that someday we will die. Although some species react when one of their own has died, it is unlikely that they actually think about death, that of others or their own.
The knowledge that we are mortal can be both terrifying and motivating. Whether one agrees or not with Church that religion exists because of that knowledge, the truth is that, unlike any other species, many of us believe in a supernatural higher power and practice a religion. It is through religious community and/or doctrine that many of us find meaning, strength, and direction as to how to live this finite life. Even for those among us who don't regularly attend a religious institution or are atheists, our lives are often shaped and marked by a culture that recognizes religious and symbolic rites, rituals, and holy days.
The knowledge of death also spurs us on to great achievements, to make the most out of the life we have. Some social psychologists maintain that without the knowledge of death, the birth of civilization, and the accomplishments it has spawned, might never have occurred.
Humans also have unique memories, that Suddendorf calls "episodic memory." He says, "Episodic memory is probably closest to what we typically mean when we use the word "remember" rather than "know." Memory allows human beings to make sense of their existence, and prepare for the future, increasing our chances of survival, not only individually, but also as a species.
Memories are passed on through human communication in the form of storytelling, which is also how knowledge is passed from generation to generation, allowing human culture to evolve. Because human beings are highly social animals, we strive to understand one another and to contribute our knowledge to a joint pool, which promotes more rapid cultural evolution. In this way, unlike other animals, each human generation is more culturally developed than the preceding generations.
Drawing on the latest research in neuroscience, psychology, and evolutionary biology, Jonathon Gottschall's enlightening book, "The Storytelling Animal," delves into what it means to be an animal that relies so uniquely on storytelling. He explores why stories are so important, some of the reasons being: they help us explore and simulate the future and test different outcomes without having to take real physical risks; they help to impart knowledge in a way that is personal and relatable to another person (that is why religious lessons are parables); they encourage pro-social behavior, since "the urge to produce and consume moralistic stories is hard-wired into us."
Suddendorf writes this about stories:
"Even our young offspring are driven to understand others' minds, and we are compelled to pass on what we have learned to the next generation.... Young children have a ravenous appetite for the stories of their elders, and in play they reenact scenarios and repeat them until they have them down pat. Stories, whether real or fantastical, teach not only specific situations but also the general ways in which narrative works. How parents talk to their children about past and future events influences children's memory and reasoning about the future: the more parents elaborate, the more their children do."
Thanks to our unique memory, acquisition of language skills, and ability to write, humans around the world, from the very young to the very old, have been communicating and transmitting their ideas through stories for thousands of years, and storytelling remains integral to being human and to human culture.
Defining what makes us uniquely human can be tricky as we learn more about the behaviour of other animals and uncover fossils that cause us to rethink the evolutionary timeline, but some scientists have discovered certain biochemical markers that are specific to humans.
One factor that may account for human language acquisition and rapid cultural development is a gene mutation that only humans have on the FOXP2 gene, a gene we share with Neanderthals and chimpanzees that is critical for development of normal speech and language.
Another study by Dr. Ajit Varki of the University of California, San Diego, found another mutation unique to humans — this one in the polysaccharide covering of the human cell surface. Dr. Varki found that the addition of just one oxygen molecule on the polysaccharide that covers the cell surface differentiates us from all other animals.
No matter how you look at it, humans are unique, and paradoxical. While we are the most advanced species intellectually, technologically, and emotionally, extending our lifespans, creating artificial intelligence, traveling to outer space, showing great acts of heroism, altruism and compassion, we also continue to engage in primitive, violent, cruel, and self-destructive behavior.
As beings with awesome intelligence and the ability to control and alter our environment, though, we also have a commensurate responsibility to care for our planet, its resources, and all the other sentient beings who inhabit it and depend on us for their survival. We are still evolving as a species and we need to continue to learn from our past, imagine better futures, and create new and better ways of being together for the sake of ourselves, other animals, and our planet.