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 behavior 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), that 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.