How humans recognize one another and receive cues from one another are very complex subjects in both neurology and psychology with incredibly significant implications in both sociology and political science. Understanding what makes us see a human (not necessarily what makes us human) is something which I will leave to the experts in these fields. I will only give a description of what a human is and what our ancestors were rather than how we identify each other. In many ways, this is a matter of identifying what attribute or combination of attributes mark all humans as unique from other species. It would certainly be simple to say “our evolutionary history is what makes us unique,” but this is like saying “this red car is different from this other red car of exactly the same make and model in that it was made in another factory.” We must examine what actually differs between individuals rather than what ancestral differences exist. It would also be an oversimplification to state “our specific genomes make us unique” without specifying what aspect of the genome is uniquely human. Strangely enough, most of the protein products of many genes are so similar as to be interchangeable with the proteins of many other organisms; it is the regulatory sequences which need to be examined in closer detail for their impact on making a “human.” This regulatory sequence is a major part of embryology, which I will discuss in the future. As for the final insult to the idea of definitions is the insane variability of human individuals; this variability is so pronounced that it seems to be mocking any attempt to produce a universal category in which all humans fit.
- Humans exhibit considerable behavioral variability between individuals (hermits to socialites; hoarders to
- Humans are capable of manipulating their bodies in ways which diverge from the expected
- Humans can survive and function in society with extensive divergences from the norm
- Humans may have physical developmental differences which cause them to depart from the modal type
In short: “humans vary substantially.” As such, I shall attempt to produce an inclusive definition of all organisms from the MRCA of all living humans today in the matrilineal line (since this is the older of the mitochondrial and y-chromosome lineages).
- All humans have an embryology similar other placental mammals
- All humans develop a brain approximately 10% of total body mass by the time of birth (0.7-0.88 lbs)
- All humans are involved in some degree of intentional social interaction (he or she must be exhibiting the intentionality).
These would fall into, from Pliny’s wonderful discussion of “the human onion,” the “developmental regulation” and “classical genetics” sections. I cannot place them in either one because classical genetics does provide the raw material for developmental regulation to act upon which results in our unique embryology, brain mass, and social interactions. Indeed, these three things are not just outcomes, but also feed back into the developmental regulation section as causes of other typical human attributes. Without social interactions, language and culture do not arise and specific regions of the brain which utilize these features never develop. This has been coined “prepared learning” by psychologists. It becomes increasingly difficult to distinguish cause from effect particularly in these two typical attributes as language facilitates the acquisition of cultural knowledge, but language is itself a form of cultural knowledge and reliant upon a specifically developed region of the brain. Apart from these universal attributes are the typical attributes of physiological adults:
- Humans typically have four limbs and a highly reduced tail (usually internal) which is a trait of hominines.
- Humans typically exhibit bipedal locomotion, culture, and social cooperation
- Humans typically have reduced body hair (compared to other hominines)
- Humans typically have increased mobility of the glenohumeral joint (also compared to other hominines)
- Humans typically exhibit dentition indicative of omnivorous diets (at least when teeth are present)
- Humans typically possess 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar*, 5 (fused) sacral*, and 4 (3-5) caudal
- Humans typically have a non-abductable hallux (big toe)
- Humans typically exhibit social specialization
- Humans typically exhibit higher degrees of fine motor control than other primates
*some individuals have what is called “lumbarization of S1” or “sacralization of L5” where the first sacral vertebra develops as a lumbar vertebra or the last lumbar vertebra becomes fused into the sacrum.
These typical aspects of “human” are variable, but the vast majority of individuals possess all of these traits, although amputations, developmental disorders of the limbs, paralytic injuries and diseases can cause a not insignificant number to lack some of these traits. I would still consider a paraplegic or amputee “human,” but an extremely lifelike doll is not, although it may be extremely expensive. Similarly, a beating heart cadaver is not a “human,” but an arrangement of cells an molecules which was a human. If electrical activity in the cerebral cortex is observed for more than 24 hours on a body with no pharmacological cause for this; you have an extremely lifelike doll.
This may all be interesting, but it doesn’t succinctly explain what a human is. It is a very lengthy explanation, but not particularly memorable.