Humans are panyparic bipedal mammals, found in most known worlds and cosmoi. Humans are used as baselines for many descriptions in the Wongery, partly because of their panyparic nature, but more because most readers of the Wongery are assumed to be human. Which, of course, renders most of the detailed description of humans below completely unnecessary, but it is included anyway for purposes of completeness.
Humans are tetrapods, with two arms and two legs. They are about 1.6 meters tall, on average, though heights vary from world to world and region to region depending on diet, genetics, and possibly other factors; males tend to be taller than females. In contrast to most mammals, humans have very thin hair, if any, over most of their bodies, with the exception of the tops of their heads and a strip over each eye, called an eyebrow. They have flattish faces, with slightly protruding noses but otherwise very little in the way of a snout; except for the eyebrows, the tops of their faces are completely hairless. Female humans have little or no hair on the bottoms of their faces as well; most male humans do grow hair there after puberty, but many choose to keep the area shaved.
Humans have a plantigrade posture, standing and walking with the heel and ball of the foot both touching the ground. They have relatively long legs, their waist generally being at about half their height, and the knee being about halfway up the leg—that is, the human's upper and lower legs are about equal in length. The same is true of the elbow and the arms. The human has two mammary glands, positioned on the upper thorax; on the male, these glands are rudimentary, their locations marked only by (usually) nonfunctional nipples, but the mammary glands of adult females swell into large breasts.
Diet and anatomy
Humans are omnivorous by nature, able to consume both meat and plant matter, although they cannot process cellulose as ruminants can. It is possible for a human to live on a purely vegetarian diet, but it takes careful planning to ensure that the individual will get all the nutrients it needs. Like other apes (and bats and cavies), humans require an external source of Vitamin C. There is evidence that some aspects of the human digestive abilities have evolved relatively recently; adult humans were generally lactose intolerant until the development of domestication provided a ready source of milk and an impetus for the evolution of lactose tolerance, and there is some evidence suggesting that something similar occurred with the ability to metabolize alcohol a few millennia ago. Both lactose intolerance and the inability to metabolize alcohol remain very common on modern-day Earth among those people that did not historically generally drink milk and alcoholic beverages, respectively. There have even been some studies suggesting that the predilection toward spicy food of those in hot climates might originate because those spices kill off harmful microorganisms more prevalent in those climes.
Humans have a relatively simple digestive system with a single stomach, the gastrointestinal tract being about five meters long, most of that being the heavily coiled intestine. One unusual feature is the vermiform appendix, a small dead-end pouch jutting off from the large intestine. The appendix is prone to potentially fatal infection or rupture, and its origins and purpose are still under debate, the most common (but not universally accepted) explanation being that it is a now-useless vestige of a larger organ in human ancestors. Another possible vestigial structure is the wisdom teeth, a third set of molars in the back of the human's mouth. Perhaps Because human jaws have shortened over the course of the species' evolution, there isn't room for all the teeth, and the wisdom teeth are often "impacted", that is, come in at the wrong angle; in industrial societies, surgical removal of these impacted teeth is common. Some humans never develop wisdom teeth, although claims that humans are evolving to lack wisdom teeth are controversial.
The scientific name for humans is Homo sapiens. The s on the end of "sapiens" is sometimes misconstrued as a plural ending, and the singular assumed to be "Homo sapien", but this is a mistake; the correct species name is unquestionably Homo sapiens, with an s, even in the singular. (Technically, the plural of the Latin words involved would be "Homines sapientes", but scientific names are not generally pluralized, so "Homo sapiens" may be used in the singular and the plural.) Attempts have been made to defend "Homo sapien" as a valid back formation which has gained acceptance through use and can now be considered correct English despite its questionable etymology, but this argument fails on the grounds that scientific names are not English words. They are specifically designed to be independent of language, and are not subject to the usual forms of linguistic evolution.
Humans are apes, their closest remaining Euterran relatives being the chimpanzee and the bonobo. There were other species in the genus Homo, more closely related to the human, but they are all extinct on True Earth (though they may still exist on other worlds). Perhaps the best known are Homo erectus; Homo habilis; and Homo neanderthalensis the Neanderthal; though there is still some debate as to whether the first two were really ancestral to Homo sapiens. (The Neanderthal, however, was definitely not.) Other genera related to humans (and now extinct) include Australopithecus and Paranthropus. These, as well as the other species in the genus Homo, are sometimes considered to be humans as well, albeit different species of human, the human clade encompassing the entire subtribe Hominina. Others, however, confine the word "human" to refer to Homo sapiens, and ultimately the matter is purely semantic.
A few subspecies of Homo sapiens may have formerly existed as well (and may still exist on worlds other than True Earth). Many taxonomists believe that some species within the genus Homo are better classified as subspecies of Homo sapiens, including the aforementioned Neanderthal, which is sometimes classified as Homo sapiens neanderthalensis. Aside from these, another possible subspecies, Homo sapiens idaltu, has been identified in Ethiopia. Modern Euterran humans are classified as Homo sapiens sapiens (and are thought to be descended from H. sapiens idaltu).
Origins and evolution
Though there was long debate as to whether bipedalism preceded the human's well-developed brain and intelligence or vice versa, the available evidence now strongly points to the former conclusion: human ancestors (originally probably arboreal) became bipedal long before their brains evolved to anything near their current state. Other important evolutionary developments setting the human lineage apart from their forebears include the development of the human hand for strong gripping, the reduction of the jaw and canine tooth, and the descent of the larynx.
This last item is tied into one of the most significant events in the development of humanity, and one that's not entirely physical at all: the establishment of language, and of the concomitant abstract pursuits such as art and music, collectively known as "behavioral modernity". The suddenness of this event is much debated, with some anthropologists believing that all of these behaviors arose abruptly at about the same time possibly due to some genetic mutation, and others believing that they represent only a gradual amassment of knowledge and tradition over hundreds of millennia of development. In any case, on True Earth humans achieved most or all aspects of behavioral modernity by about 50,000 B.C.. Of course, their current stage of development depends on the world (and to a lesser degree on the culture within the world), with humans in some areas remaining preagricultural and in others having passed through an Industrial Age and beyond.
One of the major processes of development that separated humans from their forebears is that of neoteny—the adult human retained many characterstics of the juvenile form of its ancestors. Many of the differences between the human and the adult form of other apes can in fact be found in the juvenile form of related species, including the flat face, the angle at which the skull meets the spine, and the relative hairlessness. (Of course, it would be going too far to say that humans are nothing more than giant ape fetuses; humans also have many characteristics not found in their ancestors in either adult or juvenile form.) One of the human's neotenic characteristics, its relatively large head, has led to unusual developments in the human birthing process. Because the head of a fully developed juvenile human is too large to fit through the female human's birth canal, human babies are birthed while still not fully developed; a newborn human's skull is not completely hardened, among other signs of incomplete maturity, and a human neonate is much more helpless than newborn apes of most other species. Even with the early births, however, the passage of the baby through the birth canal is still a tight fit, and the birthing process is much more difficult and painful for the human female than for the females of most other species. (Not all, however; the human female still perhaps has an easier time of it than the female spotted hyena.) It has been proposed that the human's early birth has a beneficial side effect, in that it exposes the human juvenile to rich external stimuli at a very young age when such experience can boost its development; this, however, is quite debatable.
While humans vary in the amount of melanin, a dark pigment that blocks sunlight, in their skin, most light-skinned humans have the ability (to varying degrees) to temporarily build up more melanin on exposure to ultraviolet light. This process, known as suntanning, is in some cultures voluntarily undergone in the belief that "tanned" skin is more attractive.
Although humans lack the sharp claws and teeth and other natural weaponry that many other animals have, and are muscularly weak compared to their simian relatives, they are not without physical advantages. Humans are notable for their endurance; while many other animals can run much faster in short bursts, humans are peculiarly adept at long-distance running. The arrangement of the tendons in the human leg, the balance of the moving legs with the swinging arms, and human hairlessness and ability to sweat (and therefore to readily disperse heat), all contribute to the species' skill at long-distance running. It has been theorized that in the past, human ancestors used this cursorial ability to run down prey; even if their intended quarry could outrun the human in the short term, the human could keep up the chase longer, and eventually the fleeing animal would overheat and be easily caught. In technological societies, the human's unusual endurance is less immediately useful, but still sometimes applies, as, for example, when people decide to run a marathon.
Humans possess an acute sense of vision, able to clearly see motionless objects as well as those in motion, although they need a good amount of light to see well and their vision is poor in low light conditions. The human range of visible wavelengths runs from about 400 to 700 nanometers. Human hearing is also relatively well developed, though the range of frequencies humans can hear is less than for many other mammals, only from about 20 to 20 thousand Hertz, with individuals losing the ability to hear the higher frequencies as they age. In contrast, the human senses of smell and taste are very underdeveloped compared to those of most other mammals. There have been accounts of some humans developing much keener senses of smell when deprived of their other senses, but the extent to which this is true is unclear.
In addition to the aforementioned senses and those of touch, balance, pain, and proprioception, there is some inconclusive evidence that some humans may have a rudimentary sort of direction sense, an ability to use the surrounding magnetic field to discern their direction. This may inhere in the ethmoid bone of the nose, which is known to contain some magnetic iron deposits.
Behavior and life cycle
Humans are gregarious creatures, forming complex social structures. The fact that they have also developed sophisticated means of communication makes it possible for them to form much more complicated such structures than other mammals, the more so once sufficiently advanced magics or technologies allow them to communicate with far distant places on the other side of the world, or on other worlds entirely, thereby forming relatively tight-knit communities of global scale or larger.
Unlike most mammals, humans have no estrous cycle, and no preferred mating season; like rabbits and hyenas, they can reproduce at any time of the year. Human females do have a menstrual cycle not entirely dissimilar to the estrous cycle of other mammals, and averaging about 28 days in length, but while they are more fertile during a certain window in their cycle, they are completely infertile at no point in it. (Human males also have a hormonal cycle of their own, but for whatever reason it is less documented.) In all, humans constantly have the potential to be sexually active, a fact that has consequences on their society and behavior. Sex plays an important role in human culture, if not by its presence then by the means taken to suppress and deny it. Sexual imagery (often implied or concealed) is a common component of human entertainment and advertising.
Without extraordinary means of preservation, a human usually lives to a maximum of around eighty to a hundred years.
On most worlds, there is relatively little variation in the human species; slight differences exist in facial structure, skin color, and other superficial traits, but there are no significant and consistent difference between human subgroups. (Some thedy worlds are an exception, and may have much more diversity among humankind.) Of course, that hasn't stopped the rise of prejudice and hostility between "races" defined along the lines of these visible traits.