Human Evolution: What Are Humans?

Before starting on anything, let’s go through an entire phylogenetic review of where humans are, listing the characteristics of each major clade they belong to. Within the vertebrates, they belong to the Gnathostomata, the vertebrates with jaws and characterised by their skull. Within this clade, they are osteoganthostomes. In other words, they use bone, not cartilage, to build their skeleton.

Within this group, humans belong to the Sarcopterygii, the fleshy-finned fishes. Besides the various muscle-covered fins (that will later become the extremities of the skeleton), they have teeth made of prismatic enamel, a skull made of many fused elements, as well as other characteristics. It’s from this group that the tetrapods are derived: they have two pairs of extremities with five fingers each (a recent development, evolutionarily speaking), a separate shoulder attached to the skull and axial skeleton only by muscles, and connected to the humerus (upper arm), allowing free movement. The pubis is fused to the vertebral column through the ilium, the one character crucial to supporting locomotion on land. The skull, which is composed of less elements than in the sarcopterygian ancestors, is only attached to the spine via a joint, allowing independent movement of the head. Besides the considerable differentiation in the throat, all the inner gills from the ancestors are lost, while the gill arches instead build a hyoid bone (right under your tongue), the larynx and the trachea. The middle ear also underwent plenty of differentiation to become linked to the pharynx. There are two nose openings, each one having an anterior and a posterior opening, allowing breathing with the mouth closed. We’re not done. The skin with its keratin epidermis and epidermal glands is also a tetrapod character, as is the heart being split in two part (atrium and ventricle).

Of the tetrapods, we belong to the amniotes: we undergo internal reproduction (i.e. all males have a penis), and the fetal life cycle is characterised by embryonal sacs (amnions) and an egg shell (making reproduction independent of water possible). The epidermis is hardened as drought protection. The ureter connected to the kidney is also an amniote character and allows for liquid excretion, while the urethra is turned into a canal for reproductive seeds. The skull is made even more freely moveable by the development of the atlas-epistropheus complex (the first two vertebrae).

Of the amniotes, we are mammals, characterised by milk glands, hair, the three ear bones (maleus, incus, stapes), the secondary jaw joint (squamodental), red blood cells with no nucleus, the thoracic diaphragm (why your chest heaves up and down when breathing), muscular cheeks and gums, tribosphenic molars (a specific groundplan for how the bumps and hills of your molars are), among others.

Of the mammals, we belong to the Placentalia, characterised by the placenta, a simple vaginal structure, a corpus callosum separating the two brain hemispheres, milk teeth, and brown fat, among other characters.

Of the Placentalia, we are one of the 376 species of primate (note: this number contains cryptic species), a group characterised by stereoscopic vision, a relatively large brain, a reduced 3rd incisor and 1st premolar (making for 36 teeth in total), nails instead of claws, highly-innervated hand and foot skin, an especially high number of mechanical receptors, freely-hanging penis and testicles in a scrotum, long life expectancies and slow growth. Among others, of course.Now we’ll look at the primates in a bit more detail. We don’t know who they are most closely related to: it’s either the colugos (Dermoptera) or the tree shrews (Scandentia). I cannot make any comment here. Within the primates are two groups: the Strepsirhini, containing the lemurs, galagos, sifakas, loris and the indri. The other group, the Haplorhini, contains the tarsier and the anthropoids (apes), and the main difference between these two groups is the nostril: Strepsirhines have a wet nostril, while haplorhines’ nostrils are dry. The difference is physiologically significant: the wetness is caused by a gland-rich region of the upper jaw, hidden by the gums, but visible when one looks into the mouth of of a strepsirhine: there is a gap between the medial incisors. In haplorhines, this is not present and the difference is quite remarkable, as it allows the upper lip to be more musclated – a key modification allowing the mimicry so common in apes (just look at any stand-up comedian). Other differentiating characters are found in the dentition: strepsirhines have straight lower canines and incisors used for taking care of the fur and for eating soft vegetation. Haplorhines have broad, cutting incisors adapted for fruit eating, and the upper canines are long and pointed (except in the humans).

The Anthropoidea, also called the Simiiformes, are the commonly-recognised apes and are characterised by the U-shaped upper row of teeth, a fused mandibula, the lack of a claw on the second finger (as in the lemurs) and a simplified uterus. There are two groups of anthropoid: the platyrrhines (New World apes) and the catarrhines (Old World apes), with humans belonging to the latter.

The catarrhines are characterised by the nostrils being close to each other and directed downwards, the reduction of a premolar, large, sexually dimorphic canines, a gap between the incisor and the associated canine when the mouth is closed (to aid chewing), the ability to bend the thumb, an often patriarchal society, rising importance of mimicry and vocalisation with the noteable extreme reduction of pheromonal and olfactory communication. The catarrhines are split into the Cercopithecoidea and the Hominoidea, humans obviously belonging to the latter.

The still-extant hominoids represent but a fragment of a once much more diverse group (in the Miocene). With the exception of humans, all hominoids are restricted to Southeast Asia and Africa. They are characterised by large changes in the postcranial skeleton related to the development of a straight posture. These include the broad chest, the dorsal shoulder blades and, related to those, the very long clavicula; in addition to those, the vertebrate column is longer and the tail is reduced. The hand is much more flexible, being only connected to the radius. There are two groups of hominoids: the Hyloblatidae (gibbons) and the Hominidae (apes).

The Hominidae are characterised by bipedalism and a large, rounded skull (among other things). There are two groups here: the Ponginae (orang-utans) and the Homininae, the latter including humans, chimps and gorillas, with the humans and the chimps forming a sister group to the gorillas.

Humans are characterised by their cultural evolution, and biologically by several features: small canines with no sexual dimorphism, broad premolars, a reduced third molar, an upper tooth row that is parabolical, not U-shaped, a large telencephalon, obligate bipedalism (with associated characters, such as the S-bent vertebral column and the growth and development of the pubis to support the added weight and the length of the legs being longer than that of the arms). Contrary to popular perception, humans are not naked apes: they have the same amount of fur, just the hair is much shorter (except on the head, where it is sexually dimorphic). Humans show geographically-dependent melanin concentrations, with melanin serving as protection against UV.

Socially, humans are distinguished by long-lasting couples and increased copulation – the latter a result of the lack of external exhibitions of ovulation (no swelling of the genitals, etc). Humans have the largest penis of all apes, and females have the largest breasts of all apes – a result of sexual selection. Since human males don’t have to compete for females, they can invest much more in their offspring, which has led to the retardation of development and the prolonged childhood and adolescence of the youth.

Of course, cultural evolution is also a characteristic of humans, as is the related evolution of language (and consequent enlargement of the telencephalon).

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