martes, junio 17, 2008

Evolutionary design of the human face

In a previous post in Spanish, I conjectured that a great deal of the human face traits are protections against another human adaptation: the capacity to strike with the free hands. In male-male competition and in war, the face protections would have been a very important evolutionary pressure, where intersexual aggression may also have been present.

The chin, the cheekbones, the low and prominent eyebrows seem to be designed to protect sensible areas of the face. Our teeth are protected within the prominences in the lower jaw, the chin and the cheekbones. The eyes are protected also by a strong reaction that produces accumulation of blood around the eyes, like blood airbags. The entire face tends to inflate itself in response to strokes. This reaction is also particularly intense around the brain. These traits I don´t know if are present in other primates. I guess not. Male faces have such defensive traits more prominent, as is expected from the agressive nature of males.

What I found recently is that the nose also responds to two different adaptive pressures. one of which is the protection. No ape has sus protuberances as the human nose. It is also possible that it is a protection against strokes. The protuberance protects the eyes. together with the chin they also protect the teeth. The cartilage, at the end of the nose is a natural shock absorber. The hard bone of the nose is the hard protection when the stroke is too close to the eyes. The apes have not such protection because they do not launch strokes ones against others like men. In the ape skull it is visible the lack of protection of the teeth; there is no chin and no lateral protuberances in the ape´s jaws. No protective noose too.

The other evolutionary pressure that is responsible fo the shape of the nose is the need of a wide angle of stereoscopic vision. If the nose were more prominent or more wide, the stereoscopic vision would be not possible when looking towards one side or the other. The part of the nose the less prominent is at the heigth of the eyes;
From the front to the nose tip, very often there are a concavity at the height of the eyes. This seems to be designed to permit lateral stereoscopic visibility. Men often have more priminent nose bones and sometimes this concavity is more evident. the nose bone prologates itself from this point below, where the interference with vision is less critical. The prominent front and nose of very masculine men are effective protections against strokes and projectiles. This is in agreement with the differential defensive needs of men and women and the defensive nature of the nose. It may be possible that the stereoscopic vision at the sides would be more important in men than in women, so this concavity has further reasons to exist in men. The stereoscopic lateral vision could help to avoid projectiles and strokes at the last moment.

In the other side, the cheekbones, being protections, have ticker dermis that the rest of the face , this happens also at the chin and around the teeth in the sides. The protective nature of this dermis match with some facts. for example, the fact that the cortisone, a stress hormone, which is supposed to prepare the body for dangerous situation, produces the increase of the dermis tickness (trough water acumulation) in these areas.

Another phisiological evidence are the bumps in the head, that are fast reactions to relative small hits. They act as liquid "airbags" thar protect the head from further shocks. All of this points to a specific selection in humans, specially males, against shocks in the head.

2 comentarios:

  1. ¿Qué opinión te merece la hipótesis de que la nariz evolució gracias a nuestros antepasados homínidos que vivieron una etapa en el agua (pues supuestamente las fosas nasales verticales impiden la entrada del agua, o algo así)?

  2. Hola Klaus,
    La hipotesis me parece muy interesante, por la orientacion hacia abajo de las fosas nasales y no es incompatible con los función de protección.


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