Why are human newborns so big and fat?

Abstract

Human newborns and infants have morphological and physiological traits protecting them against hypothermia. These traits are unusual for primates, with some of them rarely seen in other mammals evolving in an African environment. We can include the following: 1.) A non-allometric, bigger size of the newborn, resulting in the decrease of surface to body mass ratio (S/W). 2.) Greater amount of subcutaneous fat tissue (SFT) increasing insulation. 3.) Greater amount of brown fat tissue increasing nonshivering thermogenesis. 4.) Active thermoregulation when sleeping. 5.) Thermal balance moved in the direction of heat conservation in a few months' old babies.

I here present a hypothesis that it was the risk of nocturnal hypothermia in open habitats of late Pliocene that was the selective pressure promoting evolutionary emergence of these traits inHomo erectus. The inverse radiation at night in open habitats causes strong gradient of temperatures. In effect the temperatures near the ground (even in the tropics) can be low enough to endanger newborns and infants with hypothermia. If earlyHomo was naked, slept on the ground and if mortality of their babies caused by hypothermia was high, then selection pressures could have promoted those traits protecting infants against the risk of hypothermia. Since the most important traits (1. & 2.) in respect of heat conservation, depend on mother size, it is postulated that they appeared when female body size increased dramatically i.e. duringHomo erectus stage of human phylogeny.