As biological organisms, we are the products of our evolutionary history. This history involved the interaction of natural or sexual selection (which optimize design features of an organism) with those structural constraints which limit any genetic lineage to the exploration of a small segment of biological “design space.” For example, it has occasionally been argued that our hominid ancestors were subject to selection for an endurance-running capacity, which is thought to be advantageous in running down prey animals in tropical grassland environments. However, even an Olympic athlete cannot run faster than a maximum speed, nor sustain that maximum speed for an extended time, due to physiological constraints on muscle activity. Any adaptive design features of the human locomotor system operate within the evolutionary constraints of a system that is far more easily “tinkered with” than fundamentally redesigned by the mechanisms of selection. Natural selection is the process by which environmental pressures determine the rates of propagation of genetically controlled traits; sexual selection is the process by which such rates of propagation are determined by individuals' choices of mates.
There are many obvious design features of humans, as organisms, that also constrain or channel the cultural evolution of skills, artifacts, and behavior. We can only survive within a limited range of ambient temperatures. We must maintain a fairly constant intake of water and nutrients to enable the growth and maintenance of our body tissues over the life cycle. We are vulnerable to colonization by disease organisms, many of which are life threatening. Our physiology is geared to a diurnal cycle, and we