There is a simple explanation for how it is that we trash our exquisitely efficient innate uprighting system. We trash it by losing touch with, and hence control over, the force of our falling. We lose touch by egregiously and repetitively mis-directing our falling in the common act of sitting back against a chair-support. As children, we do this over and over and over again, every day of our lives. And we do it without noticing what we are doing.
The problem is, in sitting back, we are sending the tremendous force of our falling in the exact opposite direction that it needs to go. That we all function normally while sitting back, and seemingly without negative consequence, is testament to how we can abuse ourselves and not realize it. We fail to notice the mal-coordination we create - the muscular strain and skeletal distortions that flow from falling backwards.
As a result of habitually falling backwards in sitting, we lose access to our innate uprighting system - replacing it with an alternative system that wastes our engery, leaves us with 'bad-posture', limits our freedom of movement and makes us susceptible to injury.