So our ancestors were shaped by 'wild' forces. Unpredictable by nature. There was no program to follow, no 'meso-cycle' or 'progression' - just the unpredictable demands of survival married to to and ability to adapt.
I try to keep variety in my workouts. Over the course of a week I hit the basic planes of motion (vertical pull/press, horizontal row/press, leg press), coupled with a few static holds and some sprints. Add in a few big lifts (including those that go above your head, and some carry work and "job's a good-un".
Your body does not really know the difference between many of the exercises used - a Military Press or a HSPU...the body just knows it is having to work and which muscles are required to do that work.
But your body is shrewd. It does not want the overhead of growing bigger with its associated increase in metabolic expense. Its first line of adaption is to increase efficiency - to make better use of what it has. Growth is at best best your body's plan B. Your body will only grow so if it is challenged appropriately - if it HAS to.
Although I have kept away from 'going to failure', in retrospect I think I am too often stopping well short of where the most gains can be made. I like working out in a short and intense fashion, but it is often a kind of aerobic exhaustion that gets me (the traditional 'cardio' idea), as much as anything approaching muscular failure.
Now this has made me pretty lean and mean (IMHO), but I am curious as to whether I could get a bit bigger. In going paleo I have grown some - I sit at about 80kg most of the time from a pre-paleo mode of 77kg, but most of this is probably to do with the increase in muscle in my legs and butt from sprinting.
So in the interests of experimentation, I am going to push a bit harder towards muscular failure and see where that takes me....of course function remains my primary goal!