Of any system such as a machine is defined to be the ratio of the work done to the heat input:
Thermal efficiency = W/QH = Qh - Qc/Qh
All this talk of evolution and God is great for uncovering the motive behind why we do a lot of the small 'human' things we do. But all things have a law to govern them and to govern the outcome of their existence. In the human world there is no difference. This is where we expand upon the laws of thermodynamics in regards to the human system and what all this has to do with our biomechanical evolution.
Thermodynamic law applies to the properties of heat or energy in a system and in the system of life, energy is always being transferred, utilized, and transformed. Because the whole of energy is based on the power of light energy -- with its heat, matter, and wave particle properties -- we find that we are the inefficient end users in the long process of energy procession. The reason that thermodynamic law is vital to biomechanical integration is based on the key notion that any society on the brink of self-destruction must realize that efficiency is vital to prolonging the existence of their species.
Without a clean, renewable source of 'life' energy, all creatures in an ecosystem will exhaust or degenerate a vital link in the cycle, in the end. That is if the source that powers the entire system does not exhaust itself, in our case that would be the sun, and we have plenty of time to enjoy its rays.
In the laws of thermodynamics we find two vital understandings we must incorporate to create processes efficient enough for prolonged use. As well as the reason why the current situation - state of the world as some say - is so critical.
The first law states that in any system energy can neither be created nor destroyed. It is for this reason we can know that throughout the universe all matter and atomic structure is similar to the structure we find in the Milky Way galaxy. This is also supported by the second part of the first law in the fact that all energy in the system -- in this case, the universe as a whole -- is constant never increasing or decreasing. From the death of one star, comes the birth of others. From the death of one life comes another. These laws are not only mathematical but physical, mental, and metaphysical.
The second law states how energy is processed. In a system, energy will follow the path of natural occurrence (ex. hot air flowing into cold), least resistance (ex. electric conductors), and conversion of energy (gasoline engine). What this law defines is the apparent decrease in usable fuel in a system and the efficiency of a given system.
This is where some would argue evolution is flawed in that it does not follow this second law. "Look at a wooden table, it does not evolve into a tree!" I recall a misguided colleague of mine saying when I brought up the topic of evolution. What must be understood is the difference in the laws that govern energy and the laws that govern reality. In reality the table will never evolve into a tree.
But in the second law we can see how the eventual deterioration or conversion of the table could make it into something to be perceived as better. But to stay scientific, we want to look at things in matters of efficiency. As a tree, the wood use to make the table was efficient in converting CO2 into O2, now it is efficient in supporting a surface weight of 425 pounds against the pull of gravity, so which is more efficient? It is up to your definition of efficiency given a set realistic condition.