Many years ago I watched a wonderful lecture by someone called Richard Dawkins. This lecture was one of a series given by Richard that years, and was hosted by the Royal Institution. The RI maintain a long tradition of such lectures, shown each Christmas, in order to encourage young people to experience the wonder of science. Richard's lectures were quite definitely some of the best that had been given, and remain so to this day.
The particular lecture that caught my imagination was on the subject of 'Mount Improbable', the supposed improbability barrier to the evolution of extremely complex structures such as eyes. Richard showed how the great height of improbability that would be a sudden appearance of an eye in a lineage that had nothing resembling an eye before isn't the way evolution works. Peaks of improbability can be climbed in small steps, through individual mutations followed by selection. The eye doesn't have to (and didn't) appear in one generation.
This was a useful way of describing how evolution works, but I wasn't completely satisfied. What I could not see from the 'Mount Improbable' metaphor was why evolution would bother to climb up the the mountain at all. Looked at from the point of view of biology, the tiny steps up the mountain lead to arrangements of genes (which we call 'organisms') being better able to survive than alternative arrangements. But even so, what is actually going on, overall, in terms of probability rather than biology? What actually is the force that drives evolution up the slopes of Mount Improbable?
I have both a degree and post-doctoral work in biochemistry and published work on thermodynamics (although this was some time ago, and I have forgotten quite a lot of it). But, surely, I thought, there must be a thermodynamic way of looking at all this. So I'm going to present such a way of looking. It's going to be wrong in many ways, but I hope it can give some idea of what is probably going on with the journey up Mount Improbable.
Life needs energy to maintain itself and to grow and reproduce. But energy alone isn't enough. Life can't grown on heat energy, for example. Just as important as energy is the form in which the energy is supplied. The energy has to be ordered; it has to contain some kind of structure, and life feeds off that order. Plants use the energy of sunlight, energy which peaks at certain wavelengths. Plants use those wavelengths of light to drive reactions which split water into hydrogen and oxygen, providing chemical energy that is used to form carbon compounds from CO2 allowing the plant to maintain itself and build new cells.
There are other sources of ordered energy that can fuel life; there are single-celled organisms deep in the rocks of the Earth's crust that grow incredibly slowly using hydrogen released by reactions between iron and water. A large fraction of the mass of all life on Earth probably consists of these cells. In these different environments on Earth, and probably in many other kinds of environment throughout the Universe, life maintains itself and grows where there are flows of energy and order.
If we only consider the organisms and their complexity we don't see the whole picture. There is a much broader view that can help us understand why life's complexity can appear and increase. That view includes what life leaves behind. Living organisms make a mess. Their biochemical reactions and their physical actions throw waste products and heat into the environment, and as a result life produces disorder. Life increases entropy. The structure of plants produced by tapping the low entropy of sunlight is broken down by decay and digestion. A cow turns grass into cow, but also into cow-pats, methane and heat. The key thing here is that life produces more disorder than there would be without life. Life is a catalyst for raising entropy.
So, Mount Improbable isn't the full picture. At the same time as life progresses up the mountain, life erodes deep valleys of entropy, of probability. Overall, taking into account both the improbable mountains and the probable valleys, life lowers the landscape. Given the probability landscape before life and the landscape with evolving complex life, the landscape with life is thermodynamically favourable. It's something that will spontaneously happen. Life, including highly evolved life, is probable, perhaps even inevitable.
There is more. We can ask where does the order in the energy used by organisms such as plants come from? It comes from stars; directly when it comes to photosynthesis, and indirectly in the form of chemical energy from reactions with elements that have been formed in long-dead stars and spread throughout space by supernovae.
Where do stars come from? They come from the collapse of vast gas clouds because of gravity. Gravity amplifies the effects of slight irregularities in these clouds, and huge volumes of gas and dust gather into hot spheres that ignite to become stars like our Sun.
Ironically, Mount Improbable is climbed because of the cosmic power of gravity.