It's hard to fully comprehend the depth of time past. The universe came into existence some 13.8 billion years ago (BYA). The earth was formed around 4.5 BYA. The first signs of life – simple microbes – appear about 3.5 BYA. But as presented in a wonderful book about just how complex and essential they are – Life's Engines: How Microbes Made Earth Habitable by Paul Falkowski – microbes are anything but simple. Microbes – bacteria and archaea – are prokaryotes, single cell life without a nucleus or organelles. Everything else – single cell or multi-cell plants and animals – are eukaryotes: cells containing a nucleus and organelles such as mitochondria. The prokaryotes developed the ability to extract energy from the chemical environment and, eventually, from the sun. It took another two billion years for them to evolve into complex cells: the eukaryotes.
Two billion years is a long time. Why did it take that long to go from bacteria and archaea to the first eukaryotes? The machinery to convert chemicals such as hydrogen sulfide or ammonia, and then the much harder task of using sunlight, to fuel life would have taken a long time to develop. But not just that. Extracting energy from the environment meant a complex process of freeing electrons from chemical bonds, transferring those electrons around within the cell and using them ultimately to create other chemicals that would store those electrons (i.e., serve as “food”) to provide energy for cellular processes. Photosynthesis is an even more complex process that uses sunlight to crack electrons from water and combine them – through intermediate steps – with carbon dioxide to produce carbohydrates and, as a waste product, oxygen. This complex machinery had to evolve step by step through the repeated random changes in DNA and RNA as winnowed through natural selection. (A good part of the first billion years after the formation of earth would have been used for the construction of the RNA/DNA mechanism itself.) As Falkowski argues, the processes for producing and consuming biologic energy work as tightly as a complex and precise system of interlocking gears: one out of place and the whole won't work. All the parts of the machinery had to come on line more or less at once or it would not function. Somehow, the machinery evolved anyway, implying that a lot of time was required for vastly more failures – in which the resulting organism from random mutation simply died – than successes.
That the machinery was there to be evolved – that the givens of the universe allowed such a thing to come into existence – is also worth pondering. As is the fact that we would not be here otherwise.