A History Of Life, Part II

During the early period in the history of life, an enormous number of different kinds of biochemical pathways evolved. These are what we today would call metabolic pathways. There had to be developed biochemical pathways to obtain energy, process food, build macromolecules and carry out the functioning of the cell. It’s clear that some of the biochemical pathways, which are still central to the functioning of the cells today, arose very early.

As prokaryotes multiplied and diversified, competition for limited resources must have led to the evolution of increasingly diverse ways to acquire materials from the outside. So, the early evolutionary history of prokaryotes is really the evolution of a variety of metabolic pathways, which in turn is really the evolution of proteins. All of these metabolic functions are mediated by protein enzymes, which are what are catalyzing the biochemical reactions.

This kind of evolution isn’t something we can observe in the fossil record, but it is something we can deduce from the analysis of the structure of macromolecules and the DNA that codes for them.

It’s clear that nearly all of the metabolic processes that we find in modern cells today evolved in prokaryotes before eukaryotes even appeared on the scene.

The evolution of one biochemical process in particular had an overwhelming effect in the subsequent history of all life on Earth. Because early prokaryotes must had been competing for organic resources that would provide energy, the evolution of a process that could tap into a new boundless source of energy would had been a tremendously successful adaptation. One such source of energy is sunlight.

The Miracle and Curse of Photosynthesis

Photosynthesis is a biochemical process by which the energy of sunlight can be captured and used to build sugars, which in turn store that energy for the cell. Photosynthesis also arose very early in the evolutionary history of prokaryotes. Because of the obvious selective advantage of this trait, those organisms that possessed it soon became a dominant force on the planet.

There are several different kinds of photosynthetic pathways existing in prokaryotes today. The most efficient kind of photosynthesis, which is found in modern plants and cyanobacteria, has an interesting property: its efficiency is coupled with the fact that it generates oxygen as a waste product. This means that beginning with the evolution of photosynthetic pathways, oxygen began to be produced in abundant quantities.

The fossil record and other evidence suggest that cyanobacteria probably appeared around or before 1.7 billion years ago. Beginning about that time, generation of oxygen increased as this photosynthetic adaptation increased. So, the entire atmosphere of the planet Earth transformed from one which had no oxygen in it to one that is composed of about 20% oxygen. What’s the significance of this? Oxygen is a highly reactive molecule that interacts with organic molecules breaking them down. The presence of oxygen proved to be an environmental disaster of global proportions for most of the organisms that lived before oxygen appeared, because they simply could not live and function in an oxygen-rich environment.

This was the key to how organic molecules could evolve in the first place, as you may remember from my articles on the origin of life. The theory that Miller tested in his famous experiment assumed that the atmosphere had to not have oxygen for life to arise. When oxygen appeared on the scene, that kind of spontaneous synthesis of organic molecules could no longer occur. Furthermore, those organisms that had evolved in a non-oxygen environment now had a very hard time.

The biochemical and physical adaptations of organisms that had accumulated up to that time couldn’t cope with the oxygen revolution, but a few kinds of organisms did evolve some mechanisms that allowed them not only to cope with it, but in fact to take advantage of it. Specifically, some organisms developed a completely different process for methabolizing energy. This is a process we call cellular respiration.

The methabolic pathways of cellular respiration actually take advantage of the presence of oxygen to enormously increase the efficiency with which energy can be extracted from organic molecules. The evolution of photosynthetic pathways radically and permanently changed the Earth’s atmosphere and the biological inhabitants living on it. In the long run, also, it made certain changes that were key milestones in the history of life. Notably, it increased the amount of energy that can be produced by cells in two ways. Cells had found a new fuel, the sunlight, and a better way to burn the old kind of fuel with cellular respiration.

This increase in the amount of energy available permitted cells and organisms to become ever larger and complex. Another positive consequence of the accumulation of oxygen was the development of a layer of ozone gas in the upper atmosphere of the Earth. Ozone absorbs the radiation that hits the planet, and in so doing it made it possible for organisms to inhabit environments that previously were unavailable to them, most notably leaving the water and entering a land environment. To be continued…