A History Of Life, Part I

“All living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction. (...) Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from one primordial form, into which life was first breathed.”
Charles Darwin, On the Origin of Species

Darwin’s view of descent by modification led him to the conclusion that there existed sometime in the very remote past an organism that is literally the universal common ancestor of us all. I’ve already written some articles about how life could have arisen from non-living matter, you may be interested in reading some of them. At some point in the remote past, life was somehow “breathed” into a primordial form. An entity appeared we’ll all agree was living. Once this entity had gained a foothold on life, it became the common ancestor to everything that has ever lived on the planet.

Exactly when this organism arose is uncertain, but we know a few things. We know that it has to be around 3.9 billion years ago, when the surface of the Earth was no longer been bombarded, and the crust of the planet had cooled sufficiently for liquid water to appear. Also, it had to be sometime before 3.5 billion years ago, which is the date of the oldest fossil cells that had been found so far. These oldest fossils are found in rocks in Western Australia. They’re actually quite advanced in their appearance, resembling certain kinds of modern bacteria, so most biologists assume that cells must have appeared even earlier than this.

Let’s review the characteristics of this first living thing. This organism was certainly a single cell. It had to have a membrane that created a compartment separating it from the rest of the world, allowing the accumulation of specialized organic molecules within. Among the organic compounds that made up this first organism there must had been complex polymers capable of catalyzing biochemical reactions, the kinds of reaction that would be necessary for this primitive cell to process materials, grow and divide.

The most specialized of these biological polymers must have been some kind of molecule that could not only catalyze reactions, but also serve as a template for making more of itself. In the earliest cells, this information probably was in some form of RNA. Later, some kind of genetic system was established in an RNA world. DNA then took over the role of storing and transmitting genetic information, while proteins took over the role of the functional molecule responsible for everything else the cell might do.

Without doubt, at this early period in time, the major effect of natural selection on this primitive organism was to refine its genetic system and to ensure that the different kinds of molecules within it would cooperate. There had to be some selection so that these different kinds of molecules (lipids, proteins, nucleic acids and sugars) would be able to specialize to do their own jobs in coordination with each other.

A Prokaryote World

By the time we get to the oldest fossils, evolution had established a recognizable cell, which is not too different from what might be a typical bacteria we find today. There’s a rich fossil record that continues from that period up to the present day. This fossil record makes a couple of things clear. First of all, it shows that when life did emerge, it emerged very rapidly and quickly filled the planet.

It also shows that only prokaryotic cells existed for about 1.5 billion years, until about 2 billion years ago. In other words, the more complex eukaryotic cells didn’t exist for quite some time. The early evolution of life was all in the context of prokaryotic cells. The fact that prokaryotic cells ruled the Earth for 1.5 billion years might sound boring, but it’s nothing of the sort. There were at least two major and exciting evolutionary events that occurred during this period.

The first was that the prokaryotes themselves split into two major lineages. We call these the bacteria and the archaea. The differences between bacteria and archaea are relatively subtle in the modern world. There are biochemical differences in their internal composition and other things. In fact, there are enormously more bacteria in the world today than there are archaea. The archaea largely have been relegated to very extreme environments. These are the ones that we find in extremely saline water, for example.

In a sense, archaea are not necessarily an important form of life to understand today, but they are very important for understanding the origin of eukaryotic cells, as we’ll see later.