Based on fossil evidence, the current model for life on Earth is that for a period of ~2 x 109 (billion) years the only forms of life on Earth were microscopic. While the exact nature of these organisms remains unclear, it seems likely that they were closely related to prokaryotes, that is, bacteria and archaea. While the earliest organisms probably used chemical energy, relatively soon organisms appeared that could capture the energy in light and use it to drive various thermodynamically unfavorable reactions. A major class of such reactions involves combining CO2 (carbon dioxide), H2O(water), and other small molecules to form carbohydrates (sugars) and other important biological molecules, such as lipids, proteins, and nucleic acids. At some point during the early history of life on Earth, organisms appeared that released molecular oxygen (O2) as a waste product of light-driven reactions, known generically as oxygenic photosynthesis. These oxygen-releasing organisms became so numerous that they began to change Earth’s surface chemistry - they represent the first life-driven ecological catastrophe.
The level of atmospheric O 2 represents a balance between its production, primarily by organisms carrying out oxygenic photosynthesis, and its removal through various chemical reactions. Early on as O2 appeared, it reacted with iron to form deposits of water-insoluble Fe (III) oxide (Fe2O3)- that is, rust. This rust reaction removed large amounts of O2 from the atmosphere, keeping levels of free O2 low. The rusting of iron in the oceans is thought to be largely responsible for the massive banded iron deposits found around the world.49 O2 also reacts with organic matter, as in the burning of wood, so when large amounts of organic matter are buried before they can react, as occurs with the formation of coal, more O2 accumulates in the atmosphere. Although it was probably being generated and released earlier, by ~2 billion years ago, atmospheric O2 had appeared in detectable amounts and by ~850 million years ago O2 had risen to significant levels. Atmospheric O2 levels have changed significantly since then, based on the relative rates of its synthesis and destruction. Around ~300 million years ago, atmospheric O2 levels had reached ~35%, almost twice the current level. It has been suggested that these high levels of atmospheric O2 made the evolution of giant insects possible.50
Although we tend to think of O2 as a natural and benign substance, it is in fact a highly reactive and potentially toxic compound; its appearance posed serious challenges and unique opportunities to, organisms. As we will see later on O2 can be “detoxified” through reactions that lead to the formation of water; this type of thermodynamically favorable reaction appears to have been co-opted for a wide range of biological purposes. For example, through coupled reactions O2 can be used to capture the maximum amount of energy from the breakdown of complex molecules (food), leading to the generation of CO2 and H2O, both of which are very stable.
Around the time that O2 levels were first rising, that is ~10 9 years ago, the first trace fossil burrows appear in the fossil record. These were likely to have been produced by simple worm-like, macroscopic multicellular organisms, known as metazoans (i.e., animals), capable of moving along and through the mud on the ocean floor. About 0.6 x 109 years ago, new and more complex structural fossils begin to appear in the fossil record. Since the fossil record does not contain all organisms, we are left to speculate on what the earliest metazoans looked like. The first of these to appear in the fossil record are the so-called Ediacaran organisms, named after the geological formation in which their fossils were first found.51 Current hypotheses suggest they were immotile, like modern sponges but flatter; it remains unclear how or if they are related to later animals. By the beginning of the Cambrian age (~545 x 106 years ago), a wide variety of organisms had appeared within the fossil record, many clearly related to modern animals. Molecular level data suggest that their ancestors originated more than 30 million years earlier. These Cambrian organisms show a range of body types. Most significantly, many were armored. Since building armor involves expending energy to synthesize these components, the presence of armor suggests the presence of predators, and a need for a defensive response.
Viruses: Now, before we leave this chapter you might well ask, have we forgotten viruses? Well, no - viruses are often a critical component of an ecosystem and an organism’s susceptibility or resistance to viral infection is often an important evolutionary factor, but viruses are different from organisms in that they are non-metabolic. That means they do not carry out reactions and cannot replicate on their own, they can replicate only within a living cell. Basically they are not alive, so even though they are extremely important, we will discuss viruses only occasionally and in quite specific contexts. That said, the recent discovery of giant viruses, such as Mimivirus, suggests that something interesting is going on.52