A day on Earth… 2.4 billion years ago

When the Earth formed, its atmosphere was very different from today. In fact, in the beginning it almost had no atmosphere at all. It took millions of years for the planet to cool down enough to start building up an atmosphere composed mainly by water vapour, carbon dioxide and nitrogen. Notice anything missing?

Exactly, no oxygen! The components of this early atmosphere came from the gases released from the molten rock. And one important detail: there was no life yet.

An early Earth may have looked like this. Peter Sawyer / Smithsonian Institution

The oxygen that is found in the atmosphere (O2) comes primarily from photosynthesis. The earliest evidence for life on Earth dates to about 1 billion years after the Earth formed. Early organisms were anaerobic, likely using methanogenesis or anoxygenic photosynthesis. Neither of these mechanisms release oxygen, but rather methane or sulphur components and water (Catling & Claire, 2005). The exact moment that oxygenic photosynthesis appeared is debated, especially considering that many fossils found are poorly preserved (Sessions et al., 2009). However, to get an idea, in fig. A there is a timescale showing the ages of fossils found, and possible moments when photosynthesis could have appeared, since cyanobacteria produce O2.

Fig. A: Timescale and ages of fossils. Sessions et al. (2009)

So what happened 2.4 billion years ago?

The oxygen levels in the atmosphere increased in what is known as the "Great Oxidation Event" (fig. B). The atmospheric oxygen went from 0.001% of present day levels to 5-18% by the time everything stabilised, some 1.8 billion years ago (Sessions et al., 2009). There was a second event that finally took the oxygen levels to present day levels about 500 million years ago.

Fig. B: A simplified view of the evolution of oxygen in the atmosphere. Sessions et al. (2009)

Now, cyanobacteria had already been doing photosynthesis and producing oxygen for some time (at least some 300 million years), but it wasn't building up in the atmosphere, at least not in the amounts shown by the geological records. This means that around this time there must have been a change: either there was an increase in the production of oxygen, or a change in the oxygen sinks to allow the O2 to remain in the atmosphere.  

One hypothesis relates the Great Oxidation Event with the changes in the tectonics of the planet that happened around the same time. Continental land masses stabilised towards the end of the Archaean (~2.5 billion years ago). Kump (2007) suggests that an increase in subaerial vulcanism is related with a reduced sink for oxygen, since before the emergence of continents most volcanic activity was submarine and more reducing (used up more oxygen). This is a compelling hypothesis because changes on the Earth of this scale (mayor continental change and substantial increase in atmospheric oxygen) happening at the same time are generally more than a simple coincidence. 

Why is this important?

Besides the fact that WE need oxygen to breathe, it is likely that complex multicellular life was able to evolve thanks to this: aerobic respiration is much more efficient than anaerobic fermentation (Sessions et al., 2009).

Catling & Claire (2005) also mention a couple of important things to consider:

• An early ozone layer in the stratosphere emerged once there was enough oxygen in the atmosphere (approximately 0.1% present day levels).
• Methane levels where high (simulations indicate 100 to 1000 times higher than present day), which kept the Earth warm, despite a "weaker" Sun. The rising atmospheric oxygen caused a reduction in methane, cooling the Earth down significantly, perhaps even causing  Snowball Earth conditions.  

This is only one of many mayor changes the Earth has gone through. In this case the intervention of newly appeared life played an important role in changing the atmosphere. However, as we will see in the next posts, one change can be followed by others that are unexpected.

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Other bibliography:

Tarbuck, E. and Lutgens, F. (1997). Earth science. Upper Saddle River, NJ: Prentice Hall.