Where did we come from and are we alone?

Paul Falkowski

Board of Governors Professor of Marine and Dept of Earth and Planetary Sciences
Institute of Marine and Coastal Sciences and
Dept. of Geological Sciences

Abstract

Two gases overwhelmingly dominate Earth's atmosphere: N₂ and O₂. The former is primordial; it accumulated on the surface as the planet formed, and its presence and abundance are not driven by biological processes. Indeed, N₂ is virtually inert and has an atmospheric lifetime on the order of 1 billion years. In contrast, O₂ is continuously produced biologically via the photobiological oxidation of H₂O, a process driven by energy from the sun. The gas almost certainly was not present in abundance when the Earth was formed, is highly reactive, and has an atmospheric lifetime of ~4 million years. Yet, despite the ~2500 ratio in lifetimes, O₂ came to comprise between 10% and 30% of the atmospheric volume for the past several 500 million years. How did O₂, a gas critical to the evolution of animal life, become the second most abundant gas on Earth? The story isn't as simple as it might first appear. Here we briefly examine the story of oxygen.

This very brief exploration of how oxygen came to be the second most abundant gas on this planet clearly shows the incredible contingencies required. Biological, geochemical and geophysical processes conspired to produce the planetary atmosphere that allows complex animal life to have evolved. Perhaps ironically, although it has been known for over 200 years that oxygenic photosynthesis is responsible for the production of oxygen, we still do not understand the basic mechanism responsible for water splitting, nor do we understand what controls the concentration of the gas in our planetary atmosphere. The former issue almost certainly will be resolved within a decade, as increasingly higher resolution structures of the photosystems become available, together with increasingly sophisticated biophysical approaches to measuring electron transfer reactions. The latter issue will be more difficult to constrain, but a better understanding will emerge from more complete (not necessarily, complex) models coupled with better integrated biogeochemical measurements. We do know however, that the gas is not present in abundance on any other planet or their moons in our solar system. Hence, we left to search for the existence of life on those objects based on a compromise between our technological capabilities and our unfulfilled dreams of searching for proxy evidence. As technological capabilities develop, we can search for the existence of oxygen on planets outside of our solar system. While the identity of the gas on distant planets is difficult to confirm, it is a critical component in understanding how complex life can exist on Earth and how rare this planet really is.