Title:
How Earth's early oceans and atmosphere help guide the search for life beyond our solar system

Abstract:
In recent years, researchers have embraced the full dynamic range of our planet's long history as a catalog of 'alternative Earths.' These planetary states were wide ranging but unified by the persistence of habitable conditions for perhaps four billion years. Life and life-sustaining environments prevailed in the face of a cooling Earth interior, a warming sun, shifting tectonic modes, large and small impacts, a stabilizing and varying magnetic field, changing surface redox, climatic extremes, and a multitude of other contingencies, challenges, and opportunities expected at the hands of stellar, solar system, and planetary evolution. In the end, enduring habitability is a testament to the power of feedbacks at the interfaces between biotic and abiotic processes on Earth. Each of Earth's widely varying planetary states translates to a particular atmospheric composition that could one day be detected on an exoplanet.
From ongoing studies of past atmospheric compositions on Earth, we learn about possible false positives in life detection. Methane (CH4), for example, can be tied easily to multiple abiotic pathways. On the other hand, evidence for high early methane levels in disequilibrium with other atmospheric gases, such as carbon dioxide (CO2), or in the absence of appreciable ozone (O3) shielding would likely demand biological inputs. Similarly, seasonal patterns in gas abundances, mostly notably for oxygen (O2) and CH4, can point convincingly to biological origins. Recent lessons about time-varying methane stability also point to the potential need for additional gases, such as nitrous oxide, to maintain our liquid ocean over most of Earth history.
The most recent results from early Earth are teaching us equally about the concept of the false negative--that is, an absence of detectable atmospheric biosignatures above an ocean brimming with life. An example from the very early pages of our history is the abundant free O2 that was likely confined to the surface waters of the ocean where it was photosynthetically produced in disequilibrium with an essentially O2-free atmosphere. In fact, if Earth had been viewed remotely using current technology, O2 may not have been detectable in our atmosphere for more than two billion years following its first biological production. And the famous O2-CH4 disequilibrium biosignature may not have been demonstrable at any point in Earth history.
Studies of modern and ancient organisms on Earth also allow us to explore the limits of life under very high CO2 levels (hypercapnia), thus helping us to refine the outer edge of the habitable zone--particularly in terms of complex life. Despite previous claims, high (detectable) levels of carbon monoxide (CO) need not be an anti-biosignature. In other words, even though CO can be a source of energy to microbes, our latest results reveal that CO accumulation is possible under certain planet-star scenarios even for inhabited biospheres. Finally, findings from early Earth are informing decisions being made now about next generation telescopes. Telescopes optimized for O3 detection could, for example, elevate our ability to recognize the presence and temporal variability of O2 on distant planets.