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Did respiration evolve before photosynthesis?

June 6, 2011

Animals today stay alive by breathing in oxygen-rich air through a process known as oxygenic respiration, which consumes oxygen (O2) and releases carbon dioxide (CO2) as a byproduct. Most plants, on the other hand, convert sunlight and carbon dioxide into energy through a process known as photosynthesis, which consumes CO2 and releases O2 into the atmosphere. Because photosynthesis is a source of oxygen, it seems intuitive that photosynthesis evolved first: once enough O2 was in the air, then respiration would be able to arise in the newly oxygen-enriched atmosphere. However, some biologists have argued since the 1970′s that respiration in fact evolved first. There are many reasons that this might be the case, and new measurements of bacterial respiration at very low levels of O2 have revived this “early-respiration” hypothesis.

In a recent paper written by myself and my two graduate advisers, we argue that small quantities of O2 could have reached the surface of early Earth through transport by atmospheric dynamics. This transport would primarily occur in the Wintertime hemisphere, where a “polar Winter vortex” develops near the polar region, because the lack of sunlight in Winter would allow for greatest amount of O2 to accumulate. Our calculations show that enough dissolved O2 could have accumulated in polar Winter waters to allow early forms of marine life (i.e. microbial life) to develop and use respiration–without needing to wait for photosynthesis to oxygenate the atmosphere. Although our model calculations cannot prove that respiration did in fact evolve first, they least demonstrate a proof-of-concept that the “early-respiration” hypothesis is in fact viable.

Our paper is titled “Availability of O2 and H2O2 on pre-photosynthetic Earth” and appears in the May issue of the journal Astrobiology.

Would contact with aliens be dangerous?

May 9, 2011

Humans have yet to encounter any extraterrestrial beings, and, although speculations abound in Hollywood and in science fiction, we really have no idea what contact with aliens would be like. In one scenario, a population of aggressive and malevolent extraterrestrials invade Earth in order to enslave us or eat us as part of their conquest of the galaxy. Still another hypothetical extraterrestrial civilization might welcome us with open arms into peaceful communication and help us solve many of our global problems. With such a wide range of possibilities for an as yet unknown encounter, can we say anything at all regarding the risk of contact with extraterrestrials?

We explore this question in a recently published article titled “Would contact with extraterrestrials benefit or harm humanity? A scenario analysis”. This article was written by Seth Baum, Shawn-Domagal Goldman, and myself and appears in the current issue of Acta Astronautica. Rather than focusing on one particular outcome of alien contact, we take a broad approach of categorizing a wide range of contact scenarios as either beneficial, neutral, or harmful to humanity. In doing so, we draw on scientific as well as ethical analysis to demonstrate that there are a wide range of responses to contact with extraterrestrials, which may depend at least in part on human actions in the near and distant future.

Our television and radio signals have been leaking away from our planet for decades now, detectable by any nearby extraterrestrials willing to listen. The light from our planet, too, shows not only signs of biological life but also signs of rapid warming and climate change. Although we cannot be certain that any of our behaviors will necessarily invoke the wrath–or solicit the aid–of advanced extraterrestrials, perhaps it would behoove us to give thought and care to our future trajectory, just in case someone is watching.

Sulfur Can’t Keep Mars Warm

June 28, 2010

Satellites and rovers sent to Mars keep giving us compelling geologic evidence that liquid water flowed on the surface of the red planet in the distant past. Three billion years ago, when oceans may have existed on parts of Mars, the sun was about 30% fainter. Mars today is well below the freezing point of water, so any lakes or oceans would be frozen over. In the distant past, then, this problem is even more pronounced: how was Mars able to stay warm enough to sustain liquid water?

Many attempted resolutions have been proposed to this problem, but none has provided a complete solution for a warm, wet early Mars. In a recent paper published in Earth and Planetary Science Letters, on which I am a co-author, we argue that greenhouse warming by sulfur dioxide could not have kept early Mars warm enough. Sulfur dioxide has been suggested in the literature because it is an effective greenhouse gas, similar to carbon dioxide or methane. However, we show that atmospheric photochemistry with sulfur dioxide leads to the production of sulfate aerosols in the upper atmosphere that absorb incoming sunlight and cool the surface. Thus, sulfur dioxide may have caused net cooling on early Mars, rather than warming.

We’re still trying other mechanisms to explain a warm, wet early Mars. Most likely, it was some combination of processes, including several greenhouse gases and warming by clouds. A negative result for sulfur dioxide is not as exciting as a solution to the early Mars problem, but it’s still a small step forward.

Sky & Telescope article: Where have all the aliens gone?

January 20, 2010

If intelligent life is common in the galaxy, then the fact that we have not yet observed any extraterrestrials raises the question: where are they? The upcoming March issue of Sky & Telescope magazine features an article written by myself and Seth Baum where we discuss how the conspicuous absence of extraterrestrials may relate to the problem of sustainable development on Earth.

Funny story, we were initially slated for the January issue (we even made the table of contents), but a printing error lost the last 10 pages which sadly included our article. Nevertheless, the Sky & Telescope staff are excellent people to work with and made a fast turnaround. Be sure to check out the March issue!

A Hazy View of the Early Earth

February 6, 2009

My MS paper, “A Revised, Hazy Methane Greenhouse for the Archean Earth”, just appeared in the journal Astrobiology! You can view a PDF of the article on my research page.

We argue that the warm, ice-free climate of the early Earth (2.8 billion years ago) was maintained by a water vapor/carbon dioxide/methane/ethane atmospheric greenhouse effect that offset the ~20% reduction in solar luminosity from the faint young sun. Furthermore, a stabilizing feedback between life and the climate system may have resulted in a thin stratospheric organic haze that maintained above-freezing temperatures and shielded ultraviolet radiation. An excellent write-up of our work is available at The Planetologist.

I’ve given this talk several times over the past couple years, most recently on the Forum for Astrobiology Research (which should eventually be available as a podcast), and it feels good to finally see the paper come out.

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