Follow the Energy

How living things draw energy from the environment may be a clue to their history.

The Eridania Basin on Mars is about 3.7 billion years old. Seafloor deposits, which likely contain evidence of hydrothermal activity, are shown in color.

Astrobiologists have known for some time that the quest to understand how life originated on Earth is relevant to the search for life on other worlds. Now Joseph Michalski from the University of Hong Kong and his colleagues are suggesting that a good way to understand life’s origins might be to follow the energy sources, such as molecular hydrogen, sulfur, and iron, that the earliest organisms used. And using this strategy, evidence of early life might be easier to find on Mars than on Earth.

This seems puzzling at first, given the richness of biota on Earth and the cold, dry, and seemingly lifeless environment of Mars. But hardly any piece of Earth’s crust is left from the time when life is thought to have first appeared—about four billion years ago. Plate tectonics has recycled nearly all of it. And considering that Earth’s surface was taken over by photosynthetic life starting about three billion years ago, finding biosignatures from earlier organisms would be extremely difficult.

Michalski calls attention to the fact that Mars has an iron- and sulfur-rich crust, with abundant evidence of ancient hydrothermal activity and production of molecular hydrogen gas, which could have fueled an early biosphere. Also, since Mars transitioned from a warmer, wetter phase to an ice ball around four billion years ago, any biosignatures from that earlier period should be well preserved just below the planet’s frozen surface. Because Martian rocks are not recycled by plate tectonics, most of them are much older than rocks on Earth.

The similarity of some of the (fossilized) hydrothermal environments on Mars to those on Earth is intriguing. But we can’t jump to conclusions. Michalski and co-authors assume that terrestrial life originated in these kinds of settings, which is far from established, and still very much under discussion. Even if it did, Mars was, and is, a different place. An alkaline deep ocean hydrothermal vent probably did not exist there, and there may have been other missing ingredients.

Nevertheless, investigating ancient energy sources that aren’t commonly in use today is a step in the right direction. In another paper published in Nature Geoscience, Kazumi Ozaki from the NASA Astrobiology Institute and colleagues claim that the rise of a microbial community using hydrogen gas as an energy source, and interacting with other microbes using an ancient type of photosynthesis that didn’t involve oxygen, would have caused global warming and increased global temperatures by many degrees Celsius. That may be how Earth avoided becoming a global snowball.

The biological use of ancient energy sources might even help explain the Faint Young Sun Paradox: the long-standing puzzle of why the early Earth’s global temperatures were at times higher than they are today, despite the Sun’s radiation output being 25-30 percent lower now.

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