When trying to decipher the origins of life, there are many paths to follow. Chemical, physical, and occasionally for some, spiritual. For me, one of the most puzzling questions is what exactly defines life? It is a difficult philosophical question that is nearly unanswerable, and often not useful in studying the actual origins of life, but it is entertaining to postulate the different constraints that we place on what is “alive”. One simple designation is “You know it when you see it”, but that strays away from the actual matter at hand. At which point does something have enough interaction with the world around it to be considered alive? Some like to define life as a system that uses free energy in a system to propagate itself. I personally like this viewpoint, but it is a bit vague and leaves some open ends. Fire consumes fuel, taken from its nearby environment, and grows from this process. But is fire alive? Or does it just follow characteristics of living things. Another argument put forth is that there is a scale of life. Humans being at the top, can perceive the ‘life’ of other organisms lower down on the ladder. This implies that animals are more alive or conscious than plants, plants more so than simple multicellular organisms, and those still more complex than single sell organisms. This leads us to question a higher order organism, if in existence, would view us as less alive. Would they have some discerning power enabling them to draw differences between us? The other end of the spectrum can also be analyzed. What is more simple than a single celled organism, but is still considered alive? Through exploration of other planetary bodies, we can hope to observe new examples of life, maybe based on compounds other than water and carbon. These are just some ideas to ponder, they may not even have real answers. But thinking about difficult ideas is how new ones are born. What is your simple definition of life?

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Alexis Templeton is the Principal Investigator of the Rock-Powered Life team with the NASA Astrobiology Institute (NAI). She is a geomicrobiologist with expertise in microbe/mineral interactions, biomineralization, chemical imaging, spectroscopy, and isotope geochemistry.

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