3 - The Likelihood of Other Trees

Summary

Here, I examine whether the definition of life that we arrived at earlier needs to be modified when considering the possibility of alien life. I also note that the search for alien life of any kind – astrobiology – is a much broader venture than SETI – the search for extraterrestrial intelligence. Then I deal with the question of whether we should expect trees of life, rather than a non-tree-like pattern, to characterize other inhabited planets. In order to focus in on those parts of the galaxy that may host life, I start by excluding most parts of the galaxy from consideration – life is unlikely to be found in interstellar space or on stars, which means that more than 99% of the galaxy is lifeless in terms of both its volume and its mass. I then focus on planets. Next, I argue against the Rare Earth hypothesis – that animal life is vanishingly rare in the galaxy (and the universe). Instead, I propose the alternative Common Earth hypothesis. Finally, I ask the question: are alien trees of life likely to run in parallel to the tree of life on Earth, or might those alien trees and their constituent life-forms be very different?