Second genesis: The search for life on other worlds

The primary goal of astrobiology is the search for evidence of life beyond the Earth. It is worth asking why is this search of interest and how can it be done? I submit that the answer to the first question is also the answer to the second question and the answer is rooted in biochemistry.

The search for life in our Solar System and the implications for science and society

The search for another type of life in the Solar System addresses the fundamental question of life in the Universe. To determine if life forms we discover represent a second genesis, we must find biological material that would allow us to compare that life to the Earth’s phylogenetic tree of life. An organism would be alien if, and only if, it did not link to our tree of life. In our Solar System, the worlds of interest for a search for life are Mars, Europa, Enceladus and, for biochemistry based on a liquid other than water, Titan. If we find evidence for a second genesis of life, we will certainly learn from the comparative study of the biochemistry, organismal biology and ecology of the alien life. The discovery of alien life, if alive or revivable, will pose fundamentally new questions in environmental ethics. We should plan our exploration strategy such that we conduct biologically reversible exploration. In the long term we would do well, ethically and scientifically, to strive to support any alien life discovered as part of an overall commitment to enhancing the richness and diversity of life in the Universe.

Searching for a shadow biosphere on Earth as a test of the ‘cosmic imperative’

Estimates for the number of communicating civilizations in the galaxy, based on the so-called Drake equation, are meaningless without a plausible estimate for the probability that life will emerge on an Earth-like planet. In the absence of a theory of the origin of life, that number can be anywhere from 0 to 1. Distinguished scientists have been known to argue that life on Earth is a freak accident, unique in the observable universe and, conversely, that life is almost bound to arise in the course of time, given Earth-like conditions. De Duve, adopting the latter position, coined the phrase that ‘life is a cosmic imperative’. De Duve’s position would be immediately verified if we were to discover a second sample of life that we could be sure arose from scratch independently of known life. Given the current absence of evidence for life beyond Earth, the best way to test the hypothesis of the cosmic imperative is to see whether terrestrial life began more than once. If it did, it is possible that descendants of a second genesis might be extant, forming a sort of ‘shadow biosphere’ existing alongside, or perhaps interpenetrating, the known biosphere. I outline a strategy to detect the existence of such a shadow biosphere.