Abstract EANA2025-2

We are in the age of searching for extraterrestrial life directly and indirectly. The major question is: what are we looking for and to what extent can life on Earth provide an analogy for extraterrestrial life.

The hypothesis that life exists on other planets is predicated on a number of fundamental, Earth-centric considerations: (1) the nature of life; (2) how life appeared on Earth; and (3) the co-influence of geological evolution on its evolution. My approach to the search for extraterrestrial life is grounded in the geological record of the Earth and the evolution of terrestrial life, and the implications for biosignatures that could be recognised from space. If an extraterrestrial observer were to observe the Earth from space using the telescopes presently available, I conclude that, although life emerged probably about 4.3 Ga, it would not have been possible for the observer to detect traces of terrestrial life until the advent of planktonic eukaryotic algae after the Neoproterozoic Oxydation Event about 800 Ma. However, there may have been a possible exception of a couple of hundred million years during the Great Oxydation Event 2.3-2.1 Ga. Thus, for most of the history of life on Earth, there would be no externally observable evidence of its existence. I will conclude with a short discussion on the consequences of these reflections for the search for extraterrestrial life.