Free Astronomy Magazine May-June 2018

40 MAY-JUNE 2018 ASTROBIOLOGY A n Earth life scene of 56 million years ago, corresponding to the so-called Pa- leocene-Eocene Thermal Maxi- mum − a period of thousands of years in which there was an un- usually rapid global warming. [National Geo- graphic, Aldo Chiappe] Below, a marshy landscape of the Eocene. [Science History Images/ Alamy Stock Photo] Earth becomes. Going back tens or hun- dreds of millions of years, we have more and more fragmented information about the many species that populated the plan- et. For example, we know very little about our most distant hominid ancestors, whose evolution up to us only covers the last mil- lion years, roughly 1% of the period during which complex life developed on land. Much of the remaining 99% escapes us, a period of about 400 million years in which unimaginable scenarios may have occurred. The limitations in our knowledge of life’s history are due to the fact that the fraction of living beings that turn into fossils is ex- tremely small and varies widely depending on the weather, the consistency of the or- ganisms, and the habitats in which they lived. The fossil records that have come down to us are a tiny sample, representa- tive of only a small part of the living species that have appeared and disappeared on our planet. We are far from having a complete picture, and you have to consider that over 99% of all species are extinct. On the other hand, it is true that the biological ties be- tween ascending and descending species allow us to fill many gaps − but we cannot rule out that entire evolutionary lines, tens of millions of years long, if not more, may have been irretrievably lost. If a very advanced civilization existed on Earth before ours, it may have left no fossil evidence of a biological type, but it may