Jan Zalasiewicz is a geologist at the University of Leicester. I have been trying to avoid reading his book The Earth After Us: What Legacy Will Humans Leave in the Rocks? (OUP, 2008), because I could tell from the title alone that it would wind me up. Now that I have read it, I find I was not mistaken.
Zalasiewicz’s intention, stated in his opening chapter, is to offer a geologist’s perspective on present-day human society. This is achieved by imagining that, 100 million years in the future, our planet is visited by extraterrestrial aliens, geologists like himself, who are faced with the puzzle of working out who we were from the 100-million-year-old fossils which is all that our cities, our machines and our bones have left in the rocks.
“The arguments put forward”, he states disingenuously, “will not be affected by whether we become extinct over such a timescale [...] No special pleading need be involved; one can simply apply normal geological principles to studying the preservation potential of humans and their handiwork. The estimates used will stay sober and conservative. Where different trajectories or options are possible, these will be spelled out” (p.4).
This sounds very fair and even-handed. So what are the “different trajectories” possible for the human species over the next 100 million years? Clearly there are in principle three broad options (which may be combined to create more complex histories):