Recently I came across a 2008 article, the authors of which argue that in fact the space of protein sequences is not as large as it might seem, and that life on Earth has most likely already explored this entire space (maybe before the appearance of eukaryotes).
Dear experts, can you explain what this means?
That life on Earth has already explored all the possible space of genomes, and all possible living organisms that are generally possible have already appeared? But after all, only the human genome consists of about 3 billion base pairs, and all possible combinations are a gigantic number, how could life explore them all, not to mention other genomes? And if life before eukaryotes tried every possible genome, then how did other life forms with a different genome come about?
I'm confused, please help.