# Has life on Earth explored the entire space of genomes?

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?

• The idea that all possible living organisms have already appeared is readily disproven, by for instance plant breeding. Just with flowers, we see things like artificially-induced polyploidy, and interspecies hybrids that only survive because of things like embryo rescue. WRT that 3 billion base pairs, note that in addition to the combinations, you can make it 3 billion +1 and get a new set, then repeat with 3 billion + 2 &c. – jamesqf Jun 14 at 16:19
• The article argues about protein space, which is orders of magnitude smaller than genomic space. 1) There are 20 AAs for 64 codon sequences. 2) Proteins average 300-600 aa depending on the organism (source); note that this is much larger than your cited article's claim of <100aa, upon which their math is dependent 3) Only a fraction of most organisms' genome codes for proteins – Punintended Jun 14 at 17:13
• @Punintended How many orders of magnitude is the space of proteins less than the space of genomes? – Arman Armenpress Jun 14 at 17:28
• @ArmanArmenpress To compare the two, you'd need to make approximations for both. The amount of work in that task depends on your assumptions/approximations, and would likely be a paper (or two) in its own right. A constraint like "stability" will depend heavily on your precise definition, and since proteins and DNA have different chemical properties (and therefore stability), stability constraints on one will manifest quite differently in approximations of the other. Eg, "stability = 6 months at 4dC" will give very different results for different proteins, never mind comparing it to DNA – Punintended Jun 14 at 17:41
• @Punintended Thank you. That is, it is not known what proportion of all possible genomes are stable. – Arman Armenpress Jun 14 at 17:45