Both domains consist entirely of prokaryotes, minute single-celled organisms that have neither a distinct nucleus nor other specialized cellular organelles. Prokaryotes would dominate the biosphere for more than seven-eighths of its history, until about six hundred million years ago. If we meet living organisms elsewhere in our galaxy, we probably won’t be shaking hands with them but peering at them through a microscope. So small are prokaryotes that one hundred thousand of them could have a party inside the dot at the end of this sentence. Prokaryote
