Eukaryotic and prokaryotic organisms
The original organisms occurring on the Earth were prokaryotic, i.e. their cells did not have a classical nucleus and a number of other organelles. Of contemporary organisms, two groups, bacteria and the less well known archaea, are prokaryotes. Eukaryotes developed much later from prokaryotes. They have much larger cells, in typical cases their volume is greater by 3–4 orders of magnitude; they contain a nuclear cell wrapped in a double membrane and a number of specialized organelles, of which some, specifically mitochondria and chloroplasts, were formed at some time in the past by “taming” prokaryotic organisms – possibly parasitic bacteria related to present-day rickettsia (mitochondria) and algae (chloroplasts). A eukaryotic cell is actually a sort of conglomerate (chimera), formed in the past by the combination of several prokaryotic organisms belonging to both the group of archaea and the group of bacteria. For this reason, mitochondria and chloroplasts continue to bear their own genome – residues of the DNA of the original symbiont. In the framework of eukaryotic organisms, multicellular organisms developed over time, for example plants and animals. Prokaryotic organisms remained single-celled, but frequently form colonies of cooperating cells (belonging to a single species or to several species).