There are four main groups of primitive lifeforms: viruses, viroids and prions, prokaryotes, and protists. Viruses are obligate parasites that reproduce by taking over a host cell's machinery. Prokaryotes include bacteria and archaea, which were some of the earliest lifeforms and lack membrane-bound organelles. Protists are a diverse group of eukaryotic organisms that include algae, protozoans, slime molds, and water molds.