Problem:
Archaeans evolved billions of years ago but were not known to humans until the late 1970s. Major new lineages are still being discovered. Species in the domain Archaea share important characteristics with bacteria: All of the species studied to date are unicellular; All of the species studied to date have a single circular chromosome; many also have small, circular, auxiliary DNA molecules called plasmids. Like bacteria, archaeans lack a nucleus. This observation inspired the name prokaryote, or "before-kernel," to refer to the bacteria and archaea (here "kernel" refers to the nucleus). Life scientists will sometimes also use the term microbe to refer to archaea, bacteria, microscopic eukaryotes, and viruses. In addition to the features they share with bacteria, they also have a key synapomorphy: Cell membranes dominated by phospholipids that have several unique structural features. As animals with particularly large bodies, it is hard for humans to understand why it's a big deal that bacteria have a unique type of cell wall or archaea have a unique type of membrane phospholipid. But consider that for single-celled organisms, the cell wall and membrane form the boundary between life and non-life, between "me and them." The chemistry of these structures matters-enough that life scientists will refer to the difference in archaeal and bacterial membranes as "the lipid divide." Need Assignment Help?