Amoeba is a way of moving, not a single branch of life
There is no amoeba family on the tree of life. Shape-shifting cells that crawl on pseudopods turn up in every major group of complex organisms, from fungi to algae to animals. They range from bacterium-sized specks to deep-sea shells 20 cm wide, and many may have been having sex since life's early days.
An amoeba is any cell that changes shape by pushing out and pulling back temporary lobes called pseudopods. Older schemes lumped these organisms into a group named Sarcodina, but molecular studies showed its members do not share a common ancestor, so they are no longer classified together. Classroom favourites include Amoeba proteus and Chaos carolinense. Other notorious kinds are Naegleria fowleri, dubbed the brain-eating amoeba, Entamoeba histolytica, which causes amoebic dysentery, and the social slime mould Dictyostelium discoideum.
With no cell wall, an amoeba moves freely as actin filaments shove its membrane outward. Pseudopod shape helps tell groups apart: blunt, tube-like lobes in Amoeba, thin threads in Euglypha, merging nets in foraminifera and stiff needles braced by microtubules in radiolarians. Some species live naked; others build shells of silica, chitin, calcium or glued sand grains and diatom fragments. Freshwater species carry a contractile vacuole that pumps out water seeping in by osmosis, without which they would swell and burst; marine species, balanced with seawater, generally manage without one.
Lacking a mouth, amoebae engulf bacteria, other protists or dead matter by wrapping pseudopods around it at any point on the cell, and some also sip dissolved nutrients through tiny membrane pockets. Sizes vary enormously. Massisteria voersi measures only 2.3 to 3 micrometres, while giant amoebae such as Pelomyxa palustris can be seen with the naked eye.
Amoebae were long assumed to be asexual, but genetic evidence says otherwise. The Acanthamoeba genome contains meiosis genes such as Spo11 and Dmc1, and purified Dmc1 from E. histolytica performs the DNA swapping typical of meiosis. Dictyostelium mates when food runs short. Because Amoebozoa branched off early, meiosis probably arose early in complex life, in line with Lahr and colleagues' suggestion that most amoeboid lineages are anciently sexual.
Source: Amoeba
