▁layer ▁absor bing ▁harm ful ▁ult rav io let ▁radiation , ▁life ▁colon ized ▁the ▁surface ▁of ▁Earth . ▁ ▁Micro bes ▁ ▁The ▁first ▁form ▁of ▁life ▁to ▁develop ▁on ▁the ▁Earth ▁were ▁micro bes , ▁and ▁they ▁remained ▁the ▁only ▁form ▁of ▁life ▁until ▁about ▁a ▁billion ▁years ▁ago ▁when ▁multi - cell ular ▁organ isms ▁began ▁to ▁appear . ▁Micro organ isms ▁are ▁single - cel led ▁organ isms ▁that ▁are ▁generally ▁micro scop ic , ▁and ▁smaller ▁than ▁the ▁human ▁eye ▁can ▁see . ▁They ▁include ▁B acter ia , ▁F ung i , ▁Arch ae a , ▁and ▁Pro t ista . ▁ ▁These ▁life ▁forms ▁are ▁found ▁in ▁almost ▁every ▁location ▁on ▁the ▁Earth ▁where ▁there ▁is ▁liquid ▁water , ▁including ▁in ▁the ▁Earth ' s ▁interior . ▁Their ▁reprodu ction ▁is ▁both ▁rapid ▁and ▁prof use . ▁The ▁combination ▁of ▁a ▁high ▁mut ation ▁rate ▁and ▁a ▁horizontal ▁gene ▁transfer ▁ability ▁makes ▁them ▁highly ▁adapt able , ▁and ▁able ▁to ▁surv ive ▁in ▁new ▁environments , ▁including ▁outer ▁space . ▁They ▁form ▁an ▁essential ▁part ▁of ▁the ▁planet ary ▁e cos ystem . ▁However , ▁some ▁micro organ isms ▁are ▁path ogen ic ▁and ▁can ▁post ▁health ▁risk ▁to ▁other ▁organ isms . ▁ ▁Pl ants ▁and ▁animals ▁ ▁Origin ally ▁Arist ot le ▁divided ▁all ▁living ▁things ▁between ▁plants , ▁which ▁generally ▁do ▁not ▁move ▁fast ▁enough ▁for ▁humans ▁to ▁notice , ▁and ▁animals . ▁In ▁Lin na eus ' ▁system , ▁these ▁became ▁the