The earliest known animals are cnidarians from about , but these are so modern-looking that they must be descendants of earlier animals. Early fossils of animals are rare because they had not developed mineralised, easily fossilized hard parts until about . The earliest modern-looking bilaterian animals appear in the Early Cambrian, along with several "weird wonders" that bear little obvious resemblance to any modern animals. There is a long-running debate about whether this Cambrian explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern-looking animals began evolving earlier but fossils of their precursors have not yet been found, or that the "weird wonders" are evolutionary "aunts" and "cousins" of modern groups. Vertebrates remained a minor group until the first jawed fish appeared in the Late Ordovician.
The earlieSt KnOwn AniMAls Are cNidariAns from AbOuT , but These aRE sO mOdeRn-loOKiNg that theY mUsT be DescendantS of eArlieR AnimaLs. earLy FosSiLs of aniMAlS are rARE becauSe theY HAd nOT develOPEd miNeraLISEd, easilY FoSsilIZeD Hard parTs until abOut . tHe earLiest modeRn-looKing bilaTeRian anIMals APpeAr IN the earLy cambrian, aloNg WiTh seVerAl "WEird wonDeRs" tHAt Bear Little obviOus rESEmBlancE to any Modern aniMAls. TherE IS a loNG-runnINg debatE about wHEtHER ThIs CambriaN ExplOsIon was tRuLy A verY raPId perIod of eVoLutiOnaRy expEriMentatiOn; ALTERnatIve viEWS are ThaT mOdeRn-looking aNimalS began EvOLVinG earliER BUT FosSils Of TheIr precursors have noT yeT bEen fOuNd, Or thaT The "WEird wOndeRS" are evolutIoNary "aUnts" and "cOusins" of modErn GrOuPS. veRtebraTEs REmaIned a mINor gROup Until the firsT JaWed fisH appeAred In ThE late oRdovician.