ed ▁the ▁Camb rian , ▁the ▁first ▁period ▁of ▁the ▁Ph an ero zo ic ▁e on , ▁which ▁is ▁named ▁after ▁Camb ria , ▁the ▁Latin ised ▁name ▁for ▁Wales , ▁where ▁rocks ▁from ▁this ▁age ▁were ▁first ▁studied . ▁The ▁P rec amb rian ▁accounts ▁for ▁ 8 8 % ▁of ▁the ▁Earth ' s ▁ge olog ic ▁time . ▁ ▁The ▁P rec amb rian ▁( color ed ▁green ▁in ▁the ▁tim eline ▁figure ) ▁is ▁an ▁inform al ▁unit ▁of ▁ge olog ic ▁time , ▁sub div ided ▁into ▁three ▁e ons ▁( H ade an , ▁Ar che an , ▁Pro ter oz o ic ) ▁of ▁the ▁ge olog ic ▁time ▁scale . ▁It ▁sp ans ▁from ▁the ▁formation ▁of ▁Earth ▁about ▁ 4 . 6 ▁billion ▁years ▁ago ▁( G a ) ▁to ▁the ▁beginning ▁of ▁the ▁Camb rian ▁Period , ▁about ▁ ▁million ▁years ▁ago ▁( Ma ), ▁when ▁hard - sh elled ▁cre atures ▁first ▁appeared ▁in ▁abund ance . ▁ ▁Over view ▁Rel atively ▁little ▁is ▁known ▁about ▁the ▁P rec amb rian , ▁despite ▁it ▁making ▁up ▁roughly ▁seven - e ighth s ▁of ▁the ▁Earth ' s ▁history , ▁and ▁what ▁is ▁known ▁has ▁largely ▁been ▁discovered ▁from ▁the ▁ 1 9 6 0 s ▁on wards . ▁The ▁P rec amb rian ▁foss il ▁record ▁is ▁poor er ▁than ▁that ▁of ▁the ▁succeed ing ▁Ph an ero zo ic , ▁and ▁foss ils ▁from ▁the ▁P rec amb rian ▁(