▁ ▁Tra cks ▁from ▁an ▁animal ▁with ▁leg ▁like ▁append ages ▁have ▁been ▁found ▁in ▁what ▁was ▁mud ▁ 5 5 1 ▁million ▁years ▁ago . ▁ ▁Planet ary ▁environment ▁and ▁the ▁o xygen ▁cat ast rop he ▁ ▁Ev idence ▁of ▁the ▁details ▁of ▁plate ▁mot ions ▁and ▁other ▁t ect onic ▁activity ▁in ▁the ▁P rec amb rian ▁has ▁been ▁poor ly ▁preserved . ▁It ▁is ▁generally ▁believed ▁that ▁small ▁proto - cont in ents ▁existed ▁prior ▁to ▁ 4 2 8 0 ▁Ma , ▁and ▁that ▁most ▁of ▁the ▁Earth ' s ▁land mass es ▁collected ▁into ▁a ▁single ▁super cont inent ▁around ▁ 1 1 3 0 ▁Ma . ▁The ▁super cont inent , ▁known ▁as ▁Rod in ia , ▁broke ▁up ▁around ▁ 7 5 0 ▁Ma . ▁A ▁number ▁of ▁gla cial ▁periods ▁have ▁been ▁identified ▁going ▁as ▁far ▁back ▁as ▁the ▁Hur onian ▁epo ch , ▁roughly ▁ 2 4 0 0 – 2 1 0 0 ▁Ma . ▁One ▁of ▁the ▁best ▁studied ▁is ▁the ▁St urt ian - Var ang ian ▁gla ci ation , ▁around ▁ 8 5 0 – 6 3 5 ▁Ma , ▁which ▁may ▁have ▁brought ▁gla cial ▁conditions ▁all ▁the ▁way ▁to ▁the ▁equ ator , ▁resulting ▁in ▁a ▁" S now ball ▁Earth ". ▁ ▁The ▁atmosphere ▁of ▁the ▁early ▁Earth ▁is ▁not ▁well ▁understood . ▁Most ▁ge olog ists ▁believe ▁it ▁was ▁composed ▁primarily ▁of ▁nit ro gen , ▁carbon ▁dio x ide , ▁and ▁other ▁relatively