A Homo habilis skull and a Homo erectus skull , discovered in 1972 and 1975 , respectively , were among the most significant finds of Leakey 's earlier expeditions . A Homo habilis skull ( KNM ER 1470 ) and a Homo erectus skull ( KNM ER 3733 ) , discovered in 1972 and 1975 , respectively , were among the most significant finds of Leakey 's earlier expeditions .