The possibility of salvation is there for all mankind. Those who existed before "history" would fall into this category.
Jesus remains the only way to salvation for all mankind because He journeyed to "Abraham's Bosom" after He died on the Cross. This was to preach the gospel to those who died but we're not already condemned to eternal damnation. Judgement is carried out by God based on your conscience. So if the person knows Jesus and rejects the truth, that is not good. But if a person follows the goodness of what is revealed to them, even if not Christ (prehistoric mankind or maybe another religion, etc.) then the possibility of salvation is given them.