Answer:
We believe that Adam and Eve were not the first and only persons on earth at that time. If we had only the statements in Genesis concerning Adam and Eve, we too would believe they were the first, and once only, two people on earth.
However, consider the following:
When Cain was condemned, and he said, “My punishment is greater than I can bear” (Gen. 4:13), he said that he would be a fugitive and a vagabond in the earth: “and it will happen that anyone who finds me will kill me.” Would this be said (“anyone who finds me will kill me”) if his family were the only people on earth? Again we read, “The Lord set a mark on Cain, lest anyone finding him should kill him”—this sentence would be without meaning if there were not other people on the earth.
Besides getting a wife in Nod, Cain built a city there. We read that “Cain went out from the presence of the Lord, and dwelt in the land of Nod…and he built a city, and called the name of the city, after the name of his son Enoch” (Gen. 4:16-17). How could he build a city if his family were the only people on earth? There is no indication God is interested in the entire human race There were many on earth in the time of Adam.
We are not prepared to discuss the rate of decay of radioisotopes over time and its effect on dating methods used by scientists. However, even without going into detailed discussion, remains unearthed by geologists indicate that there was animal and human population on the earth long before six thousand years ago.
Nevertheless, there are MANY things we are not told that we would love to know! But just as Moses said to ancient Israel, “the secret things belong unto the Lord our God,” so it is true today. Yet we can be sure He has revealed all that we need that we may “live soberly, righteously and godly in this present world” (Tit. 2:11-12).