This Bible contradiction is from the Skeptic's Annotated Bible.
Yes, everyone is descended from Adam and Eve.
Genesis 3:20 View context
And Adam called his wife's name Eve; because she was the mother of all living.
Some are not.
Hebrews 7:3 View context
Melchisidec just appeared, without father or mother. So he wasn't descended from Adam and Eve.
Melchisedec, king of Salem, priest of the most high God, was Without father, without mother, without descent, having neither beginning of days, nor end of life; but made like unto the Son of God; abideth a priest continually.
Genesis 6:2-4 View context
The Nephilim were the giant offspring of the sons of God (angels?) and the daughter of men. So they weren't desscended entirely from Adam and Eve.
The sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. ... There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Numbers 13:33 View context
And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight.
And at least some of the Nephilim survived the flood, according to the book of Numbers. So maybe there are still some giant offspring of God's sons around today!