For example, it is written that God created the "Day" and the "Night" on the first "day". But he did not create the "greater and lesser" lights (the Sun and the Moon) to give light upon the earth until the fourth "day", the same day he created the stars. Therefore, the first three "days" had no 24 hour clock as we know it (the Sun). Those "days" could have been any length of time - even millions or billions of years.
Therefore, it is easy to assume that a "day" with the Lord is not the same as a "day" with man. It is just as easy to assume that when, on the fifth "day", God said, "Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven . . . etc., that God could have taken millions of years for that event, also.
That said, assume that on the sixth "day", when he said, "Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind" . . . etc. , that event could have also taken millions of years.
Later on the sixth "day", God created Adam, the Son of God, in his likeness, along with his wife, Eve. Adam is referred to as the "Son of God" in the genealogy of Jesus in Luke, which ends with this passage: ". . . Which was the son of Enos, which was the son of Seth, which was the son of Adam, which was the son of God." -- Luke 3:38
Now, that event (the creation of Adam) could have been instantaneous; and we all know that Adam had a son named Cain. Cain married a woman from the east of Eden (the land of Nod), and I assume she was not his sister. So it is possible there were other "men" and "women" on the earth at that time.
My conclusion? The other "men" (those from whom Cain's wife was born) evolved, either from the "waters" as many scientists believe (with the evolution beginning on the fifth day), or from the earth (on the sixth day). But God created Adam, the Son of God, directly, and in his own likeness. And from Adam came both Israel, and Jesus, our Lord and Savior. That is, there are two distinct sources of "mankind": the descendents of Adam and Eve who were created directly by God, and the descendents of man who evolved. This is easier to believe if you read this verse: "And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose." -- Genesis 6:1-2
And then there is this: "O Timothy, keep that which is committed to thy trust, avoiding profane and vain babblings, and oppositions of science falsely so called: Which some professing have erred concerning the faith. Grace be with thee. Amen --1 Timothy 6:20-21
Finally, I bolded the above words "open firmament of heaven" for another purpose. I believe Heaven to be the earth's lower atmosphere -- the air we breath -- the air where the "fowls" fly (see Genesis 1:20) -- that which separates the waters above the heaven (the clouds) from the waters below the heaven (the lakes, rivers, and seas) (see Genesis 1:7).
And, of course, with God, all things are possible.
With the lifespans and prolific childbearing of that time, it could have been a niece. I wouldn't bet on her being an ape.