As to Genesis : Where did Cain and Seth's WIVES come from? God didn't make them, like Adam and Eve, from the dust. So where did THEY come from? Simple answer : Adam and Eve were made/placed in a world already populated with people.
As to "eve", DNA evidence, the known drift rate in mutations thereof, points to a cro-magnon "eve" between 125,000 and 250,000 years ago, as the mother of all people living today.
Also, mitochrondial DNA points to a "bottleneck" of just a few hundred people surviving some great extinction between 70,000 and 80,000 years ago. 74,000 years ago the indonesian SUPERvolcano(15 mi x 15 mi caldera)blew up, creating a 3.5 year long nuclear winter. The human race was almost completely wiped out(like the BIG dinosaurs).
Gen 2:6 briefly describes the .5B year long age(4.4B to 3.9B)between the perturbation-out-of-orbit of the original terran ring system onto the earth's surface by the captured moon, and the first ocean. No, the incoming mars didn't directly hit the earth, and thus slow down(the I-S theory), it "hit" the Roche Lobe, and was torn into 2 parts(comma head and tail).
The head slowed down by less mass(g force)and greater orbital radius, was further rightward deflected, and thus went into a highly elliptical but CAPTURED orbit around terra. It then quickly destroyed the ring system , dumping most of the wet-gravel planetisimals onto the equator; the origin of our later continents/oceans. Tectonic forces then massaged that ring-debris layer for 500,000,000 years, ie, Gen 2:6.
The long tail of mini-moon/asteroids from the equatorial lunar bulge was thrown at high velocity entirely OUT of the terran gravity well, much like the 20 SL9 comet-train fragments in 1994 that first "hit" jupiter's Roche Lobe, then later impacted it directly.
Drop a spinning oil blob onto the eddy of a draining bathtub and you'll see the comma shape for a brief instant.
"So quick to judge, so slow to learn" was how an old black librarian described young blacks. It applies to far more people....esp, those who deny science and its ability to discern TRUTH, from biblical clues.
"Simple answer : Adam and Eve were made/placed in a world already populated with people."