Ah, if only the Bible was so!It's pretty plain in the black and white...
It's things like this on IO that come up, things I'd never heard of, that send me looking to the texts and the commentaries.
In short, the 'eighth day creation' has found favour in certain quarters, and sometimes with significant racist overtones! The idea being that God created man on the sixth day, but the white man on the eighth. This then solves the problem of who Adam's sons took as wives, etc. and stems from a too-literal reading of the text.
I can't find any credible scholarship that follows the eighth day idea, although the notion first appears as far back as Philo of Alexandria (20BC-50AD). Then again, Philo is never given credence in Torah commentaries, although he is referenced by some of the early Christian sources.
The scholarly consensus follows the Documentary Theory of difference, that of a number of oral traditions fed into a document that was compiled at a later date. The 4-source JEPD theory (Jahwist, Elohist, Deuteronomist and Priestly) held sway until the close of the last century, but more recent scholarship has favoured alternative explanations. Deuteronomist scribe(s) still hold as the significant compiler(s) of the text, but the E and P sources have become increasingly fragile.