"How come there are so many different cultures, ethnicities, colors, of man/woman on earth?"
--Simple population genetics process of speciation and pronunciation of characteristical traits. If this is unacceptable, you may want to give punctuated equillibria another pass.
"When did God introduce these people?"
--Technically, God did not introduce these people, isolated populations by migration to various countries.
"Which ones came first?"
--Who knows.
"What ethnicity would Adam and Eve most resemble?"
--Who knows.
"How did they all originate from just two people in such a short time (4-6K years)?"
--Reproduction?
"Please don't respond that it's all irrelevant, because its not! In order for the bible to be a historical document, showing origins of mankind, then I would think this information is very important."
--Yes your questions are relevent, though they should also be a bit realistic (ie, pertaining to your questions of color and the like), unless ofcourse you would like to show me the color of an extinct reptilian skin such as a dinosaur.
"Were Adam and Eve more intelligent than we are today?"
--Who knows in all fact, unless you have a brain sample. Though my best guess is that he was very intelligent.
"If so, how come they ate from the Tree of Knowledge after being specifically instructed not to?"
--Temptation, an act of deceit, not knowledge, and with how old they may have been, they didn't have very much wisdom pertaining.
"If you argue that they didn't know it was the tree of knowledge, then why would this be considered a sin if they didn't know?"
--If this were true, then yes, it would not have been a sin.
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