I am adding an answer to this question because most of the other ones do not offer sources to support their claims. And, seeing how this claim can easily be seen as derogatory by anyone, I invite everyone to re-edit their answers to offer the views of scholars rather than personal views, thank you.
Sons' and daughters' of men, women, huris, and jins:
According to Shi'a Islam, there was no incest.
There are multiple narrations that provide an explanation where incest, God forbid, did not give rise to mankind. After we examine this explanation and its validity it easy to see that does not result in contradiction.
These narrations vary in the names of the sons, but, they have one thing in common: the sons' of our Father and Prophet Adam (a) where ordered by the Almighty (j) to marry huris and jins. When they married huris and jins it provided a generation of cousins. These cousins were also ordered to marry each other. And, now mankind was able to reproduce amongst themselves, without any illegitame relationship. So to reiterate, the first generation consists of the son's of Prophet Adam (a) marrying huris and jins.
In another reliable tradition, it is said that Adam had four sons.
Allah sent four houries for them. When children were born from them,
Allah recalled the houries. Then the same sons were married to jinn
women and in this way generations grew. Therefore, forbearance is from
Adam and every beauty and perfection is due to four houries. Whereas
ugliness, bad behavior and evil is from jinn.
According to reliable chain of narrators Sulayman ibn Khalid said to Ja‘far as-Sadiq, “May I
be sacrificed for you, people says that Adam married his sons to his
daughters?” Imam said, “Yes people do say that, but O Sulayman, you
might not know that Holy Prophet said, ‘If Adam had married his
daughters to his sons then certainly would have married Zaynab to
Qasim without having left the religion of Adam.’” Sulayman said, “May
I be sacrificed on you, they say that Qabil killed Habil because Habil
was ashamed that his sister was being given to Habil. Imam said, “O
Sulayman! You also attribute such filthy things to Adam and do not
feel ashamed?” He said, “May I be sacrificed on you. What was the
reason that Qabil killed Habil?” Imam replied, “Because Adam had
appointed Habil as his successor. Allah revealed to Adam to hand over
the successorship and the Grand Name to Habil while Qabil was elder.
When he knew this, he was angry and said that he was the rightful heir
to successorship and bounties. Under divine instructions, Adam asked
both of them to present a sacrifice to Allah. Allah accepted Habil’s
sacrifice and rejected Qabil’s. So he envied Habil and killed him.”
Hayat-al-Qulub Vol 1. Stories of the Prophets
Origins of the "incest hypothesis":
This idea comes from non-Muslim thinking and has found its way in some of the Muslim books of hadith and stories as here is a video talking about origin of this hypothesis (I have provided primary sources here). This "incest hypothesis" is accepted as the "truth" amongst some of the followers of the Abrahamic religions. Here is one example from one of the sects of Christianity, in answering this question Creation Moment's article Did Adam and Eve's Children Practice Incest?" says:
Nevertheless, Adam and Eve's immediate offspring would have been very
close to physical perfection while brother-sister marriages were the
only unions possible!
Also here is an example from one of the sects of Judaism, Chabad's article Whom did Cain and Abel marry? says:
Indeed, Cain and Abel had to marry their sisters, considering that
there were no other women around. King David writes, "The world was
built with kindness." Our sages explain that this verse is referring
to G‑d's kindness in allowing Adam and Eve's children to marry their
own sisters in order to populate the species.
Jack, as for the question "if there were other humans on earth?", I will reply as soon as I find sources to support any claim. But, I think it is important enough to be a standalone question. I hope this answers your question about the "incest hypothesis" as it is important to address this issue in the correct matter, after all it is the story of our origin! If there is any confusion please let me know.
