How did sexuates come to exist ?

Question

Early creatures used to reproduce asexually. How did they evolve into creatures ? Did one of them become a male and another a female and it just so happened that each of their bodies fit with the other's ?

Answer 1

Sexual reproduction evolved long before male and female evolved.  You don't have to have "male" and "female" for there to be s*x.

Sexual reproduction only means that the DNA from two different (usually) individuals combines to make a new, genetically distinct, individual.  There are organisms around now that reproduce sexually but don't have male and female, including algae and fungi.  In these organisms, haploid cells from two different individuals fuse to form a zygote.  

The reason they aren't male and female is because all individuals produce gametes of the same size.  (This is called isogamy.)  The definition of a male is the individual that produces small, motile gametes.  The definition of a female is the individual that produces large, non-motile gametes.

Once sexual reproduction evolved, there were selective pressures to make sure your gametes were successful.  Some individuals started to become more successful by making smaller gametes, which meant they could produce more, and making them very motile so they could get to other gametes quickly.  Other individuals became more successful by going the opposite route and producing fewer gametes but making them really big so they'd be easy to find.  These eggs also produced chemical attractants to tell the sperm where they were.

So, now we have sexual reproduction and we have two sexes, but still organisms were not copulating, but spewing their gametes out into the water where they would mix.  Copulatory organs evolved much later under a similar scenario as the evolution of the two different gametes.  Selection favored those males who were most successful at delivering their sperm and those females who were able to retain it.