Zebra mussels are hermaphrodites...only takes one to tango. I think the same is true for asian clams (
Corbicula fluminea) That's why they are so invasive.
Lots of freshwater snails are hermaphrodites.
Aphids and water fleas (
Daphnia magna) spend most of their lives as asexual female critters by producing clones of themselves, until they come under environmental stress, at which point they crank out a male instead of another clone.
Certain sea slugs are both male and female, and when they meet up, they have this neat little battle to decide who gets to be the male and who has to be the female. The male being the one who jabs his business into the other one first.
These are just a few examples and variations where asexual and sexual reproduction are seen within single creatures that are alive today, so extrapolation to how it all started can begin with what we can observe today.
clpMINI