All humans in known existence were once female. This doesn’t mean in a past life, though that may be very likely as well, instead – we were once female upon growing and evolving our physical form within the womb before birth.
This is possible due to the way DNA joins together, from both the mother and father, during conception. When two people mate, they combine half their genetic material with another, 23 chromosomes each. One pair of the chromosomes helps to determine the sex of the fetus. Depending on if the chromosomes contain two X’s – the child will be female. Yet if the chromosomes contain an X and a Y, they become male.