“...Or single sex organisms.”
there are some creatures and plants that reproduce through asexual reproduction called parthenogenesis. for instance, the komodo dragon is one such creature who can reproduce either thru sexual or asexual reproduction.
“Parthenogenesis is a natural form of asexual reproduction in which growth and development of embryos occur without fertilization. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell. In plants parthenogenesis is a component process of apomixis.”
[source: https://en.wikipedia.org/wiki/Parthenogenesis]
Could conceivably be life forms that “spontaneously” appear, as life here, according to some, once did. Then, did it spontaneously appear only once in one location, or did it repeatedly appear in many locations over a period of time.