section_path: Evolution of sex
section_name: Evolution of sex
paragraph_text: Evolution of sex
  
  It is generally accepted that isogamy was ancestral to anisogamy and that anisogamy evolved several times independently in different groups of eukaryotes, including protists, algae, plants, and animals. The evolution of anisogamy is synonymous with the origin of male and the origin of female. It is also the first step towards sexual dimorphism and influenced the evolution of various sex differences.
  
  It is unclear whether anisogamy first led to the evolution of hermaphroditism or the evolution of gonochorism, and the evolution of sperm and eggs has left no fossil evidence.
  
  A 1.2 billion year old fossil from Bangiomorpha pubescens has provided the oldest fossil record for the differentiation of male and female reproductive types and shown that sexes evolved early in eukaryotes. Studies on green algae have provided genetic evidence for the evolutionary link between sexes and mating types.
  
  The original form of sex was external fertilization. Internal fertilization, or sex as we know it, evolved later and became dominant for vertebrates after their emergence on land.