99% of vertebrates are gonochoric, and the remaining 1% that are hermaphroditic are almost all fishes. Plants The majority of plants are bisexual, either hermaphrodite (with both stamens and pistil in the same flower) or monoecious. In dioecious species male and female sexes are on separate plants. About 5% of flowering plants are dioecious, resulting from as many as 5000 independent origins. Dioecy is common in gymnosperms, in which about 65% of species are dioecious, but most conifers are monoecious. 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. However, the evolution of anisogamy has left no fossil evidence and until 2006 there was no genetic evidence for the evolutionary link between sexes and mating types. It is unclear whether anisogamy first led to the evolution of hermaphroditism or the evolution of gonochorism. But 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. 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. Sex-determination systems The biological cause of an organism developing into one sex or the other is called sex determination. The cause may be genetic, environmental, haplodiploidy, or multiple factors. Within animals and other organisms that have genetic sex-determination systems, the determining factor may be the presence of a sex chromosome. In plants that are sexually dimorphic, such as Ginkgo biloba, the liverwort Marchantia polymorpha or the dioecious species in the flowering plant genus Silene, sex may also be determined by sex chromosomes. Non-genetic systems may use environmental cues, such as the temperature during early development in crocodiles, to determine the sex of the offspring. Sex determination is often distinct from sex differentiation. Sex determination is the designation for the development stage towards either male or female while sex differentiation is the pathway towards the development of the phenotype. Genetic XY sex determination Humans and most other mammals have an XY sex-determination