They were first used to denote the effective sex of plants (i.e. sex of individual in a given crossbreed, since most plants are hermaphroditic) by Carolus Linnaeus in 1751.
The shape of the Mars symbol has been likened to an iron-tipped spear (i.e. a weapon mainly used by men) and shape of the Venus symbol to a bronze mirror or a distaff (stereotypically associated with women in former centuries).
Numerous variations of gender symbols have been developed in the context of LGBT (lesbian, gay, bisexual and transgender) culture since the 1990s. Some of these symbols have been adopted into Unicode beginning with version 4.1 (2005).
From the symbol of Mercury (U+263F ☿). This symbol is used to indicate a virgin female (for example, in genetic analysis). Also used in botany to indicate flower with both male and female reproductive organs.