5 November 2014 Last updated at 18:02
By Melissa HogenboomScience reporter, BBC News
Lizard males have paired genitals called hemipenes - now research shows their genetic origin comes from limbs
In order for vertebrates to evolve from the sea to the land, some drastic evolutionary changes were needed.
Their ancient sea-dwelling ancestors had no need for external sex organs whereas their land relatives did.
Now a new study offers insights into the genetic changes that allowed land-dwelling animals to develop sex organs.
The Nature research suggests the key to the origin of genitalia lies in the limbs, at least in snakes and lizards.
For their genitals - called hemipenes - to develop, a signalling centre instructs the relevant genes to switch on.
Initially the researchers wanted to understand why snakes do not develop limbs but then soon discovered that the earliest stages of genital development closely resembled limb formation.
They found that when a given nudge, embryonic limb cells of lizards and snakes could then be turned into genitals. In mice, tail bud cells could be manipulated in a similar way.
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