a single origin
... that venom is thought to have had a single origin in the evolutionary history of snakes?
... that <a href="./Venom" id="mwB1I" rel="mw:WikiLink" title="Venom">venom</a> is thought to have had <b id="mwB1M"><a href="./Evolution_of_snake_venom" id="mwB1Q" rel="mw:WikiLink" title="Evolution of snake venom">a single origin</a></b> in the <a href="./Evolution" id="mwB1U" rel="mw:WikiLink" title="Evolution">evolutionary</a> history of snakes?