The emergence of the two bioluminescent shark families (Etmopteridae and Dalatiidae) appear to be somewhere just before the Cretaceous/ Tertiary (KT) mass extinction event (~65 mya). Now, whilst they are both present in the fossil record, it is unclear if they had acquired the ability to glow. After the KT mass extinction, the deep-sea sharks radiated into deeper waters as larger predators went extinct, thus opening up new niches for the deep-sea sharks.

It is widely believed that bioluminescence first evolved in Dalatiid sharks (cookie cutters) and as they became part of the deep/open water environment, adopting counter-illumination would have been highly beneficial. The Etmopterids also adapted the counter-illuminating bioluminescent patterning but then developed it into a more complex signalling tool as they colonised deep-water niches. Wether Dalatiidae and Etmopteridae acquired their luminescence independently remains, however, in debate.

The fact that so many deep-sea organisms adopt bioluminescence is an indication that it has the ability to evolve independently in many different animals. I am unsure on the genetic mechanisms that are in place in order to allow such a phenotypic trait to evolve but its emergence has clearly played a fundamental role in the radiation and diversity seen within the deep-sea.

(p.s. you have used it’s (contraction of it is) when you meant its (the genitive form) e.g. near it’s “neck” )