[thedoc (OP)]

Oscar asked the Naked Scientists:
   What was the first animal on earth that grew a penis?
What do you think?

[the5thforce]

ultimately theres male and female components in every cell and organism- fusion is a universally 'fertile' chemical process, i would imagine some single celled organism developed increasingly complex male and female-like organic reactions inside itself and perhaps eventually its offspring started randomly inheriting dominant aspects of one or the other 'structure' until they required exchanging chemicals in order to reproduce, the evolution couldve happened many ways- maybe one cell divided itself incorrectly into two distinct but conjoined cells which may have eventually separated completely

as for your original question, a penis is only a tool to facilitate the injection of organic material, a fairly simple rudimentary phallic object likely wouldve re-evolved multiple times given the chance, also consider that in most species the female possess a less developed analogous 'phallic' object called the clitoris and a pair of ovaries which is analogous to the testicles, nature rarely wastes DNA meaning the genders are never entirely unique from eachother

[Colin2B]

I have a vague memory that they called the fossil Dickus somethinglatinus

[evan_au]

Microbrachius dicki, is the earliest example found so far, estimated at 385 million years ago.

See: http://www.abc.net.au/radionational/programs/scienceshow/small-bony-fish-earliest-example-of-copulation/5837178

[Colin2B]

Microbrachius dicki, is the earliest example found so far, estimated at 385 million years ago.

Ah, that's the one. Bet he's not happy about the micro bit!