Flatworm ejaculation is triggered by dopamine… a lesson in ultraconservation.
In biology, “ultraconservation” does not mean wanting to go back to the Dark Ages. Instead, it’s a remarkable fact about remotely related species — say, the round worm C elegans and your aunt Molly — that show striking similarities. The biggest traditional example is the parallel evolution of lensed eyes among far-distant species.
Dopamine is a very simple molecule that shows up in many places in the biosphere — most famously in the human brain, where dopamine is the main signal molecule in our reward (pleasure) system. But dopamine has numerous functions, and it shows up in many different species as well. It is a molecular “key” that fits many locks in many different places.
The tiny flatworm C elegans has been used as a model species in thousands of basic biological experiments. Because so much is known about “him/it” the flatworm is a good test bed to study things like… mating, and specifically, ejaculation from the male to the hermaphrodite, the two sexes C elegans is blessed with.
(Or, as Woody Allen almost said, being a hermaphrodite doubles your chance of a date on a Saturday night.)
Now it turns out that dopamine, the human pleasure molecule (among other functions) is also part of flatworm ejaculation.
We can now learn a great deal about the human brain from C elegans, because we now have the blueprints for both species — their genomes. While there is still much to learn about our respective blueprints, in some cases we can see clear parallels between genes, their functions, and how they are expressed in proteins in our bodies. Gene expression also controls time-triggered events like sleep and waking, and environmentally triggered events like mating.
Because reproduction is one of the two big Darwinian evolutionary drivers, it would make some sense that life early on figured out how to pass “germ-line” cells (in humans, eggs and sperm) from a male to a receptive animal. Once evolution settles on a good solution to a universal problem, it tends to stay with it, until a change in environmental conditions favors a mutation.