You, a non-synthesizing-chromista, consume a rhodophyte, and to everyone’s surprise, it doesn’t die, but you both live happily ever after. You provide the rhodophyte with chemicals, and it converts those, via sunlight, to sugar for you. Marvellous. It’s a little engine living right inside you. Amazingly, it also has little engines living inside it[1].
The chlorophyll gained from the rhodophyte helps a lot, but it could still be improved. For example, it doesn’t work in deep water, where the sunlight is reduced.
Another of those little copy errors could make a different kind of chlorophyll that works better in the deep ocean and gives you the opportunity to colonise areas where nothing lives at the moment. But that will mean having less of the tried-and-tested chlorophyll that works so well near the surface.
[1] This is the only time that this sort of engulfing event seems to have happened: a eukarypte has engulfed another eukaryote, rather than a simpler prokaryote. That means that the original eukaryote already had its own, smaller, engulfees, like a Russian Doll.
We can be sure that it happened this way, because the resulting organelle has four membranes, where all the others have two.
