Nice video! I was aware of this theory but I did not know that we had a good idea of what the actual endosymbionts were: Rickettsia for the mitochondria and cyanobacteria for the chloroplasts.

There are also some cool examples these days of animals that will consume algae and incorporate their cells or their chloroplasts into their own cells, and then use them for photosynthesis. For example: Elysia chlorotica, salamander embryos, and hydras.

In the case of Elysia chlorotica there has been some debate in the literature about whether horizontal gene transfer has occurred, allowing the Elysia to pass on some of the photosynthetic genes directly to their offspring… More recent full-genome data suggests it hasn’t, but I think there are still some unsolved controversies being debated on this topic.

It is possible that these species at the moment could in the future give rise to truly photosynthetic animals - so we could very well be co-existing with some very cool evolutionary intermediates, which in the next few millions of years could give rise to photosynthetic animals, assuming we humans don’t succeed in driving them to extinction…