About the evolution of complex multicellular life - endosymbiotic theory and related questions of how eukaryotic cells and their complexity arose.
Looking at life, the visible diversity we see actually is a small part of the massive diversity of bacteria and archaea. All Eukaryotic cells look really similar, it’s the bacteria and archaea that look very different, even though we wouldn’t think it.
There’s perhaps an analogy to the layers of abstraction in engineering / computing. Looking at an application on a computer nowadays, you couldn’t really tell the difference between a video game and a tax software at the lowest level (whether it’s at the code, or the chips that run it). In some ways, the mechanisms that existed before computers (clocks, watches, turbines, etc) have a lot more diversity in physical structure than computers / software do. It depends on what level you’re measuring diversity at - diversity builds at different levels.
Endosymbiosis happens twice - with an Archaeal host cell and a bacteria that gets eaten. Once for all eukaryotic life (mitochondria), and a second time for plants (chloroplast).
Energy limitations - proton gradients are what power cells - by pumping protons across a membrane voltages are created.