Oldest found so far are between 1.5 - 1.4 billion years ago. Reproduce independently of whole cell. However, totally dependent on some nuclear-coded proteins and cannot survive independently. The respiring procaryote eventually became mitochondria . In contrast, most eukaryotes have the same type of metabolism, because they rely on mitochondria.
Eukaryotes have the high energy efficiency needed to develop complexity.
Soft bodies, therefore not easily fossilized. Trophic level refers to the organisms position in the food chain.