The engulfment of a bacterium by a eukaryotic organism approximately 400 million years ago led to improved energy production for the eukaryote, enabling more efficient use of oxygen.

“one of these amazing things that we think happened was it at some point in this evolution 400 million years ago one of these eukaryotic organisms gulped up a bacteria and what did that become what happened yeah so that's what's so cool so that by gulping up that bacteria and that bacteria feeling really good about what was going to be instead of killing that you carry out actually made it better it was able now to give it more energy so before that we couldn't use oxygen the way we use it today we didn't make as much”