Question: Someone (i Think zoe) mentioned that mitochondria where their own bacterial cell before they were taken in by another cell. How do we know this? How long ago did that happen and which kind of cell was the first to do that?

Zoe Vance answered on 29 Apr 2024: last edited 29 Apr 2024 3:18 pm

You guys are really putting me through my paces here, I’ve had to go do some reading to be sure of the timing! We can tell mitochondria used to be their own cell because they have their own genomes – their own little loop of DNA (mtDNA) that has instructions within it for mitochondrial proteins – and they also do some metabolic processes separately to the rest of the cell. mtDNA is used in genetics studies sometimes because it’s quite stable in terms of how much it mutates, and it’s much smaller to work with than the entire genome. It also can only be inherited from your mother which makes it doubly interesting – the sperm doesn’t contribute any mitochondria during fertilisation.

Estimates vary but this happened at least 1.45 billion years ago – the mitochondria getting taken in is a key event in eukaryotes (more complex cells than bacteria) first evolving so we know once we start seeing fossils of those in the rock record that it must have taken place before the age of that rock. Also some debate over what exactly the mitochondria ancestor and the cell engulfing it looked like exactly, but people seem to be agreed that it has only happened once to give rise to the mitochondria in all eukaryotes including us.

If you want to look into this more it’s called ‘endosymbiosis’