Comment by cwmma

ok so we knew about bacteria, tiny cells that don't have a nucleus, responsible for lots of disease. Bacteria do not look like the bigger cells that have a nucleus, they noticeable differences in how they work. Those big cells do have mitochondria in them which very much do look like bacteria that started living in those other cells.

Later on archaea were discovered they are the size of bacteria but the look like the bigger cells in how some of the key things in side of them work. Archaea don't cause any known diseases so that's one of the reasons it took so long to find them, but we can now find them all over the place by their DNA and it's now pretty clear that the ancestor of all big celled (and multi cellular life) was an archaea and they've even found the group of archaea that multi cellular life evolved out of.