The most widely accepted scientific explanation for the arrival of all complex life on Earth has had an unsolved mystery at its heart. According to the theory, all plants, animals and fungi, known collectively as eukaryotes, are thought to have evolved after two very different types of microbes came together. The problem was in figuring out how the two were in such close proximity in the first place, given that one of the microbes requires oxygen for survival and the other was known to live in spaces without oxygen.