The surface area/volume ratio is an important factor for one celled organisms. as a cell's volume grows, its surface area/volume ratio making it more difficult to acquire enough nutrients via diffusion.

What did not occur in the proterozoic eon a)the oxygen levels increase greatly in the atmosphere, b)the emergence of the eukarya , c)muti cella’s eukaryotic organisms began to diversify, d)anaerobic bacteria kept thriving with more oxygen