Why is the human heart so complicated?

What did hunter gathers do to alter the environment?

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

When the oxygen produced by photosynthetic plants in the water dissolves in the water, which of these is being formed?