Why is damage caused to an ecosystem by adding fertilizer

Why is it important to reduce the number of chromosomes in half when forming gametes for sexual reproduction?

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

What does the positioning of transcription factors determine?