Are there evolutionary reasons for reproduction?

Water is made of 90% oxygen so how could it be that there was no oxygen in early earth? The water at the deep vents does not have oxygen. How does this make sense?

This is the biggest question I wish someone could answer. Oxygen being so toxic for early life should have stop the ones producing it.

According to a recent genetic study the biomarker for the first cyanobacteria and the first eukaryote is 2.7 Ga. However, the first eukaryote did not have mitochondria until after the oxygen crisis occurred and this was 1.8 Ga.It is known today that cyanobacteria do not have a problem with living in their own waste production. It has not been determined if this was always the case since its first appearance.Edited by barbara, : No reason given.

Once plants got reasonably established is why oxygen was able to build up in the atmosphere?Plant cells resemble colonies of cyanobacteria and plants have mitochondria cells for respiration. This makes it more interesting since the first cyanobacteria and the first eukaryote without mitochondria originated at the same time.Are you saying that cyanobacteria could not by itself produce the oxygen that we depend on today? Ocean plants that are eukaryotes with mitochondria and chloroplasts is why we have an o2 environment?

New question: Did the retrovirus that form the placenta the same in all mammals that have a placenta?

How do you know it only happened once?

Since it only occurred once and you are sure of it then is there a tree diagram linking all species that have placenta formation of this identical retrovirus sequence?