What organism(s) and processes led to o2 accumulation in our atmosphere?

Plants are the organisms that produce oxygen, or O2. To do this, they go through a process called photosynthesis. They take carbon dioxide, water, and sunlight, and convert it into oxygen and glucose (sugar). Without plants, you, me, your pet, and all other living organisms wouldn't have oxygen :) hope this helped!

Ifound this on google “according to the findings, a foramen magnum positioned toward the base of the skull is found not only in humans, but in other habitually bipedal mammals as well.” i hope this you for what your looking for