Answer:The answer is tiny organisms known as cyanobacteria, or blue-green algae.
Explanation:
These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and oxygen increase. Additionally, some sources you could use that I used to answer this question was
https://www.scientificamerican.com/article/origin-of-oxygen-in-atmosphere/
They increase in number and they are more specialized
1. peripheral resistance
2. vessel elasticity
3. blood volume
4. cardiac output
I think its called an Axon.
