For giving you understanding ; we need Free O2 but Oxygen is also available in H20 right why dont we use it !!
As the early atmosphere was reducing as there was no free oxygen but it was later produced by chemoautotrophic bacteria !! Living thing release O2 in atmosphere thus making it oxidising in nature and then life evolves !!
Nerves - transmit impulse of sensation to brain or spinal cord, then to muscles and organs
B. The prefrontal cortex is used to plan complex cognitive behavior, personality expresison, and moderate social behavior.