Humans, alongside all other life on this planet, have carbon as the basic element found in every thing in their body. One may say that carbon is omnipresent within living beings. With us, we breathe oxygen as we need it for basic PROCESSES of life, whereas carbon provides the actual structures of the things which take in that oxygen. Without carbon as the base element of life, Earth's creatures and general environment would be very different. I honestly hope that this answer makes sense to you.
-M