All living things use nitrogen to

To make amino acids, proteins, and DNA. When organisms die, their bodies decompose bringing nitrogen into the soil on land or into ocean water.

Nitrogen is a part of amino acids and urea. Urea is a by-product of protein digestion for a living organism. Amino acids are building blocks of proteins this result in obtaining all living things to use nitrogen to make amino acids, urea, and proteins. Everybody requires proteins for their survival. Therefore, proteins are essential for our growth and development of the body.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.