All tissues of animal bodies, including our own, have quite a rapid turnover. Although we maintain our general appearance from year to year, every tissue in our body is constantly being broken down and replaced with newly made molecules derived from the food we eat. All the atoms and molecules in our bodies are replaced on the average every four years. Thus we and every other form of life are constantly participating in the cycles of all the atoms.
Imagine one individual carbon atom making up the protein of the skin on your hand. Where did it come from? From the food you ate a few weeks ago. And before that? From carbon dioxide in the atmosphere that was incorporated into a plant by photosynthesis. Where will it go? In a few weeks, it will in all likelihood be back in the atmosphere as CO2 as the top layer your skin is sloughed off and oxidized by microorganisms. This carbon atom will go on to new cycles. If this or any other atom in your body could tell us its life story, it might go something like this:
“I was created by a distant star before the creation of Earth. Here on planet Earth, in my countless cycles from the air through living things and back, I have participated in the bodies of virtually every species that has ever existed, including trees and animals of the forests, seaweeds, fishes and other creatures of the oceans, and birds and other creatures of the air, and the dinosaurs that roamed the land 100 million years ago. In more recent times, travels along food chains have led me through quite a few humans as well as plants and animals who share Earth …”
All life is interconnected through sharing and recycling a common pool of atoms. Generations come and go, species evolve, but atoms remain the same.
Adapted from Environmental Science, Nebel Wright, 4th edition, 1993