atom replacament+bacterial biosphere

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Body Rejuvenation 101
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In the early '50s, researchers discovered the body's rejuvenating power by – yes, really – feeding and injecting subjects with radioactive atoms and observing their movement. They found that, on average, 98 percent of the Wade].]atoms inside the body – the smallest units of matter, which form the molecules that help comprise bodily cells – are replaced each year. Most new atoms are taken in through the air we breathe, the food we eat and the liquids we drink [source: NPR].

More than five decades later, Swedish molecular biologist Dr. Jonas Frisen studied body tissue renewal by measuring levels of a radioactive material called carbon-14. This material was released in the air before testing nuclear weapons aboveground was banned in 1963. Carbon-14 is breathed in by plants, which humans and animals eat every day, and is part of our DNA. But unlike other atoms and molecules that are constantly changing, a person's DNA remains the same from the day of a cell's birth – which occurs when a parent cell divides – throughout its life span. When a cell divides, in other words, the DNA incorporated in the new cell includes a certain level of carbon-14 that corresponds to the level of the material in the air around us at the time. This serves as a time stamp of sorts, by which researchers can determine when the cell was created based on the level of carbon-14 in its DNA [sources: Wade, Science Update].