Question

If something has been dead for a long time, how can you tell if it was ever alive to begin with?

This is an excellent question. The answer is that it is difficult to tell if an organic substance was ever alive. However, there are some tests that can be done. One good way is to look at amino acids in the material. Amino acids come in two "flavors", right handed and left handed, depending upon the arrangement of atoms. You would expect abiotically formed amino acids to be an equal mix of D and L chirals (left and right symmetries). Living things, for reasons we don't really know, only produce L amino acids (they are homochiral). You could also look at the carbon isotope signature of the organic matter. You could say that living things discriminate in favor of light isotopes. So a signature lighter than the average carbon in the universe might be biogenic. Lastly, although simple organic molecules can be abiotically produced in the universe, more complex polymers or organic compounds, like proteins and complex carbohydrates and fats, are the result, usually, of biological (living) processes. However all of these observations are interpretations and are not proof positive. Scientists are now looking at other isotopes, such as isotopes of Fe and Mo, to determine if other isotopic signatures might be useful in interpreting the presence or absence of life. Scientists insist that several lines of evidence be consistent in indicating that a fossil or chemical observation is indicative of life. Even with several lines of evidence, however, it is hard to prove that something now dead was once alive many billions of years ago. Answered by Prof. Susan Brantley
May 1, 2002