-------------------------------------------------------
answers:
St Lusakan say: We look for the space craft before we agree to anything?
-
Elizabeth say: There are a few ways.
One such way is to look at chirality of amino acids. For some reason we don't yet fully understand, all of the biology on earth has a preference for 'left-handed' amino acids. These are molecules that could be assembled in two mirror forms like your right hand and left hand. But when we look at earth's biology we find only the left-handed molecules in use. Now there might be some underlying reason for this preference, but from a chemical standpoint the molecules behave identically. If we found lifeforms using the reverse chirality it might be an indication that that life was alien to this world.
Most organisms are comprised of cells that have mitochondria - these are the chemical power plants of the cell. Mitochondria have their own DNA that is separate from the DNA inside the nucleus. We can look at this mitochondrial DNA and compare the differences and mutations and errors from one species to the next. For example, we know that every human being on the planet shares mitochondrial DNA and, based on the slight changes around the globe, how many people have those differences, and how many generations it takes to accumulate that difference in the population, we know that every human being alive today is related to a single woman who lived about 200,000 years ago (we call her Mitochondrial Eve). If an organism had some radically different energy producing structure, or had radically different mitochondrial DNA to organisms today, we might be able to identify it as a alien life form.
The problem that we have, though, is that it unless the chemical processes taking place in that organism are entirely different (for example, the life form is based on silicon rather than carbon, has an equivalent to DNA that uses different base pairs or more of them) then we're always left with three possibilities: