One requirement for natural selection would be the variation or differences of heritable traits being observed in a select group of organisms, allowing for better adaptability and ultimately survival of the said group or category of organisms.
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They would know if they see anything like traces of water or anything else that would be nessecary for a living organism to have. They look for these with technology like the hubble telescope as well as the drone on Mars to gather as much information as possible.
Answer:
They don't have DNA common. In prokaryotic cells there is a single piece of DNA whereas in eukaryotic there are multiple chromosomes.
