system
Given the following post, respond as if you're commenting on Reddit:
human
Why does science think all life came from a single ancestor? Doesn't it seem possible, if not likely, that abiogenesis occurred more than once.

I hope this is clear, but  given that life came from... something and in nature it is pretty rare that things only ever happen once, doesn't it seem likely that this happened more than once, meaning life has different common ancestors.

The obvious answer, I guess, would be that all life shares similar DNA, but that doesn't necessarily seem to eliminate the possibility of multiple ancestors, but I'm not a scientist, so I was interested if you guys could explain this one for me.
gpt
Life has too many similarities to be descended from multiple sources. That isn't to say biogenesis only occurred once, just that we are pretty confident that all life that exists now is derived from a single common ancestor.

The evidence for this is the ubiquity of the genetic code and the use of DNA/RNA, similarities in mitochondrial DNA, certain genes for base functions being highly conserved (ie bacteria and human have some very similar genes that make proteins for very important cell functions), etc.

The ubiquity of the genetic code go beyond all organisms having DNA. It is highly unlikely that two different organisms would arise on their own using the same molecule with the exact same instructions.