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[–] littul_kitton 1 points 3 points (+4|-1) ago 

The more I learn about the evolution of life on Earth, the less likely exogenesis seems. The key is the viruses.

If exogenesis is true, then only a small fragment of another world arrived on Earth as the seed of our creation. Assuming it was ripped from the parent world by some cataclysm, it could only have carried a tiny slice of the ancestral ecosystem. The most ancient fossils show this, the handful of organisms known as the Ediacaran biota.

And yet we find many families of viruses that are radically different from each other. Plant viroids that are tiny loops of DNA. Single-stranded RNA viruses. Double stranded RNA viruses. Single- and double-stranded DNA viruses. Circular viruses. Chromosomal viruses. Stripped down tiny viruses. Viruses more sophisticated than many bacteria. Retroviruses. Viruses that infect other viruses.

It beggars belief that a chunk of an alien world transported to Earth could have carried all of this. The evidence seems to suggest that the rollicking, baroque ecosystem that produced these viruses was entirely present on our Earth.

The second line of evidence is the viral defenses. If you have heard of CRISPR-Cas9 genetic engineering, you need to know that it is from bacteria and they use it to defend against viruses. That's right, the simplest cells on Earth can adapt to and destroy viruses. The plants also have an adaptive viral defense - they spread snippets of the virus and use RNA silencing. Every organism studied in depth seems to have a strong defense against viruses. I doubt this could have happened if a few alien cells had been dropped on Earth by accident. It would only have happened if Earth started out with a huge viral ecosystem, which evolved from the most ancient genetic material.