According to mitochondrial gene analysis, man didn't interbreed with Homo Sapiens... - Nick Lane

[A]ll multicellular plants and animals... contain mitochondria.

I would say that if there's a probability of life being cellular, which I think there is. Life being based, which I think there is. Life starting out with CO₂ because it's so common in planetary atmospheres, and , which is very common, from the kind of s which I'm talking about... and liquid water. They need liquid water for , but we know of it on ... on Europa... [Serpentinization] is giving rise to alkaline fluids with hydrogen gas. Most hydrogen gas you find in planetary atmosphere are coming from serpentinization. , which is the mineral required for that... is ubiquitous in interstellar dust... So all of this pushes you down a certain avenue, and if that's correct it gives you bacteria... and if that's correct then bacteria have a structural problem, and they're not going to get beyond bacteria except with an endosymbiosis, and that in itself is improbable, unlikely... because it only happened once, to our knowledge, on earth.

That's why they [viruses] are not in the tree of life. They don't relate in a very direct way. ...[T]he tree of life now is not only about ribosomes. You can build trees from whole genomes, but viral genomes? They don't really fit in, in a way which makes sense to people.