Only 1 Tree of Life?

Somebody mentioned viruses. Viruses have their own trees of life, and for that reason some biologists exclude them from being "life." But I would count them as life.

I think you are right, MrJack. At least that is the reason they give, and I don't have much reason to doubt it. There was a discussion about this in my high school biology class. But I still suspect the underlying reason is that viruses don't fit into the patterns that all of the rest of life fits into. And the root cause of the mismatch is that viruses don't belong to the standard phylogenetic tree. The definitions of "life" are made by finding characters that encompass every fringe clade of the standard tree of life, which means that the definitions are likely to exclude viruses and perhaps outerspace alien beings that otherwise seem like life.