If I want to get super-reductive, I think of life as one continuous self-sustaining chemical reaction. It's an accurate, though usually not very useful way of thinking - but I still cherish it, as it reminds me that life and evolution do not obey the conceptual boundaries we set for it. The concept of a "cell" or "individual" doesn't exist in the runtime of physical reality - if we see evolution as optimization process, it's one that can and does optimize across those boundaries.