Living systems exist in the solid regime near the edge of chaos, and natural selection achieves and sustains such a poised state. - Stuart Kauffman

As I thought about this, I noted that the bacterium is just a physical system; it's just a bunch of molecules that hang together and do things to one another. So, I wondered, what characteristics are necessary for a physical system to be an autonomous agent? After thinking about this for a number of months I came up with a tentative definition. My definition is that an autonomous agent is something that can both reproduce itself and do at least one thermodynamic work cycle. It turns out that this is true of all free-living cells, excepting weird special cases. They all do work cycles, just like the bacterium spinning its flagellum as it swims up the glucose gradient. The cells in your body are busy doing work cycles all the time.

Life does not depend on the magic of Watson-Crick base pairing or any other specific template-replicating machinery. Life lies ... in the property of catalytic closure among a collection of molecular species

The onset of evolutionism brought with it the concept of branching phylogenies. The branching image, so clear and succinct, has come to underlie all our thinking about organisms and evolution.