>But society and the terrestrial ecosystem don't exist without functioning brains in them.

1) Original reply didn't say anything about society, only life an it's physical interactions.

2) Sure it does. Plenty of life forms without brains exist, and their sum is more complex than the human brain. The human brain itself is a composite of brainless life.

>And a genealogical relationship doesn't exist at all as a physical object, just as a conceptualized object (such as a graph). And conceptualized objects, such as graphs, typically aren't that complex (though can probably get there if someone dedicates the computational resources to do so).

That isn't because of what conceptual objects are, it's because of how they are used. The purpose of most conceptual objects is simplification. One can produce arbitrarily complex conceptual objects, we just don't normally because... why?

We can even throw out all of these semantics if you want and go with a much narrower version of the original statement: "The brain is the most complex physical system", and even that isn't necessarily true. The bacterial ecosystems that compose the digestive system have more interacting cells, with less defined interactions, than the brain. Maybe you meant "most complex computational system"? That's potentially true, depending on your definition of complexity.