A single person
The Earth and everything in it.
All of these are CAS - complex adaptive systems.
"System" because the whole is made up of many smaller parts (for example, all the businesses in a market; but don't forget their suppliers; and their consumers; and their employees; and...)
"Adaptive" because the smaller parts change over time depending on their situation (which organisms evolve to prosper in a particular ecosystem over time...and which die out?)
"Complex" because...well...the behaviour of these systems isn't always easy to understand or predict. If you sat down around a campfire a bazillion years ago with all of the primates in the jungle and asked, "what would happen if I gave one of these guys an opposable thumb instead of a prehensile tail?" your answer probably wouldn't have been "Well, that one guy would become the most dominant species on the planet, and along the way develop self-awareness, technology, language, etc."
And yet that very thing happened because our ancestral primate existed in a system made up of many independant, evolving parts. The interactions between all the parts of that system range from the obvious (the guy with the thumb got better at killing animals for food once he figured out how to throw a spear) to the obscure (when a single-celled organism called Phaeocystis globosa is eaten by bigger creatures, it helps reduce the amount of carbon dioxide in the atmosphere; but when it is eaten by smaller creatures, it doesn't...)
The whole point is that the universe is full of complex interactions, and yet by their nature we have trouble connecting the dots. By studying CAS, we're trying to understand some general rules about how these complex interactions work, whether we're looking at an ecosystem, a market, or the blood cells in your body. Hopefully we can get to a point where every time we learn more about one particular system, we learn more about all the other, unrelated systems too.
We're trying to connect the dots.