Complexity is a measure of how difficult it is to understand something. What is an example of complexity in the natural world?

Multicellular organism is the example of complexity of the natural world.

Explanation:

Multicellular organism such as human which is made of billions of cell. Each cell perform a specific function. The function of one organ is different from the other organ. For example, brain of human is made of millions of neurons which takes instructions from brain to the organs and bring messages from organs to the nervous system in the form of electrical impulses. In short every system in our body is full of complexity.