No species alone
Large complex multicellular life and simple single celled creatures have been evolving together. Each species requires an environment similar to the one in which it evolved in order to grow and develop normally. That includes the other creatures that have been evolving along with it. Today, we talk about how mice, deprived of their usual microbial symbiotes, show changes in their behavior as adults—changes that cannot be reversed.
Here’s the paper on the experiment and the results.