Science

Complexity can increase or decrease as a species evolves.

The concept of "devolution," which isn't a real thing, can cause us to miss some of the interesting changes that are actually occurring as a species evolves, especially in species that become less complex and more similar to distant ancestors in their gross structural appearance.

One hears claims of species that have remained "unchanged for millions of years." Structurally a modern cockroach might look very similar to a fossil cockroach, but the behavior and metabolism of a modern cockroach could be more complex or less complex than its ancient ancestor. Without a time machine it's difficult to say. The cockroaches would be certainly be different. .