An interesting fact that I learned from the book Metazoa is that biological diversity was greater in the seas than on land, until about 130 million years ago, and then the explosion of diversity on land, at that time and still now, was because of a feedback loop that developed between plants and insects, with the insects allowing greater specialization among the plants, and the greater diversity among plants allowing greater diversity among insects. This was a true feedback loop, in that it would be impossible to say what the starting point was, the insects or the plants.

https://www.amazon.com/Metazoa-Animal-Minds-Birth-Consciousn...