Why are there so many kinds of living things? Theoretical ecology offers an answer.
There aren't any reliable Precambrian fossils of complex multi-celled organisms. About 600 million years ago, most of the major phyla of invertebrate animals appeared within a period of a few million years. Why did this occur? The relationship between predation and diversity can explain this Cambrian "explosion" of life.
Environments devoid of predators have a lot of population but only a few species. The "cropping principle" states that introduction of predators increases the diversity of the environment.
The Cambrian explosion might be due to the evolution of cropping herbivores - single-celled protists that ate the Precambrian algal community that flourished for two and a half billion years. The original cropping protist, the unsung hero of the history of life, might not have been fossilized.
A well-cropped system is maximally diverse.
Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould
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