This reminds Foer of a scientist's point of view that he saw not long ago: if humans find aliens on a planet, there must be more than one creature on this planet.

What do you mean? In other words, if there is life on other planets, it must be diverse. There can't be only one creature on this planet.

The reason is natural selection. Biodiversity ensures the continuation of life on earth. If the biological species is single, once the environment of this planet changes greatly, it will easily lead to the extinction of life. Diversity improves the overall adaptability of life.