If you can overcome the horrors in this book (aliens is nothing), this is very very interesting. It’s likely that most species are parasites, parasites can control speciation, sex, move effortlessly from dinosaurs to mammals when the former die out, etc etc.
2022-12-06: A more nuanced view

“Among all known animals, there are more species that live as parasites than are free-living.” Parasitic life styles exist in all major animal groups, with the single exception of echinoderms, the phylum containing starfish and sea urchins. Parasitism is rife, too, among plants and, as you’d expect, fungi. Many organisms are what are termed “obligate parasites,” unable to complete their life cycle in the absence of a host—and obligate parasites include viruses, which, some scientists contend, aren’t even alive until they hijack a host’s cells.

Among the parasitic infections that affect humans, a large portion are caused by various species of nematodes, a phylum of worms. Nematodes account for 80% of all animal species and are so plentiful that, the authors write, one could “line them up end to end and have nematodes in every meter across our entire galaxy.” Envisioning a “parasite Olympics,” the authors award their gold medal to the nematode Ascaris lumbricoides, which has succeeded in establishing residence in the intestines of 15% of the human population, more than 1b people. The worm causes a tropical disease, ascariasis, whose symptoms include fever, abdominal pain, cough, vomiting, and weight loss. The success of the species comes in part from the fact that, unusually for a parasite, it doesn’t require an intermediate host—the way that malaria, say, needs a mosquito in order to infect a human. Instead, Ascaris is transmitted from person to person via contaminated feces.
Humans appear to have incorporated products of beneficial parasitic infection. The endosymbiosis theory holds that we may have co-opted parasitic bacteria that are now essential to life. Early in the evolution of single-cell organisms, bacterial parasites that were not destructive entered cells. These parasites ultimately became mitochondria, the organelles within the cell which produce energy—a structure essential to animal life.

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