For this reason the Yilanè society has remained a monoculture for millions of years, with no divisions. The Yilanè normally are inseparable from their society if a city leader casts out a member of the Yilanè society, in a ritual which involves taking away her name, then the cast-out member will immediately die due to a reaction in her brain to the fear of being cast out.

In order to deceive others they may only restrict their movements or go into a state of immobility until the emotion or thought has passed.

As their emotions are directly and immediately translated into the movement of their bodies, Yilanè cannot lie. The Yilanè language is incredibly complex, based on sounds, color (the Yilanè are able to alter the skin color on parts of their body, notably the hands, akin to chameleons) and body movements, and a key factor in social status among females is how well the language is mastered. Repeated matings will kill males, so they are generally very wary of the females. Males are primarily poets and artisans, and enjoy dull, pampered lifestyles. The females control all political, military, and scientific aspects of the culture and keep the males segregated. Their boats were originally squids, their submarines are enhanced ichthyosaurs (here called uruketos), while their guns are modified monitor lizards which eject projectiles using pressurised gas. The Yilanè, having had millions of years of civilization, have a very advanced society primarily based on a mastery of the biological sciences, especially genetic engineering, so much so that almost every tool and artifact they use is a modified lifeform. By the time the novel begins, the humanoids have reached a late Stone Age level of technology and culture, with a number of societies having developed farming skills. Unlike humans, which evolved from African primates, the Tanu have evolved from a lineage of New World monkey. A human-like species, the Tanu, evolved to fill the niche in North America, but are only found on that continent. However, during the evolutionary process, the species became non-viable on the North American continent and Caribbean area, leaving them free of Yilanè for millions of years and opening an ecological niche for a top predator. The intelligent reptiloid species is called the Yilanè, and represents the dominant life form on most of the planet.

Consequently, the Cretaceous–Paleogene extinction event which wiped out the dinosaurs and other related reptiles never happened, leaving the way clear for an intelligent species to eventually evolve from mosasaurs, a family of Late Cretaceous marine lizards closely related to the modern monitor lizards. In the parallel universe of this novel, Earth was not struck by an asteroid 65 million years before the present. West of Eden is a 1984 science fiction novel by American writer Harry Harrison.

West of Eden by Harry Harrison