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.

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.

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.

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 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.

The story revolves around the eventual discovery of the American continents by the Yilanè, who are searching for new resources and territories for colonization.

After Kerrick escapes he joins other humanoid tribes and journeys over the mountains, being pursued all the while by Vaintè, who wants nothing more than the absolute destruction of the ustuzou.

Dave Langford reviewed West of Eden for White Dwarf #59, and stated that "It's the density of background detail which makes this, not a Great book, but the best Harrison for some while.