After making four Booker Prize shortlists and covering literary territory as diverse as science fiction, autobiography, libretti and non-fiction, the 87-year-old Doris Lessing now speculates on the moment when the human species became, as the novel's title suggests, both forever divided and intertwined.

In the beginning there were only female humans, who were able to regulate their relationship with the Earth so efficiently that they were able to procreate without males. Incurious, these Clefts lounged around by the sea like walruses, never venturing from their little cove. And it was good, if a little uneventful.

But then, for no obvious reason, they began to give birth to Monsters, possessed of penises and all. Despite repeated attempts to kill them, these new beings with their "tincture of maleness" survived. The Monsters, or Squirts - for the characters, such as they are, are named and defined for the shape or action of their genitals - unwittingly introduce jealousy and grief and the "beginning of squirming, emotional discomfort". These new beings are adventurous and reckless. They run around and wrestle while the Clefts clean their huts, nag them and berate their carelessness. It's a bit like one of those television advertisements in which a woman, rolling her eyes, cleans up after her dopey hubby has whacked a cricket ball through the kitchen window or tracked dirt through the house.

Perhaps myths endure because of their brevity and lack of specificity - after all, who really cares what Icarus looks like? - but such a lack of detail is a large hurdle to overcome in a 260-page novel. Overall, the language of The Cleft is colourless, characters are almost non-existent and there is no internal engine of plot. What might have made a slight, if politically contentious, version of the emergence of what we understand to be gender ends up sagging under the weight of its own ambition.