Reid Malenfant is an unhappy man - trained to be an astronaut, NASA has never chosen him to go into space. Trying to reconcile himself and Emma he takes her flying over southern Africa just as the Red Moon appears in the sky. Unfortunately his daemon leads him to fly too high, and as it has lost Emma for him before, so he loses her again as she is sucked upward and he is cast down to Earth. Back on the ground Malenfant starts chivvying for a rescue mission, and he is finally given a three-seater craft in which he sets out with Nemoto (another version of a previous character), intending to land, collect Emma and return. That single sentence account is close to Baxter's brevity in describing Malenfant's efforts.

With another American woman and her young son who were untimely ripped from a passing airliner, Emma meanwhile has found herself dropped onto the edge of a savannah . Unfortunately, it is the savannah of the Pleistocene, while two-thirds of the survivors can only just cope with Rodeo Drive of today. The three follow a passing troop but, while Emma does her best, her fellow Americans cannot cope with the struggles of Homo Erectus and Australopithecines who raid into and out of the forest littoral. Semi-apes who eat their young ("their young" sometimes referring to physical off-spring, sometimes to youngsters in the troop) will have little time to care for a young American - the boy is soon not only dead, but partially eaten.

Baxter tells his story through short chapters, swapping point of view quickly, and the humans receive only a part of the story. Many other characters are introduced, each in their family or group - for the Red Moon contains examples of every branch of the evolutionary tree that has ended in Homo Sapiens alone here on Earth. For a moment, the reader may think that Emma will bring intelligence to these "people" (they have names, mostly, at least), but that is not Baxter's plotline and by the time that Malenfant and Nemoto have crash landed the reader is aware that her rescue will be no more than a minor plot motor, too. As Emma travels on she realises that each sub-species has reached the end of its evolutionary potential - some have discovered how to use fire, some knap flints, for instance, but not one has discovered how to ignite fires, none have realised that a flint can be carried rather than knapped anew when one is called for. And Emma realises that they never will. While it is in the heavens, metaphorically this means that the Red Moon with its impossibility of improvement is not purgatory, but hell.

Malenfant, once landed, suffers from an "old inner-ear problem" (a critical term found in Bored Of The Rings) common to adventure stories, and travels away from Emma. It leads him, though, to the other sources of intelligence on the moon - humanoid and pongoid (i.e. gorilla based) intelligences who have evolved on other Earths previously visited by the Red Moon and who have used spacecraft to make their own commutes to it, and he meets too the religious zealots of this globe. No good can come of it, and Malenfant suffers as much as any ape before his final rescue.

While Stephen Baxter is often hailed as one of the leading forces in hard sf, one of his stylistic features rarely commented on is that he works through pastiche. Voyage, his alternate history of the Mars probes, for instance, is constructed in the blockbuster manner of Arthur Haley and Harold Robbins, but in Origin one can see a much older influence: adventures in Darkest Africa apart. In being transported to the ape world in the sky, Emma has been raised to Laputa and has found it occupied by Yahoos. This is then emphasised by the repetitions which Swift made:- the hominids are not only violent, senseless and brutal, their patterns of dominance are also strongly scatological - in fact, they repeat Gulliver's experiences of being dunged on and of seeing (literal) arse-licking. These may be the experiences of monkeys and apes on Earth as they maintain their patterns of dominance (grooming is also mentioned by Baxter, as a binding or soothing factor), but when the Trilogy has been concerned with questions of cosmology and the construction of alternate worlds, so much attention to the minutiae of the behavioural sciences makes one question Baxter's purpose. Norman O. Brown's 1957 essay "The Excremental Vision" (in his Life Against Death: The Psychoanalytical Meaning of History) might be a starting point for an in-depth study of this, as violence and scatology are not identical, no matter how far they may be associated, and other reviews of Origin have tended to elide this distinction.

From the base aspects of life to the higher, movement comes in two ways: firstly, religion, which is presented as the fundamental beliefs of the zealots. The zealots initially appear to have gone down a line of divergence in puritan times, now to find themselves struggling in their jungle stockades, except that Malenfant finds that inside their costumes they have tails. (It is not clear if these tails are prehensile or not, and therefore not clear if the zealots's Old South-ern snake-handling, Baptist manner comes from being descended from New World rather than Old World anthropoids, or is simply an easy parody of a fundamentalist cult). Secondly, the pongoids come from an Earth in which the people have the world and themselves fully exploited and fully under control. Then, suddenly, unexpectedly, one is infected with a religious madness from the zealots (it echoes the New Testament temptation of Christ), and with almost no proof or evidence accepts that she can take over everything - events which quickly lead to tragedy.

Against all this, Nemoto keeps producing theories to account for the possible earths. There is no solution, and there is no conclusion - Emma intends to travel on, "going up-river", to search for the Red Centre. Surely, though, that is what Philip Jose Farmer described in the Riverworld series twenty years ago? Via a new means - the collision of ethology and cosmology - Stephen Baxter seems to be repeating an sf trope. Farmer's series became more diluted and unsatisfying as it went on, Origin, too, threatened to reveal its underlying weakness of theory. It stopped only just in time.