20 Things You Probably Didn't Know About Children Of Men
9. The Film's Main Divergence From The Novel's Story
In the source novel, the infertility of men worldwide (referred to as the "Ultimate Failure") is what leads to the planetary crisis at play but this is changed in the film. Here, it is the women who are unable to bear children and due to the film’s exposition-lite approach, it is never explained why this change was made.
Also, the film eschews with the book's prologue so that means we do not get to see the initial war that broke out following the realization that nearly half of humanity became infertile, the categorization of the last humans to be born as Omegas and Theo's despotic cousin, Xan Lyppiatt (who in the film is named Nigel and is not hinted to be as authoritarian as he is in the novel).
In addition to this change, Julian (Julianne Moore) was initially the last expectant woman in the world as opposed to Kee (Claire Hope-Ashitey), a character created for the film.
The change of gender, alongside the film’s surface level exploration of the book’s religious themes and removal of some of the book's worldbuilding were some of the main gripes a good number of viewers had with the film despite P.D. James herself enjoying the film according to Cuaron.