First of a new three-book series, Destiny's Children, imagining the future course of human evolution.
Upon the death of his father, middle-aged computer programmer George Poole returns to Manchester, where he meets old friend and neighbor Peter McLachlan, now a member of a weird group called the Slan(t)ers. Pondering his family—his maternal grandfather was an Italian-American GI; his sister Gina now lives in Florida—George discovers an old photograph showing him with a twin sister! Equally strange, his father made substantial monthly payments to a secretive Catholic organization in Rome. But what happened to George's twin? When questioned, Gina proves curiously hostile and professes to know nothing; but she gives George the names of other American relatives who might know more. Meanwhile, alternating chapters relate the story of Regina, a noblewoman born during the last days of Roman Britain. Early in the fifth century, the legions leave, commerce collapses and brigandry increases; Regina's father commits suicide, and her mother flees to Rome. Eventually, Regina, via King Arthur's camp, makes her way to Rome to found a mysterious organization—the same organization that, shaped by Regina's distinctive social engineering, has survived and prospered for 1600 years. But what did it want with George's twin sister, and how is George himself involved? Among the complications: a huge, tetrahedral object discovered orbiting in the Kuiper Belt, and Slan(t)er paranoia about an invisible war between all-but-imperceptible dark-matter aliens.
Baxter (Evolution, Feb. 2003, etc.) will never win prizes for style, but he's much more convincing when he writes about physical science and engineering (the Manifold series) than biology: Tepid.