Books like Coalescent by Stephen Baxter

George Poole, thirty‑five and feeling stuck, learns of a twin sister he never knew existed after his father's sudden death. Rosa, a member of a cult that has survived since the collapse of Roman Britain, invites him into their hidden, subterranean city beneath Rome. There, amid a mother‑candied colony long buried in myth, George encounters women who are both incredibly similar and disturbingly young, hinting at evolutionary divergence. Joined by his childhood friend Peter McLachlan, who arrives with his own dark secret, George uncovers evidence that this underground Order may represent an advanced genetic line capable of rendering ordinary humans obsolete. As the Order’s plans to leave their underground nest unfold, George must decide whether to stand against a future that could eliminate humanity itself.

First publish date: 2003