I am currently reading "the resuerection of the Son of God" by NT Wright and the first few chapters deal with WHERE and WHEN the notion of ressurection and after life came to be.
It was NOT there in the Penteauch but you do start to see the formation of a "life AFTER the AFTERLIFE" afterwards in Ezekiel, Isiah and of course Daniel, even more so after the second temple and the liberation from the Babylonians, as the Hebrews got "intertewined" with th e Hellenistic world, they started to but things in a more "greek" way, but there was still no real belief persay in an immortal soul, but then again, the spirit was alwaus viewed as "never dying" but just returning to God and death was always viewed as sleep and the decomposing of the body.
The hebrews and christians (early) believd that the sould can be killed/die, but that doesn't mean it wasn't immortal ( immortal and eternal are not the same, even an immortal being CAN die if it is killed).
Of course at the same time, in the OT there was a belief of SOME life in the grave, the prohibiltions against "calling on the dead" show that it was possible to communicate with them, as in the case of King Saul speaking to Samuel.
I think that it is wrong to believe it was an either/ or situation, the Saudacees believed in NO ressurection and no "interim" transformation into spirit or "angel", the pharasees believed in them.
Jesus preached resurrection, he warned that the soul CAN die and also preached that there is SOME concisouness in the "death state", Paul himself believed that upon his death he would be with Jesus and the souls of the martyrs are "alive and well" under the throne of God in Revelation.