Ehrman spends most of his time among the Greeks and the Jews, with a bit thrown in from the Babylonians--but little time with the Egyptians, which is something of a shame, since their own views had such an impact on later Greek ideas. As Ehrman notes, among the early Greeks, concepts of the afterlife were hazy and generally unappealing. Unless you were some sort of hero who went on to reign in the world of the dead, you were likely to become a mere shadow, a sort-of half-conscious/half-existent being, which left questions as to what exactly the heroes among the dead ruled over. The philosophers had their own views, including that of Socrates, who thought that there either was no afterlife or that if there was one, it was most excellent. Either way, death was not to be feared, for we had no pain or sorrow before life and thus will feel none after either way. As time went on, Plato, via various Eastern cultures, adopted views that included an immortal soul and various rewards and punishments that went with a good life or a bad one. All came from the One, in Plato's view, and all were headed back there--as one moved closer in viewpoint to that First Cause through proper living and thought.
Among the Jews, in Ehrman's interpretation, there was concern only with national restoration, which resulted in the concept of a resurrection. But the scriptures focus on that resurrection as a way of restoring God's nation, more than on individuals. In time, however, the concerns with national restoration and the nation's resurrection became individualized such that there came to be a view that there was a resurrection and a judgment for individuals. (I am not completely swayed by Ehrman's view that resurrection was solely a national prospect in the early-going, but certainly I'd agree that the emphasis was on the nation rather than the individual.) PUnishment was generally simply not being resurrected or being destroyed for all time--burned up.
By Jesus's day, the individualized view of the resurrection and judgment was a common Jewish viewpoint, though there were others--no afterlife (common among the Sadducees) or immortal soul (common among the Essenes).
Early Christians emphasized a resurrection to life on Earth (sometimes in the flesh, sometimes as something bodily but somehow more than flesh), according to Ehrman, as part of the idea that Jesus would be returning soon and establishing his Kingdom. As this did not happen on the quick time scale people expected, other theories began to predominate, including an idea that the soul would be taken to heaven to await unifying with the body at the last judgment and Jesus's return or the idea that the body would be done away with completely. Punishment for an evil life moved from being eternal death to being eternal punishment. Likewise, in time, some Christians began to believe that there was likely some kind of intermediate state, or even a set of Platonic reincarnations, to purify those who had not been that evil (or even though who had been evil) so that all would have a chance at eternal bliss.
It is this last section of Ehrman's book that I found somewhat disappointing. After setting up his discussion so well with regard to what ancients believed regarding the afterlife and what people of Jesus's day believed, he pretty much wraps up the book, paying scant attention to how we get to the views so many Christians have today. To be sure, such views vary widely, as Ehrman notes--not everyone believes in Purgatory, for example. But in leaving out the chronological development of thought after the first century, we don't get a very clear sense of just how and when certain views came to predominate. As a history of the afterlife, the book could have easily continued into the Middle Ages to give a clearer sense of how we end up with the views that are so common today.