Definitely more of a 3.5.
The author plunges us deep in the middle of this post apocalyptic world and it took me a while to gather my bearings. And while it'll probably take me a long while to ever get comfortable with all the scientific terminology associated with these kinds of sci-fi books, I was able to get the general idea and emotion behind it all. The dual timeline with the current and the ancient Mesopotamian civilization was a master stroke to make us feel invested in everyone's lives, the discussion about the ethics of time travel as well as a very short term focused banking economy on the development of the world was fascinating, and the best part was the clash of cultures we could witness - not just between the timelines, but also within the present between the generation who survived the plagues and the younger ones born in the aftermath. That cliffhanger ending though equal parts wonderful and frustrating, because I can't even guess what happens next.