This manga is based on the Akkadian myths of Usumgallu and the Atra-Hasis. Usumgallu is the royal dragon of Akkad while Atra-Hasis is the flood myth included in the Epic of Gilgamesh. The old Noah-like grandpa is Uthnapishtim/Atra-Hasis, the man made immortal after becoming the savior of life in the Great Flood. Now here is where things get interesting: Depending on which version of the myth is consulted, Uthnapishtim is either the perserver of the seed of man (the Noah route, all humans are descended from him and his kids, etc) or the last of the old humanity purged by the gods. In the latter version of the myth, the gods instead fashioned a new humanity to take the place of the old one extinguished by the Flood.
I think this manga is going to follow route 2 and make Usumgallu both Adam and Eve of the new humanity because we clearly see him being created by the old gods.