68 years ago - Thorne is born in Atlantis.
51 years ago - 17-year-old Thorne becomes one of Atlantis's Scientists.
46 years ago - 22-year-old Thorne begins proposing radical theories of Atlantean bio-adaptation.
43 years ago - 25-year-old Thorne joins the followers of Kalandro in order to advance his own resources.
42 years ago - 26-year-old Thorne is named the new chief royal scientist by Orm Marius & Kalandro after the coup in Atlantis.
36 years ago - 32-year-old Thorne begins kidnapping surface dwellers for his experiments.
32 years ago - 36-year-old Thorne's experiments with surface dwellers yield only one survivor, David Hyde, who is dumped back on land.
11 years ago - 57-year-old Thorne escapes Atlantis during Aquaman's coup. He begins setting up his experiments again in San Diego.
2 years ago - 66-year-old Thorne begins releasing his mutogenic catalyst into San Diego's ecosystem, beginning to secretly mutate the population, including Lorena Marquez.
now - 68-year-old Thorne's experiments to mutate the population of San Diego moves into it's final stage when he sinks the city. He is found by Aqualad & Aquagirl, and imprisoned in Atlantis.
There are quite a few characters whose stories made the most sense if there was a character in the world who is actively working on experiments to adapt surface dwellers to give them Atlantean physiology. At first, we made a real effort to use our newly expanded character Kalandro for the job, but it soon became clear that this wasn't a role for the religious zealot. We needed a new character, and since we had such great success finding Kalandro in Peter David's Chronicles of Atlantis, we looked to do the same with our mad Atlantean scientist.
Thorne was featured in an earlier issue; He was the scientist that invented the treatment that allowed the Atlanteans to gain the ability to breathe underwater and survive under the ocean. We aren't using that as the premise for how the Atlanteans gained those abilities... we instead made the entire transition of the Atlantean people into sea-dwellers part of a massive spell enacted by the ancient magic-wielder Atlan in the first Shadowpact, which means that Thorne was now open for us to use as part of our modern timeline.
His existence actually works really well as a lynchpin of quite a few stories. Atlantis is generally related to magic, but it's a civilization with massive underwater cities and fleets of huge submarine warships, so obviously science is a huge part of their world as well, and creating a character that can implement that science to all sorts of narrative effects is a great tool.