If you've seen the Lord of the Rings films, you have a good idea of how Smeagol turned into Gollum. But if you've seen only the films, you don't know the full story. The books bring new light to the already illuminating cinematic portrayal of Smeagol's transformation. For instance, the movies don't mention his prestigious family, his first meeting with Shelob, or his torment at the hands of Gandalf! There are many more details to divulge, as the rise, fall, second rise, and final, literal fall of Smeagol/Gollum is one of the most compelling stories ever put on page.
Here's the complete Smeagol/Gollum transformation timeline.
Note: TA means Third Age.
TA 2430: Born A Hobbit To A Wealthy Hobbit Family
Smeagol was no middle-class Middle-earthling. He was born to an elite family helmed by a venerable matriarch.
He was middle-sized, however, as Smeagol's race is hobbit of the riverland Stoor-kind.
TA 2463: Found The Ring And Choked His Cousin For It
On the day of his 33rd birthday, Smeagol was treated to a fishing trip by his friend and relative (presumably a cousin) Deagol. After hooking a big fish, Deagol was pulled out of the boat and deep underwater. It was there he came upon the One Ring, which had embedded itself in the river over two millennia prior when Isildur was ambushed by Orcs.
He emerged from the river with ring in hand and under the envious eye of Smeagol, who requested the newly discovered treasure as a birthday gift. When Deagol refused, Smeagol strangled him. He then became the fourth Ring-bearer.
Renamed 'Gollum' For The Coughing Sound He Made
The ring was quick to exert its dark influence on Smeagol. He became obsessed with his precious possession, constantly mumbling and gurgling for no reason that his disconcerted community members could understand.
Based on those sounds, they took to calling him "Gollum."
TA 2470: Banished By His People For StealingPhoto: The Lord of the Rings: The Fellowship of the Ring / New Line Cinema
It didn't take long for Gollum to become alienated in his community. The ring caused him to act erratically and criminally, which caused his peers to respond in kind.
Distancing himself, Gollum resorted to stealing in order to survive. It was his own grandmother who finally decreed his banishment.