So I decided to make up a timeline of events from TMNT pushing it back a decade from the 2012 series
1960- Janurary 7th, Hamato Yoshi Is born
1960- December 24th, Oroku Saki is born
1963- Foot Clan Massacure, Saki is moved to the custody of Hamato Yoshi's Father
1966- Oroku Saki and Hamato Yoshi Become freinds and become rivals while training as ninja
1975- Hamato Yoshi and Oroku Saki's rivarly grows more as they meet Tang Shang
1979- Hamato Yoshi is married to Tang Shang, Oroku is crushed.
1985- June 15th, Casey Jones is Born
1985- November 28th April O'neil is born
1986- Feburary 9th, Miwa (Karai) is born
1986- April, Saki attacks Hamatos home, Kills Tang Shang, And Kidnaps a 2 month year old Miwa (Karai)
1987- Janurary, Hamato fleds Japan with a broken Heart And gives up his given name using the alias "Splinter"
1987- Janurary 6th, Feburary 28th, April 19th, May 30th, The turtle brothers Leonardo,Rapheal, Donatello, Michelangelo are born to a mother turtle who are then seperated into the land and found, Given to a pet shop owner.
1987- September 29th, Hamato stops by a pet shop to get 4 new born turtles. He then spots 2 krang men in an alley, Hamato Is then spotted by the krang by a rat that ran up his leg. Hamato fights and wins but is then droped by some mutagen. That mutates him into a rat, and giving the turtles some human genes
1987- October 10th, Hamato finds an abandon train station deep in the sewers and decides to make it his lair
1994- 6 year old turtles begin training with Splinter (Hamato)
2001- September 29th, Turtles Finish there training an convince splinter to go to the outside world
You can take over, if you want. I dont feel like theres more to cover ,but I might finish later.