
Big Boss's history prior to joining the armed forces and becoming a disciple of The Boss is a complete mystery. His military career dates back to 1950, in which he joined the Green Berets and became The Boss' apprentice. In 1956, Jack was involved in the Bikini Atoll testing of the first airborne detonation of a hydrogen bomb. During Jack's time under The Boss' tutelage, she taught him everything about combat, weaponry, survival, espionage, destruction, psychology and foreign languages like Russian. They also developed CQC together. The Boss and Jack went their separate ways on June 12th, 1959, and later met for the first time in five years during Jack's first field mission in the FOX unit, on August 24th, 1964.

Solid Snake was born in 1972, a product of the Les Enfants Terribles project, a secret government project to create the perfect soldier by using the genes of the "Greatest Warrior of the 20th Century", Big Boss. By utilizing the Super Baby Method, eight clone babies, Solid Snake being one of them, were grown and transferred to an unknown woman's uterus. However, six of the clones were intentionally aborted (later revealed to be only five after the revelation of a third brother, Solidus Snake), leaving behind the two (three) children who would grow up to become Solid Snake and his brother, Liquid Snake (and Solidus Snake). A phenotype was created, with all of Big Boss's supposedly "superior" dominant genes being expressed to create Solid Snake, while Liquid received all the "inferior" recessive genes. This fact would enrage Liquid throughout his life and fuel his mission to enact revenge on his brother, although he did not realize that he infact got the "superior" set of genes. Even though Liquid had clear signs of recessivity, he had in fact gotten all the soldier genes, both the ones he had originally and the ones they took from his brother, Solid Snake.
There are no records of Solid's childhood proceeding the completion of the project. Snake claims he was "raised by many people," although he says he has no family. It is possible someone during his childhood bestowed the name "David" onto him, as Snake claims this to be his name at the conclusion of the Shadow Moses incident. But in one of the games, he mentions being raised by nuns. This may be who gave him the name David, it's probably from the bible's stories about David and Goliath, or David and the Lions.
Nevertheless, Snake eventually made his way into the military by 1991. He had been inducted into the Green Berets and took part in a mission to infiltrate western Iraq during the Gulf War. Sometime after the Gulf War he joined FOXHOUND. Like most new recruits, he was trained by Master Miller.

Liquid was raised in the UK shortly after birth and was the youngest person to serve in the Special Air Service. During the Gulf War in Iraq, he was assigned to track down and destroy mobile Scud missiles. In truth, he infiltrated the Middle East as a sleeper agent for the British SAS, but was taken prisoner by Iraqi forces and held captive as POW until 1994, when he was rescued by the U.S. Government.
Liquid Snake joined FOXHOUND in the year 2000, after Solid Snake and Colonel Campbell had already retired and left the unit. After Liquid joined, he became the field commander of FOXHOUND.

The early years of Solidus' life are unknown. The earliest bits we can piece together is that he fought in the Liberian Civil War. He also killed Raiden's parents, adopted him and taught him to be a ruthless killer, in order to fight in the Liberian Civil War at an extremely young age.
The following events occur in Metal Gear Solid.
Solidus was also the 43rd President of the USA, being known as George Sears while he was in presidency. In the year 2005, and acting outside the orders of The Patriots he started the Shadow Moses incident in complete secrecy, telling Revolver Ocelot to persuade Liquid into stealing Metal Gear REX and use it for his own twisted ways. But because Solidus accidentally revealed the existence of Metal Gear REX and the GENOME Army to the public, he was removed from Presidency and went underground when he heard the Patriots were planning to kill him off. While underground he took ahold of Dead Cell as a way to gain power and destroy The Patriots.