Jecomen was from the Convents of Health on Byulna, owned by his parents. He would inherit it from them. Jecomen was kidnapped at a very young age by Lenny Kruger, who brought him to 20th century Earth, where he grew up in Liverpool as James O'Meara inside a flesh suit, thinking himself human.
In 1957, he got together with two other locals, Mark Carville and Korky Goldsmith, and formed the Common Men. The band was not particularly popular, and — by the suggestion of the Fifth Doctor — became the backing group for Aubrey Waites in 1963. This new alliance, adopting the name John Smith and the Common Men, became hugely popular within weeks, and quickly rose up to #2 on the British charts, just below the Beatles. James never found out he was an alien.
In an alternate timeline created by Lenny Kruger, the Common Men became the most popular band in the world in the Beatles' place. They met Nyssa in 1950s Hamburg, and let her stay in their hotel for a few nights. He and Mark met the Fifth Doctor close afterwards, who explained their origins to them in the Pfannkuchen. They soon, though, had their memories wiped of the encounter by Lenny.
They then unknowingly re-encountered the Doctor on 31 October 1963, when they arrived in London after a plane ride, only to find hundreds of fans screaming for them outside. The Doctor warned them of Lenny walking through the crowd with an impulse laser, and James invited him for a drink.
The group met the Time Lord once more in 1967. They were at the time in a small Welsh town, promised a free private seminar with the guru Paravatar. James was approached by the Doctor, who informed him that they may be in danger. Paravatar, secretly a retired time traveller, hypnotised the musicians into revealing their true identities. After waking up, the Common Men were finally approached by Lenny, who quickly became their new manager.
According to Mark, James once make him do retakes of a guitar solo for nine hours, then came by the next morning and simply did it himself in twenty minutes.
Even after the Common Men split up, James had a connection with Lenny and Mark. When entering Mark's New York house around 1970, Mark had just transformed into his true Byulnian form, and killed him out of anger.
Lenny, upset that all did not work to plan, further manipulated time. In yet another timeline, the Common Men got back together and James never died. At the Doctor's request, though, the group and others pretended that James had been killed by Lenny in May of 1966, and was subsequently replaced by a look-alike. The Doctor created a false corpse from Lenny's own genetic modifier, and had Rita spread the rumour.
This spread bad word about Lenny throughout the world. Lenny, soon to die because of this, finally let the Doctor restore the original timeline, and all returned to normal. (AUDIO: 1963: Fanfare for the Common Men)