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I'm quite new to this family tree stuff and have come against a bit of a block.  I have found the marriage of my gggGranddad John Christie to Elspet Gauld in Strachan in 1855.  The only problem is that he is 22 and according to the marriage certificate he was born in Marykirk but the birth was not registered.  It has his parents as David Christie and Elizabeth Christie.  How can I go further back than John without any birth records for him?  I have found John in the 1851 Census in Fordoun working as a farm labourer.  Also possibly in Glenbervie in the 1841 Census at the age of 8 as an agricultural labourer (this census doesn't have the place of birth on it so I can't confirm it's definitely him).  Any suggestions for how I can get any further back and also on why he wasn't registered born would be really helpful.
Thanks