Human nature and its basic honest and positive response to truth, love, kindness, empathy etc were the same as old as the existence of humanity itself
And the rules of Shastras or other similar laws in any society or faith evolved for the protection of such nice qualities and to instill abhorrence to the bad qulities and the base propensities also present in human nature..
Sattwa, Rajas and Tamas were present everywhere andy basically Sastras evolved to uphold Sattwa or the good against the unlimited attack of Raja and Tama gunas..
So I am always of the view that even if we do not consciously follow the tenets of Shastras but keep a place for love, kindness, co operaton, tolerance etc in our minds, we would be following the Shastras even though without our conscious learning or knowledge..
If that happens there might not be any reason for pariharams at all.. or expiations.. because we would be doing only good and right things
But if we just follow shastras as pretensions or even as duties without imbibing the underlying spirit of love and empathy, such Shastras may be of little use either to us or others..