It might have been our default condition, but it certainly isn't now. Manual labor has evolved with the machines and techniques of civilization. Its been a while since ditch diggers weren't aided by mechanical assistance, and we've all met people who really cannot be trusted near anything with moving parts.
> Our bodies have not lost the ability to do manual labor in under 10 generations.
Our living conditions have substantially weakened the filters that prevent people likely to be unable to do manual labor from surviving to adulthood (if the problem manifests at birb) or surviving long after the condition arises (if it manifests later.)
It might have been our default condition, but it certainly isn't now. Manual labor has evolved with the machines and techniques of civilization. Its been a while since ditch diggers weren't aided by mechanical assistance, and we've all met people who really cannot be trusted near anything with moving parts.