Many analogs and alternatives have been invented in the intervening time, such as initials, crosses, phrases, wax seals, and rubber stamps. But in all cases, courts have been able to resolve whether these marks constitute a valid signature by drawing analogies to the manuscript signature.
The reason this works is that when making a contract, the law is principally concerned with the function of the signature: what it means and what it does. This function is so universally understood by both lawyers and non-lawyers alike, that in practice it is never called into question.
Accordingly (and perhaps surprisingly) there is almost never a specific requirement for the form of the signature, so long as its function is represented and understood.