My thoughts on this.  Basically, we have the ability to taste salt,
sweet, bitter, and sour.  Without eating any salt now for about 4 years
(with the rare occasional exception, perhaps once every 2 months or so),
it seems to me that the "salt" taste might be there just to encourage us
to eat meat (as in, food from an animal source, meat, birds, fish,
etc).  My ability to taste salt is very strong now and I can easily
taste the salt which is in meat.  If salt were not easy to obtain, we
could still get it in animal-source foods.  On the other hands, humans
shed salt like it was going out of style, much faster than you'd expect
for an animal whose life depends on it.  This suggests there was plenty
of salt in our source environment.  So we must have had salt available
to us in our native environment or had access to plenty of meat which
had it.  So that's my guess.

Erik