In linguistics, the terms are esoteric and exoteric.
And it helps to remember that reconstructed languages only take us back so far (where they can be reconstructed at all); there's no reason not to believe that language (as opposed to "mere" communication) has been around at least as long as the species has been, so it's had a lot of time to fester, grammaticalize common concepts, overgrow, rot, have chance encounters with other, equally-esoteric languages at levels below those that would force simplification, etc. (We only have to go back to the 15th century to find William Caxton complaining that "English" was really a set of almost mutually-incomprehensible languages, and realising that he would be guiding the language in one way or another by the words he chose when making his translations.)
Language may have started out simple, but it rarely stays that way unless it needs to be shared with "outsiders". Cree, Tsez or Nama/Khoekhoe would melt the average adult American's or European's brain, but their native speakers can handle them while tired, ill and intoxicated. If by some fluke of history any of them were to become a dominant language learned by outsiders (after childhood), they'd become a whole lot less, um, interesting.
And it helps to remember that reconstructed languages only take us back so far (where they can be reconstructed at all); there's no reason not to believe that language (as opposed to "mere" communication) has been around at least as long as the species has been, so it's had a lot of time to fester, grammaticalize common concepts, overgrow, rot, have chance encounters with other, equally-esoteric languages at levels below those that would force simplification, etc. (We only have to go back to the 15th century to find William Caxton complaining that "English" was really a set of almost mutually-incomprehensible languages, and realising that he would be guiding the language in one way or another by the words he chose when making his translations.)
Language may have started out simple, but it rarely stays that way unless it needs to be shared with "outsiders". Cree, Tsez or Nama/Khoekhoe would melt the average adult American's or European's brain, but their native speakers can handle them while tired, ill and intoxicated. If by some fluke of history any of them were to become a dominant language learned by outsiders (after childhood), they'd become a whole lot less, um, interesting.