The Chomsky hierarchy of types 0,1,2,3 grammars correspond, as he showed, to the abstract automata classified in accordance with their use of memory. The type 2 grammars, the context free phrase structure grammars, are computationally equivalent to the push-down store automata (which Chomsky was the first to characterize), which use memory as a push-down stack. Stacks are the mechanism by which computer programs call subroutines. The push-down stack is accessed by push and pop instructions, which store and retrieve memories on the stack.
The fact that a push-down stack is in use explains why both human languages and modern high-level programming languages have hierarchical tree structures, since the items below a node in a tree structure correspond to a subroutine in programming.
Yet, so far as I know, nothing neurological corresponding to memory pushes and pops has been discovered by brain scientists. This is a puzzle. Can anyone help?