This Lisp I code is from 1959/60.

http://bitsavers.informatik.uni-stuttgart.de/pdf/mit/rle_lis...

Page 101:

    DEFINE
    (((COLLAPSE,(LAMBDA,(L),(COND,((ATOM,L),(CONS,L,NIL)),
    ((NULL,(CDR,L)),(COND,((ATOM,(CAR,L)),L),(T,(COLLAPSE,
    (CAR,L))))),(T,(APPEND,(COLLAPSE,(CAR,L)),(COLLAPSE,(CDR,L))))))))

This is the Common Lisp version. Formatted, comma removed, DEFUN instead of DEFINE.

Other than that the code is unchanged. It still works in Common Lisp.

    (DEFUN COLLAPSE (L)
      (COND ((ATOM L) (CONS L NIL))
            ((NULL (CDR L))
             (COND ((ATOM (CAR L)) L)
                   (T (COLLAPSE (CAR L)))))
            (T (APPEND (COLLAPSE (CAR L)) (COLLAPSE (CDR L))))))

    CL-USER 191 > (collapse '(1 2 (3 4) ((5 6) 7)))
    (1 2 3 4 5 6 7)

Common Lisp is backwards compatible to Lisp I and Lisp 1.5 in many ways.

Code like that can be relatively easy ported to Scheme (naming and syntax is slightly different). There are examples which are more difficult. But generally simple (!) Scheme is not that far away from that old core.