Chapter II of Merkle's 1979 PhD thesis is titled "One Way Hash Functions." The chapter appears to be the first reference to cryptographic hashing. The chapter has no references. Is there an earlier reference that I am not aware of?
I think the first paragraph of this document show be helpful.
The Rabin reference at the bottom of the text is from 1978, and the impression I get from p3 of this document is that he came up with a linear iterated hash function in that 1978 paper. So maybe Rabin invented it.
I think we can be pretty sure it was either Merkle, Rabin or Yuval (if you must choose one), but that sounds like an answer correct for a good deal of history questions in this field.