In biological vocabulary, sometimes both pre- and pro- are used as prefixes to indicate something earlier in a sequence. For example, pro-B cells develop into pre-B cells, which eventually develop into mature B cells. And pro-microRNAs develop into pre-microRNAs, which develop into mature microRNAs.

When both pro- and pre- are used to mean "earlier to/prior to/before", does pro- always come first? Is this distinction used outside biological vocabulary? Does this come from some subtle difference in a classical language?

* Of course, pro-B cells come after pre-pro-B cells. And pro-microRNAs come from pri-microRNAs, but "pri" is an abbrevation for "primary".

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    To be honest, I don't think I've ever seen "pro-" used as a synonym for "pre-". Usually "pro-" implies support. – Dog Lover May 11 '16 at 1:32
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    Nonetheless, the first definition of "pro-" in Merriam-Webster's Collegiate Dictionary, 10th Edition is "earlier than : prior to : before". – Michael Hoffman May 11 '16 at 1:37
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    Actually, I'd bet that the first appearance of "pre-" in Merriam-Webster comes before the first appearance of "pro-". – Hot Licks May 11 '16 at 1:59
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    Maybe it's a shortening of "proto", or using "pro" in a sense similar to that of procreate or produce. – user0721090601 May 11 '16 at 2:40

This is a very interesting question. Unfortunately, I have not been able to find enough information to give more than a partial answer. But I hope it will be of some use.

According to the Oxford English Dictionary (OED), the prefix "pro-," meaning "before," comes mainly from the Greek prefix προ-. It is cognate with the Latin preposition pro meaning "for, before, in front of," and with the English word fore. More than 60 words with this prefix were adopted into Latin in antiquity, including prologus "prologue" and propheta "prophet," so Latin has been an important intermediate source of words with this prefix.

The OED indicates that the Latin prefix prae-, which corresponds to modern English pre-, was mainly used in Classical times with verbs and verbal derivatives, or to intensify the meaning of adjectives. There are also some cases where it modifies the meaning of nouns and adjectives, but the use before a noun to indicate an earlier version of that noun apparently only became common in post-Classical Latin and later languages.

The prepositional construction, in which pre- governs the second element, which was so rare in Latin, has in English received vast extension (as also in French), pre- being preferred to ante- as the opposite of post- in new formations, and often substituted for it, as in pre-Christian, prehistoric, pre-Darwinian, pre-reformation instead of ante-Christian, ante-historic, ante-Darwinian, ante-reformation. [...] It may also be combined with nouns in order to form compound nouns directly, as precancer n., preclimax n., [...] premyelocyte n., prepuberty n., etc. Compounds of this type are of relatively recent appearance.

So the distinction you mention does not seem to date back to Classical languages.

It does remind me of the use of hyper- in some contexts as a more extreme version of super- (discussed in the answers to this question: Which is higher — "hyper-", "ultra-" or "super-"?). In both cases, it seems like the more common term (super- or pre-) is used for the less extreme meaning. I have not found any cases where pre- and pro- are distinguished this way outside of biology.

I found one document about biology which seems to describe a slightly different naming scheme using "pre-pre-" instead of "pro-":

So-called B-ALL, for example, is based on progenitor cells of B-lymphocytes, while T-ALL forms from precursors of T-lymphocytes. A degeneracy in the early development stages is characterised by the prefix "pre". This results in the following ALL-subtypes:

  • Pre-pre-B-ALL (now commonly referred to as pro-B-ALL)
  • Common ALL
  • Pre-B-ALL
  • (mature) B-ALL
  • Pro- and Pre-T-ALL
  • Intermediate (cortical) T-ALL
  • T-ALL

(Acute lymphoblastic leukaemia (ALL) - Brief Information)

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    Which means pro does precede pre—etymologically at least – Unrelated May 12 '16 at 0:33

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