Proto-Germanic had no indefinite article, nor did Old English have one. The bare noun could be used in contexts which in Modern English require an indefinite article and also, although more rarely, in contexts which require a definite one. features a bare nominal as a predicate complement.

has an abstract noun with specific reference, still without article.

The inherited numeral 'one' was ān, invariable in the nominative, but declining for the three genders in other cases. In Old English, it is already used in contexts like , where the meaning ‘one’ is yet possible.

In a Middle English text, almost the same context as in already requires a reduced form of that numeral as an indefinite article ().

(Dean, James D. (ed.) 1996, Medieval English political writings. Kalamazoo, MI: Medieval Institute Publications (TEAMS Middle English Texts))

In the further course of the development from Middle to Modern English, the indefinite article is increasingly used for generic reference, as in a whale is a mammal.

At the same time, the former numeral ān survives in its original function and in the form one, so that this grammaticalization process involves a split ().

Already in the Middle English period, the form of the new article is reduced. Its declension is lost, and phonological erosion takes place. The /a:/ of ān was shortened. Before consonants, its final /n/ got first assimilated and then lost. The result is /ə/, the shortest word of the language. By the same token, the indefinite article has developed an allomorphy, viz. a before consonants, an before vowels, which is matched by a similar allomorphy in the definite article, but is unknown elsewhere in the language system. The position of the indefinite article in front of the noun phrase is now fixed, by contrast with the numeral 'one', which in Old English could still follow the nominal group.

This particular grammaticalization process may have been favored by language contact. Even after the stage of the indefinite article has been reached, grammaticalization may proceed. The English indefinite article is currently limited to singular individual (or countable) concrete common nouns, to the exclusion of plural nouns, mass and abstract nouns, which bear no article. In Spanish, grammaticalization of the indefinite article is further advanced, i.e., it occurs in more contexts.

Reference

Givón, Talmy 1981, „On the development of the numeral ‚one’ as an indefinite marker.“ Folia Linguistica Historica 2: 35-54.