Mighty oaks from little acorns grow
Great things may come from small beginnings.
The word acorn doesn't come from 'oak' and 'corn', as is popularly supposed, but from the Old English 'aecern', meaning berry or fruit. The tree genus Acer comes from the same root.
Before oaks were mighty they were first either great, tall, sturdy or even just big. Examples of early variants of 'mighty oaks from little acorns grow' are found in Geoffrey Chaucer's Troilus and Criseyde, 1374,
"as an ook cometh of a litel spyr" [a spyr, or spire, is a sapling]
Thomas Fuller's Gnomologia, 1732:
"The greatest Oaks have been little Acorns."
and in an essay by D. Everett in The Columbian Orator, 1797:
"Large streams from little fountains flow, Tall oaks from little acorns grow."
The Oxford Dictionary of Quotations states that 'great oaks from little acorns grow' is a 14th century proverb. Unfortunately, they don't include any details to support their view.
The 'mighty' version is known, in the USA at least, from the middle of the 19th century. It appeared in A. B. Johnson's The Philosophical Emperor a Political Experiment, 1841.
See also: the List of Proverbs.