We know very little about that.
So sometimes you'll encounter written pieces, from the eighteenth century for example, or maybe even slightly earlier...
...where people write about manual communication, or gesticulation.
But it often isn't called sign language.
But what we're pretty sure of, is that such forms of manual communication...
...that they've existed for quite a long time. Who knows, possibly even longer than spoken language.
But we hardly have any actual documents to figure out...
...what this sign language was made up of, what the grammatical structure is.
I know that for American Sign Language, there are films from the 1920s.
I don't know any old documents like that for the Sign Language of the Netherlands.
So that naturally makes research into language change very difficult.
Because in language change research you want to compare older documents and newer forms.
If those older documents don't exist, that's indeed impossible.