verbs - What is the origin of the 'do' construction?

Modern English seems to require this verb in several circumstances, where most other European languages don't seem to need it. (See? I just used it.)

For example, in questions: "Do you have a dog?" Whereas, "Have you a dog?" would be normal in other languages or in the English of days gone by.

Another example: "I do not know him." Again, "I know him not," could be used, but sounds stilted nowadays.

So, where did this 'do' come from?