SRM's answer is correct: digital signing is what you're looking for. It's single goal is to do exactly what you are asking for.
However, traditional signing only goes so far. It only proves that you chose to sign a document, it doesn't stop others from signing it too. Its entirely possible to take a signed document, strip the signature, and apply your own signature.
In most cases, this is unimportant, because the purpose of the signature is to hold the signatory to something. However, this is a bit different. The author wants to be able to prove not only that he signed it, but that he originated it. Consider the case of a malicious government that wants to erase Bob's name. Rather than trying to unsign Bob's signature from the document, they generate dozens of fake signatures. They spread documents that suggest Alice signed the document, or Carol signed the document, or Dave, or Eve. All of them with forged timestamps to confuse the question of who released the document first. Bob's name could get lost on the mud, squelching his message in a pile of fake owners of the document.
There's two ways to control information. One is to prevent information from getting out. The other is to drown the information in a sufficient volume of fake information as to hide it from anyone who is looking.
There is a recognized approach to solve this, which involves a "trusted third party." The traditional third party is a newspaper, but any widely disseminated source of information will do.
First Bob writes his paper. He then calculates a cryptographic hash of the paper. This is a "mostly unique" fingerprint for his document. It is astronomically unlikely that two documents will "collide" and have the same fingerprint, and the algorithms are designed to also make it hard to maliciously make two documents collide. We have many in use today, SHA-3 being the most recent major algorithm.
Bob then writes up a claim. He writes
I am the originator of the manifesto whose SHA-3 hash is f4202e3c5852f9182a0430fd8144f0a74b95e7417ecae17db0f8cfeed0e3e66e. He then digitally signs this claim using the digital signature approaches SRM talked about.
Now for the nifty part. Bob then takes out a classified ad in a few major newspapers. Such an ad is cheap and easy. The add contains nothing but his claim
I am the originator... and the digital signature for that claim. After submitting these, Bob waits. Usually one day is enough. All that matters is that the message is properly disseminated into print. After this, Bob can send out his manifesto. The manifesto itself can be signed or unsigned, it doesn't really matter. Just the message put in the classifieds actually needs to be signed.
Once the government sees this message, they want to squelch his name, so they want to re-sign this paper with as many people's names as possible. However, Bob can now point to the classified and say "this is the oldest record of anyone claiming to be an originator of this document, and I hold the private key it was signed with." His claim is now special because anyone can go find a copy of the paper on the date Bob says to look in, find the classified ad, compute the SHA-3 hash of the document, make sure it matches the claim in the ad, and then check his signature. The government can release any number of similar classifieds, but all of them will be dated after Bob's claim. To date it before Bob's claim would require the government to go find every single copy of the newspaper and replace it with their own (very 1984esque).
The key to this approach is that the signed affadavit in the classified comes out before the document is released. Thus, there's no opportunity for the government to create their own fake claims. The best non-1984 solution would be for them to intercept the request for a classified ad, and quickly replace it with their own ad. However, if they do this, Bob reads the paper when it comes out the next day, finds out that his ad has been replaced and doesn't release his manifesto. Instead, he simply changes something minor in the manifesto, re-hashes it, and tries again. Eventually the government will fail to intercept his work, and then he can safely release the document.