Did I just see Eric Schmidt blink? I suspect that the Google (Nasdaq: GOOG) CEO -- who has successfully stared down Yahoo Inc. (Nasdaq: YHOO), Microsoft Corp. (Nasdaq: MSFT), the European Union, and the U.S. government since he took over the company in 2001 -- is nervous. The cause of his newfound angst? Twitter, of course, the real-time short-messaging network that is, in real time, revolutionizing the same Internet that Google began to revolutionize 10 years ago.

It happened earlier this week in San Francisco, at the Morgan Stanley Technology conference, when Schmidt was asked a question about Twitter’s usefulness. Here’s how he answered:

Speaking as a computer scientist, I view all of these as sort of poor man's email systems. In other words, they have aspects of an email system, but they don't have a full offering. To me, the question about companies like Twitter is: Do they fundamentally evolve as sort of a note phenomenon, or do they fundamentally evolve to have storage, revocation, identity, and all the other aspects that traditional email systems have? Or do email systems themselves broaden what they do to take on some of that characteristic?

Yes, he blinked. Google has had a remarkable run. For 10 years now, search has dominated the technology conversation. But what Schmidt’s strained, geeky answer reveals is that the next big thing in Silicon Valley -- and thus the real threat to Google -- will be, in his words, “poor man’s email.”

In 1993, email changed my life by enabling me to instantaneously communicate with people all over the world. Back then, however, electronic mail was just that -- a digital version of traditional, private one-to-one mail. In the 15 years since then, email has remained relatively unchanged as a core piece of the Internet’s communications architecture. At the same time, the social media revolution exploded, first with Google’s crowdsourced search engine and then with social or knowledge networks like YouTube Inc. , Wikipedia, and Facebook (Nasdaq: FB).

Twitter is a huge hit because it represents the synthesis of both Web 1.0 email and the Web 2.0 social media revolutions. It is many-to-many email in which users share most of their correspondence with others on their network.

Just as Web 2.0 user-generated content revolutionized media, so Twitter is revolutionizing both the culture and economics of electronic communications. It makes what Schmidt calls “note phenomenon” the engine of a new personalized communications system. Twitter replaces email with, so to speak, tmail.

Over the past few weeks, I’ve begun to shift my mode of Internet communications from email to tmail. Yes, of course, email isn’t dead and I still use it for important personal and business correspondence that I want to keep private. But tmail has become my preferred mode of communications with the world because it is quicker, easier, and more fun.

Most importantly, Twitter has transformed email into a viral tool for business development. So most of my tmail is designed to acquire a following and readership, build my personal brand, and establish new commercial relationships.

But how does tmail, Schmidt’s “poor man’s email” (boy, that remark is going to come back to haunt him), threaten Google? The problem for Google is that Twitter could emerge as a rival network to the Internet. As Twitter goes from its current 6 million users to 60 million and even eventually to 600 million, it will begin to compete with the Internet as an informational ecosystem. Not only might this be a real-time and self-correcting informational resource, a Wikipedia on steroids, but Twitter -- which acquired the Summize search engine last summer -- will also have the capacity to become an all-knowing search engine of its users’ knowledge.

Eric Schmidt, of course, knows all this. That’s why he publicly blinked at the Twitter question earlier this week. He understands that the real threat to Google will come not from another search engine or social network or email service, but from a new product that synthesizes all these technologies into something both intimately familiar and brand new.

Twitter is the simplest and most intuitive product to emerge since Google. If it is indeed a poor man’s email, then we are all poor men now in a revolutionary new era of tmail.

— Andrew Keen, Silicon Valley author, broadcaster, and entrepreneur, can be reached on Twitter at @ajkeen.