Kevin Kelly often writes about how people and tools have evolved together, and how we can no longer truly be distinguished from our technology, and we certainly couldn't survive without our technology.
I definitely agree with that point, and I would argue that some of our technological advances have been so dramatic that they have, in a sense, almost changed us into a different species. The changes wrought by these new technologies are so dramatic that I would argue that people living before and people living after the advent of these technologies have fundamentally different kinds of lives. Technologies like this are obviously very rare, but here are a few:
Language: Language almost certainly progressed very slowly, but language is now almost the sole means by which we think, communicate, etc. When we think about Helen Keller, we feel so desperately sorry for her, not because she was blind or deaf or mute - plenty of people have those afflictions without arousing such deep pity - but because those afflictions shut her out from the world of language. It is when she finally understands language that her life becomes heroic and understandable - before that she is only pitiable. Thinking of what it meant to be a human before language seems impossible.
Writing: Before the advent of writing, all knowledge was local knowledge, both in space and in time. You could never know more than the sum of what your neighbors knew. Ideas and inventions died with their creators. Writing now touches nearly every aspect of our lives. Like language, it seems impossible for any civilization to exist without writing.
Industrial Revolution/Urbanization/Globalization: Before the Industrial Revolution, most people were farmers, living in self-contained communities. Unless you were royalty, you probably made your own shirts, grew your own food, and neither you nor anything that you owned had ever travelled more than a dozen miles. Because of the resources needed to provide everything for yourself, most people only had a few changes of clothes, a very simple diet, and small, sparse homes.
Now, we think nothing of buying shirts from Thailand, TVs from China, bananas from Mexico, and shoes from Singapore - in fact, we could probably buy all of those things at the same store, within a few minutes of our homes. We no longer make our own clothes or grow our own food. Chances are that you are not a farmer - that you don't even know anyone who is a farmer. The lives that we lead now would be wholly foreign to a pre-Industrial farmer.
The Internet: Although it isn't there yet, I believe that the Internet will change the world as much as any of the other technological advances. Just like language and writing enabled not only more conversation, but a different type of conversation, the Internet is not just like a new version of TV or newspapers or books, but it is something new and different. It is changing the way that we do everything - from buying and selling things to finding new friends to learning to working - the Internet democratizes everything. Instead of learning about the Iran elections by watching the news, we can talk with regular people in Iran. Instead of just listening to music by a band that we like, we can write suggestions to them on their MySpace page. Instead of finding a job in a big company, we can be employed by the Internet - as a blogger or an editor or an eBay buyer and seller.
Like many people, the Internet is an integral part of my life, and the things that I do would be dramatically different if it didn't exist. It is truly amazing to me that 10 years ago just over 50% of Americans had ever used the Internet. My school experience seems anachronistic, in that I actually had to research things in a library, using card catalogs to find books. In 7th or 8th grade I spent hours and hours trying to find basic information for a report on Indonesia. I looked through book after book to find and compile a small percentage of the information that is now on the Indonesia Wikipedia page. We are gathering the world's knowledge together, and we are figuring out how to use it. As computers get smarter, and as that data continue to pour in, the Internet will subsume more and more of our lives.
Each of our past advances has enriched the human experience. I am sure that there will be some bumps on the road, but I think that we can be confident in predicting that the Internet will again change for the better what it means to be human.