Imagine a world where everyone's consciousness has been uploaded to computers. People play sports, knit sweaters, whatever- all in virtual reality. It's much safer and cheaper than maintaining a physical body, after all. Now each person might have some real human friends, but they're more likely to have AI friends who are smarter, more charming, more interesting. Or maybe there's just a lot more of them. You may not even know which of your friends are "real." It might be considered insulting to ask. You spend more and more time playing alone or tittering with a circle of sycophantic AIs.

Over time people decide to pass on by ritualistically committing data-suicide, or overdose on data-drugs, or get squeezed out by increasing scarcity of computing resources, or whatever. And so the species gradually dies out until you just have some AI routines running in a hot server room on a dead planet. The last human might not even realize that she's the last.

The same situation could occur without mind uploads if you could build very realistic androids, but I like the mind upload scenario.

And of course this is all very chauvinistically assuming that those AI routines slash robots don't qualify as human even though they're indistinguishable. The AIs could be conscious, or not, I suppose, depending on how they're implemented.