Interesting, my first thought was similar but different.

Clothing.

Now I have to go poke around the Internet trying to understand the history of both, which came first, and speculate about which made a bigger impact on our species.

edit:

Yep, it was fire. By like a lot. Both have pretty big ranges, but fire seems to be in the hundreds of thousands of years ago range, and clothing seems to be in the dozens of thousands of years.