Some 12,000 years ago, with the invention of farming, humans started a war against weeds — and the weeds are still a step ahead. As farmers advanced from using hard labor to protect their crops to using chemicals and genetic engineering, the weeds survived thanks to the oldest weapon known to living things: evolution.

Now, while scientists work on new technology to ward off the weedy menace, some worry they're speeding up the development of heartier, more herbicide-resistant foes.

Weeds may seem benign compared to crop-eating insects, but they pose a major threat to agriculture. They compete for scarce resources with crops, sucking water and nutrients out of the soil they share. Ton for ton, farmers use more weed killers than any other kind of pesticide. Without weed control, some crop yields would be cut in half.