Two weeks later the mice ran the maze again, but this time nobody got the bacteria. The experimental mice were still quicker than the controls, but this time they weren't like greased lightning. After three weeks' rest the test was conducted again, without the bacteria. The experimental mice were still a tad faster, but not significantly different.
So the effect was temporary. But why did it work at all?
The bacteria stimulates a group of neurons within the brain to start secreting serotonin in the forebrain area, the part of the brain that's responsible for anxiety and for higher order thinking, and the formation of memories, Matthews said. She suspects its probably part of our evolution.
"For most of our evolutionary history we were hunter-gatherers, rooting around for vegetables, and following animals," she said. "More recently we were agricultural and also having a lot of contact with the soil and with nature," and thus exposed to the bacteria on a regular basis.
What's particularly neat about this work is there isn't any need for some company to bottle the bacteria and sell it to us. The bacteria used in this experiment was live, unlike bacteria that might be found in a supplement, and it's the same stuff we get when we rake the leaves.
"It's healthy for us to have contact with nature," said Matthews, an avid gardener. "Maybe we should design our schools and our living spaces to put us more in contact with nature. It's a healthy thing to do."
So no one needs to eat dirt. Besides, if eating dirt makes us smarter, wouldn't we all be smart enough to eat more dirt?
The better answer, Matthews said, is to take a walk through the woods.