Problem is, that increased immune vigilance has a side effect: allergies. “Our speculation is that this is some kind of trade-off, right? In the past you needed to resist some kind of pathogen, and the trade-off or sacrifice you have to make for that is some increased responsiveness to nonpathogenic allergens." So next time some of you get the springtime sniffles, blame your distant ancestor—the one with the heavy brow ridge.