BOSTON—Evolution works with what it's handed. Take the human back, for example. It went from a horizontal connection between front and hind limbs to a vertical support for the upper half of the body. The resultant balancing act has led to chronic back pain for a huge percentage of the population. Science's Ann Gibbons spoke with Bruce Latimer and Jeremy DeSilva at the AAAS (publisher of ScienceNOW) meeting here about the tradeoffs we've made and the scars of human evolution .