Check this out (Thanks Steve!)

Insight into how we tell whether something’s alive

When viewers see the unscrambled pictures, they readily discern whether the point-light display represents a living thing or a random moving pattern. In fact, the task is so easy that it’s not actually very useful for researchers trying to understand the visual system. What Chang and Troje want to know is whether viewers use a “local” system or a “global” system to identify biological motion. In other words, are viewers looking at an isolated part of the display like the human’s ankles, or are they considering the concerted motion of all the points together?
…

Other research has found that the motion of the ankle appears to be a key in identifying biological motion. This may be because nearly all walking vertebrates swing their legs forward in a similar manner: they don’t actually use their muscles, but instead simply rely on gravity, thus conserving energy. Chang and Troje speculate that perhaps it is this distinctive arc that viewers focus in on when they identify biological motion.