Regrowing a missing limb is no big deal — to a starfish or salamander, creatures that are well-known for using regenerative "superpowers" to replace missing arms and tails. But they aren't the only animals that can rebuild body parts that are destroyed or damaged. Deer can re-sprout as much as 66 lbs. (30 kilograms) of antlers in only three months. Zebrafish can regrow their hearts, while flatworms have demonstrated that they can regenerate their own heads.
For humans, though, what's lost is lost — or is it?