Humans are the only creatures that cry for both grief and happiness, although many animals shed tears that help protect their eyes. As science journalist and former CNN bureau chief Walter tells readers in this fascinating and superbly written book, there are a handful of characteristics (like crying) that distinguish us from the rest of the animal kingdom and can be explained in evolutionary terms as having been advantageous for our distant ancestors. Laughter is one: dogs may bark happily when they get to go for a ride or play with their canine neighbors, but only humans break into chortles and guffaws. Walter (who coauthored I'm Working on That with William Shatner) says that laughter helps us bond with our friends and co-workers. He points out that we give our big toe little thought until we stub it, but its evolution allowed Homo erectus to stand upright millions of years ago and led to other helpful evolutionary features, like the pharynx—which in turn made speech possible. Readers also learn why we tousle our children's hair, why kissing is so much fun and what may lie ahead as we near the end of our current evolutionary reel. (Nov.)