THE CHEMICAL SENSES

Light travels virtually infinitely fast; sound travels more slowly but is still unable to linger in one spot for any length of time. In our efforts to gather useful information about the world out there, we really could use a source of information that sticks around for much longer. This is where the chemical senses – smell and taste – prove useful. Biological systems have developed an ability to detect certain types of molecule that convey information about sources of food, other animals and possible hazards and poisons. To appreciate this information, just watch a dog sniffing for a buried bone – or tracking the path taken by another dog. Here we have a source of information that comes with a level of persistence, a spatial memory of previous events. In humans, the sense of smell seems to be less developed than in other animals. But we do have a well-developed sense of taste, which tells us about the chemical composition of the food we eat and warns us (together with smell) of toxins, for example in food that is putrid. Clearly this is a very different type of information from that provided by light and sound. It requires physical contact or at least close proximity, but the information persists for much longer.