All instances of behaviour require an explanation at each of these four levels. For example, the function of eating is to acquire nutrients (which ultimately aids survival and reproduction), but the immediate cause of eating is hunger (causation). Hunger and eating are evolutionarily ancient and are found in many species (evolutionary history), and develop early within an organism's lifespan (development). It is easy to confuse such questions – for example to argue that people eat because they're hungry and not to acquire nutrients – without realizing that the reason people experience hunger (causation) is because it causes them to acquire nutrients (function).