I'm aware that the word 'evolution' is quite a controversial one, particularly in the USA. What I'm referring to here by the 'e' word is simply natural selection of populations -- I'm not attempting to address issues such as the origin of life and the nature of humanity. Unlike these deeper questions, natural selection isn't at all controversial. It's taking place all the time. The popular notion of natural selection is of 'survival of the fittest'. However, this needs some unpacking. What is referred to by 'fittest' here is not what we commonly think of as fitness, but instead is 'reproductive fitness'. The driving force behind natural selection is differential reproductive success of individuals and their kin. The idea is that if you have a particularly good set of genes that enables you to do better than others (in this sense, have more surviving children), these genes will be relatively increased in distribution in the next generation. With successive generations, the frequency of 'good' genes will increase, and less-good ones will disappear. In different, changing environments, different constellations of genes will provide better adaptation to these environments, and this is the driving force behind the change with time that is commonly known as evolution. If it all sounds very competitive and ruthless, well, it is. That's life. It's a jungle out there. And even if you have doubts about evolution at the broader level (origin of life, etc.), the data suggesting that natural selection is taking place all the time in natural populations are pretty compelling.

Now let's take a look at the evolution of taste and smell in humans. With human populations there is a complication, and this is that we are no longer subject to the same natural selection as we were many thousands of years ago. What we think of as the characteristics of humanity, such as our behaviour, social interactions and personality where shaped in what evolutionary scientists refer to as the environment of evolutionary adaptation (EEA). This shaping of humanity is thought to have taken place tens to hundreds of thousands of years ago, when our ancestors were likely hunter- gatherers on the plains of Africa. So if we are going to think about how our abilities of taste and smell were shaped by evolution, we have to go back in time to our hypothetical EEA. This is where it gets rather theoretical.

Let's do the following thought experiment. You are assigned the task of designing a sensory apparatus for human-like beings in an unpredictable, changing and highly competitive environment. What features and design tolerances would you opt for when it comes to their sense of taste and smell to give the their best chance of success? After all, this is what hundreds of thousands of years of evolution have been doing with the human senses.

The ability to discriminate between good and bad foods

Hard-wired aversions
First, these people are going to encounter unfamiliar foodstuffs. They need to avoid eating anything potentially harmful, while at the same time maximizing intake of available food. And, in order to keep as many food options open as possible, you are going to want to have only the minimum necessary set of hard-wired aversions. Having said this, there are certain flavour profiles that indicate danger, such as the smell of rotten or decaying food and the extreme bitter flavour of many poisonous chemicals, and it would be wise to make these extremely distasteful.

Predilections for 'good' foodstuffs
Likewise, people need to be drawn to the sorts of foods that are good for them. In a hunter-gatherer environment where calories are relatively scarce, you don't want your people wasting time eating food that is indigestible or has few calories. You want people to be drawn to foods with high energy content (sweet or fatty) -- just the sort of foodstuffs we are all busy avoiding in our western, food-surplus diets. Have you ever wondered why all the tasty foods are 'bad' for us? If we were all naturally drawn to low energy foodstuffs such as green salads, we'd have had a distinct competitive disadvantage in our EEA.

Learning and memory
On the whole though, you want your people to try as wide a range of potential foodstuffs as possible, with a well developed capacity for learning from these experiments what is good and what is bad. To this end, if an unfamiliar food is nutritious, you want your people to be able to acquire a taste for it. If a potential food item is tried, and makes you sick, then you'll want to have a strong aversion to that food coupled with good memory and accurate discrimination of its precise smell (preferably you don't want it to get as far as your mouth) -- or failing that its taste. Should an ingested food have noxious properties, you want your people to possess a good stomach clearance mechanism at the first sign of trouble -- the vomiting reflex will do nicely here. Humans are particularly good at vomiting, apparently. This goes some way to explaining why we generally have an excellent memory for smells, which is rather useful for wine tasting, too. It also explains the 'plastic' nature of our preferences: while we instinctively like some foods, others we gradually acquire a taste for. And its often these tastes that are only gradually acquired (such as fine wine) that are the most enduring and rewarding.

Reward and interest
This is probably most important of all. Just like sex, finding food -- whether it is by hunting or gathering or even agriculture -- is a potentially difficult, risky, and time and energy consuming pursuit. Yet if your people don't have a strong motivation for doing this, then they won't bother, and consequently will starve. So you need to take a carrot and stick approach. First, you must make the state of hunger an uncomfortable enough one that your people will feel motivated to get off their butts and go and get some food (the stick). Then you need to give them a strong reward for doing this (the carrot): eating must be a pleasurable sensation. Thus not only will a discriminating sense of taste and smell be useful in helping people choose good food from bad, it will also add greatly to the potential interest level of the food consumed. Have you ever wondered why wine tasting can be such a compelling and absorbing hobby? The ideas presented here give some sort of biological explanation. The ability of our gustatory and olfactory abilities to be trained, the excellent long-term memory we have for tastes and smells, and the reward systems that encourage us to seek out pleasurable taste sensations all must play a part.