If you wanted to build a muticellular organism, it stands to reason that you would need to include an array of sensors that allowed this complex organism to detect its environment so it could respond. In our case, we have many senses. Consider the sense of smell and taste.

Smell and taste are very similar, in that both senses detect chemicals in the same way. With the sense of smell, molecules (which we call odorants) bind to receptors on the cilia of receptor cells in the roof of the nasal cavity. With the sense of taste, molecules (which we call tastants) bind receptors on the cilia of receptor cells in taste buds in your tongue. If enough tastants or odorants bind to the receptor, this triggers an electrical signal, known as an action potential, that is sent to the brain. The brain is thus notified about what is in your mouth or nose.

With taste, there are four primary taste sensations – sweet, sour, bitter, and salty – where each sensation represents to class of molecules (for example, sour represents acidity). Of course, the food we taste is not simply a combination of these four taste sensations (which sensation does steak taste like?). The food we taste is a combination of signals that some from both the taste buds and the more diverse receptors in the nasal cavity. This is why food tastes like it smells and why we can’t taste most foods when we have a head cold (the receptors in the nasal cavity are buried with inflammed tissue and secretions).

The two senses are also similar in that they both undergo rapid adaptation. For example, if you walk into a room and it stinks, you will stop noticing the stink after remaining in the room for some time. The stinky molecules remain in the room, but your olfactory receptors in your nose stop sending signals to the brain. Your brain, which must determine whether you will react to changes in the environment, is most concerned with new smells and tastes.

Both taste and smell receptors behave as sentries. Since the nose and mouth act at the interface of your inner body and the environment, we need some sensors that not only inform us about the environment, but whether the contents of that environment are helpful or threatening to our body. Things that smell or taste bad will trigger an avoidance response.

So how would a designer front-load such a sentry system into a single cell, something without a nose, mouth, or brain?

Once again, Tetrahymena serves as a nice model for thinking about front-loading, as a recent study measured whether or not Tetrahymena would detect and react to odorants and tastants commonly detected by vertebrates. [1]

Among the molecules analyzed were as follows:

menthol - cool taste
capsaicin – hot; active component of chili peppers
carvacrol - oregano
eugenol - cloves
piperine - black pepper
chloroquine – bitter
denatonium benzoate - very bitter
allyl isothiocyanate – horseradish

When these molecules are added to Tetrahymena’s media, they all trigger changes in the swimming behavior of the cells. In other words, the single-celled Tetrahymena also “smells” or “tastes” these molecules.

What’s more, Tetrahymena show adaptation to carvacrol, eugenol, quinacrine, and capsaicin. And previous work has shown that the altered swimming behavior is associated with generating actions potentials. So the basic foundation for the sense of smell and taste are represented in this single-celled organism: the ability to couple the binding of specific odorants and tastants in the environment with the generation of action pontentials, altered behavior, and sensory adaptation.

Remember the basic problem that a front-loading designer has – how does one design nonexistent multicellular organism for the future through a unicellular lifestyle? The logic of the design plan is laid out in my book, The Design Matrix. Yet by considering Tetrahymena alone, we can see how the foundations for both the endocrine system and two of the special senses could be implanted in a single-celled organism, such that this information could then serve as a seeing-eye dog for the blind watchmaker during subsequent evolution.

1. Rodgers LF, Markle KL, Hennessey TM. 2008. Responses of the ciliates tetrahymena and paramecium to vertebrate odorants and tastants. J Eukaryot Microbiol. 55:27-33.