Accounting for the nature of consciousness appears elusive, with many claiming that it cannot be defined at all, yet defining it is actually straightforward.

What is common between the delectable taste of a favourite food, the sharp sting of an infected tooth, the fullness after a heavy meal, the slow passage of time while waiting, the willing of a deliberate act, and the mixture of vitality, tinged with anxiety, just before a competitive event?

All are distinct experiences. What cuts across each is that all are subjective states, and all are consciously felt. Accounting for the nature of consciousness appears elusive, with many claiming that it cannot be defined at all, yet defining it is actually straightforward. Here goes: Consciousness is experience.

That’s it. Consciousness is any experience, from the most mundane to the most exalted. Some distinguish awareness from consciousness; I don’t find this distinction helpful and so I use these two words interchangeably.

I also do not distinguish between feeling and experience, although in everyday use feeling is usually reserved for strong emotions, such as feeling angry or in love. As I use it, any feeling is an experience. Collectively taken, then, consciousness is lived reality. It is the feeling of life itself

But who else, besides myself, has experiences? Because you are so similar to me, I abduce that you do. The same logic applies to other people. Apart from the occasional solitary solipsist this is uncontroversial. But how widespread is consciousness in the cosmos at large? How far consciousness extends its dominion within the tree of life becomes more difficult to abduce as species become more alien to us.

One line of argument takes the principles of integrated information theory (IIT) to their logical conclusion. Some level of experience can be found in all organisms, it says, including perhaps in Paramecium and other single-cell life forms.

Indeed, according to IIT, which aims to precisely define both the quality and the quantity of any one conscious experience, experience may not even be restricted to biological entities but might extend to non-evolved physical systems previously assumed to be mindless — a pleasing and parsimonious conclusion about the makeup of the universe.

How Widespread Is Consciousness?

The evolutionary relationship among bacteria, fungi, plants, and animals is commonly visualised using the tree of life metaphor. All living species, whether fly, mouse, or person, lie somewhere on the periphery of the tree, all equally adapted to their particular ecological niches.

Every living organism descends in an unbroken lineage from the last universal common ancestor (abbreviated to a charming LUCA) of planetary life. This hypothetical species lived an unfathomable 3.5 billion years ago, smack at the centre of the tree-of-life mandala. Evolution explains not only the makeup of our bodies but also the constitution of our minds — for they don’t get a special dispensation.

Given the similarities at the behavioural, physiological, anatomical, developmental, and genetic levels between Homo sapiens and other mammals, I have no reason to doubt that all of us experience the sounds and sights, the pains and pleasures of life, albeit not necessarily as richly as we do. All of us strive to eat and drink, to procreate, to avoid injury and death; we bask in the sun’s warming rays, we seek the company of conspecifics, we fear predators, we sleep, and we dream.

While mammalian consciousness depends on a functioning six-layered neocortex, this does not imply that animals without a neocortex do not feel. Again, the similarities between the structure, dynamics, and genetic specification of nervous systems of all tetrapods — mammals, amphibians, birds (in particular ravens, crows, magpies, parrots), and reptiles — allows me to abduce that they too experience the world. A similar inference can be made for other creatures with a backbone, such as fish.

But why be a vertebrate chauvinist? The tree of life is populated by a throng of invertebrates that move about, sense their environment, learn from prior experience, display all the trappings of emotions, communicate with others — insects, crabs, worms, octopuses, and on and on. We might balk at the idea that tiny buzzing flies or diaphanous pulsating jellyfish, so foreign in form, have experiences.

Yet honey bees can recognise faces, communicate the location and quality of food sources to their sisters via the waggle dance, and navigate complex mazes with the help of cues they store in short-term memory. A scent blown into a hive can trigger a return to the place where the bees previously encountered this odour, a type of associative memory. Bees have collective decision-making skills that, in their efficiency, put any academic faculty committee to shame.

This “wisdom of the crowd” phenomenon has been studied during swarming, when a queen and thousands of her workers split off from the main colony and chooses a new hive that must satisfy multiple demands crucial to group survival (think of that when you go house hunting). Bumble bees can even learn to use a tool after watching other bees use them.

Charles Darwin, in an 1881 book on earthworms, wanted “to learn how far the worms acted consciously and how much mental power they displayed.” Studying their feeding behaviours, Darwin concluded that there was no absolute threshold between complex and simple animals that assigned higher mental powers to one but not to the other. No one has discovered a Rubicon that separates sentient from non-sentient creatures.

Of course, the richness and diversity of animal consciousness will diminish as their nervous system becomes simpler and more primitive, eventually turning into a loosely organised neural net. As the pace of the underlying assemblies becomes more sluggish, the dynamics of the organisms’ experiences will slow down as well.

Does experience even require a nervous system? We don’t know. It has been asserted that trees, members of the kingdom of plants, can communicate with each other in unexpected ways, and that they adapt and learn. Of course, all of that can happen without experience. So I would say the evidence is intriguing but very preliminary.

As we step down the ladder of complexity rung by rung, how far down do we go before there is not even an inkling of awareness? Again, we don’t know. We have reached the limits of abduction based on similarity with the only subject we have direct acquaintance with — ourselves.

Originally published at The Science Wire