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Any self-respecting pet owner will confidently claim that their dog or cat (or rabbit, or gerbil) seems sentient, exhibiting a distinct temperament and emotional responses. I know my many beloved pets over the years could feel pain, and fear, as well as love and trust. But are our pets truly conscious creatures? Or are we merely projecting our own thoughts and feelings onto our animal companions?

It likely depends on what you mean by consciousness. On one end of the scale there is basic wakefulness and sentient awareness—which we share with all living creatures—and on the other, more sophisticated end there is self-awareness. But there are many other systems and terms advanced by various researchers to categorize and describe consciousness. Part of the confusion stems from the great complexity, largely still unknown, of how the brain gives rise to consciousness.

For a long time, scientists assumed that having a neocortex (the outer, and more recently evolved, part of mammals’ brains) was necessary to be truly conscious in the human sense—not merely aware of one’s surroundings (sentient) but also self-aware. But there’s good news for animal lovers, because that view is changing.

When we are just over one year old, we can look into a mirror and recognize the reflection as being “us,” and the brain maps that visual input, tracking the changes in our size and appearance over the years. In the 1970s, Gordon G. Gallup devised a mirror test, in which he marked the face of an animal as it slept. Once it woke and saw its reflection in a mirror, if the animal tried to wipe away the mark, Gallup took this as evidence of a certain degree of self-awareness. Animals that have passed the mirror test include chimpanzees, orangutans, gorillas, dolphins (video), and elephants, all of which possess a neocortex—and magpies, which do not. So it turns out that a neocortex might not be essential after all.

Maybe the roundworm is more sentient than we think, philosophizing about whether or not those giant humans exhibit consciousness.

Recognizing that we do share some basic neural structures and functions with many animals, a number of prominent neuroscientists signed a public statement in August 2012 to that effect during a conference on animals and consciousness. The so-called Cambridge Declaration on Consciousness [pdf; YouTube video] concluded that, while not self-aware in the human sense, “non-human animals”—including mammals and the octopus—possess sufficient brain structure and function to generate conscious states. (One signatory, Philip Low, has since become a vegan.) The Cambridge declaration isn’t talking about advanced mega-cognition or self-awareness; its focus is on what might be better termed self-representation, or the ability to distinguish between self and other.

But while the declaration suggests progress in understanding animal consciousness, the definitions and distinctions remain tricky. Consider C. elegans. The humble nematode, or roundworm, has a mere 302 neurons, barely sufficient to qualify as a functioning brain, but part of one of those neurons is devoted to distinguishing its own body from the world around it. In essence, C. elegans has a “protoself”: It can distinguish between its own movement and movement in its environment.

This does not mean that the nematode could recognize itself in a worm-sized mirror, or has a conscious sense of self on a par with human beings, although it’s not like we can ask C. elegans directly. How do we know the little creature doesn’t have a rich interior life? Let’s just say this possibility seems unlikely, given what we know about its neuroanatomy. It is the number of neurons that matters most when it comes to the richness of self-representation, and the nematode doesn’t have that many.

Neuroscientists have not yet pinpointed the specific mechanisms behind how the brain produces a distinct, persistent, subjective personal identity. That’s a much tougher neuro-nut to crack. There is a fundamental layer we seem to share with animals, onto which human beings add a second, richer layer of self representation. That extra layer of meta-cognition might just be what separates animal and human consciousness.

Or not. Thomas Nagel famously opined that we could never really know what it’s like to be a bat, for instance, regardless of what we can observe about its behavior and the structure of its brain. Maybe C. elegans is more sentient than we think, philosophizing about whether or not those giant humans exhibit consciousness—when it’s not getting down with its bad self, doing the Harlem Shake.