As one continues to descend the evolutionary ladder, the plausibility of extrapolating
from human to non-human animal consciousness becomes increasingly remote.
There may, for example, be critical transition points in the development of
consciousness that accompany critical transitions in functional organisation (Sloman,
1997). Self-awareness, for example, probably occurs only in creatures capable of self-
representation. That said, phenomenal consciousness (of any kind) might only require
representation. If so, even simple invertebrates might have some rudimentary
awareness, in so far as they are able to represent and, indeed, respond to certain
features of the world.
Planarians (flat worms) for example, can be taught to avoid a stimulus light if it has
been previously associated with an electric shock (following a classical conditioning
procedure). And simple molluscs such as the sea-hare Aplysia that withdraw into
their shells when touched, respond to stimulus "novelty." For example, they may
habituate (show diminished withdrawal) after repeated stimulation at a given site, but
withdraw fully if the same stimulation is applied to another nearby site. Habituation in
Aplysia appears to be mediated by events at just one centrally placed synapse between
sensory and motor neurons. This is very simple learning, and it is very difficult to
imagine what a mollusc might experience. But if the ability to learn and respond to
the environment were the criterion for consciousness, there would be no principled
grounds to rule this out. It might be, for example, that simple approach and avoidance
are associated with rudimentary experiences of pleasure and pain.
Is consciousness confined to brains?
It is commonly thought that the evolution of human consciousness is intimately linked
to the evolution of the neocortex (e.g. Jerison, 1985)—and it seems likely that cortical
structures play a central role in determining the forms of consciousness that we
experience. However, whether consciousness first emerged with the emergence of the
neocortex or whether there is something special about the nature of cortical cells that
somehow “produces” consciousness is less certain. As Charles Sherrington has
pointed out, there appears to be nothing special about the internal structure of brain
cells that might make them uniquely responsible for mind or consciousness. For,
"A brain-cell is not unalterably from birth a brain-cell. In the embryo-frog the cells
destined to be brain can be replaced by cells from the skin of the back, the back even
of another embryo; these after transplantation become in their new host brain-cells
and seem to serve the brain's purpose duly. But cells of the skin it is difficult to
suppose as having a special germ of mind. Moreover cells, like those of the brain in
microscopic appearance, in chemical character, and in provenance, are elsewhere
concerned with acts wholly devoid of mind, e.g. the knee-jerk, the light-reflex of the
pupil. A knee-jerk ‘kick’ and a mathematical problem employ similar-looking cells.
With the spine broken and the spinal cords so torn across as to disconnect the body
below from the brain above, although the former retains the unharmed remainder of
the spinal cord consisting of masses of nervous cells, and retains a number of nervous
reactions, it reveals no trace of recognizable mind.... Mind, as attaching to any
unicellular life would seem to be unrecognizable to observation; but I would not feel
that permits me to affirm that it is not there. Indeed, I would think, that since mind
appears in the developing source that amounts to showing that it is potential in the
ovum (and sperm) from which the source spring. The appearance of recognizable