This question brings up an important point. Yet, simple empirical data seems to reveal that consciousness is a multifaceted entity that exists in degrees. Those of us who are familiar with dogs might be unwilling to accept the assertion that when Spot buries his bone in the backyard it is the action of a non-conscious being. It might be argued that both dogs and humans have consciousness, but that the quality of that consciousness differs. It is therefore more useful to examine the differences between the consciousness of lower and higher organisms than to search for the point in evolution when some imaginary consciousness switch was turned on.
Sensations exist in both lower and higher organisms.
The most basic aspect of consciousness appears to be sensation. It has been argued that consciousness in higher organisms evolved from sensation in lower organisms. Sensation can be defined as the ability of an organism to be aware of external circumstances through biological mechanisms that respond to those circumstances. Yet even a unicellular prokaryotic bacterium is able to sense chemical gradients. What higher organisms have evolved is the ability to process those sensations to a higher level.