6 ideas
Subjective experience did not switch on at a single threshold but arose by degrees as organisms became integrated agents with a point of view. Because there is no sharp line, the question 'which animals are conscious?' has many borderline answers, and forcing a yes/no misdescribes the biology.
The explanatory gap looks unbridgeable when experience is set against inert matter such as neurons treated as circuitry. Seen instead as a feature of metabolically active, self-maintaining, sensing-and-acting bodies, experience looks less like an extra ingredient and more like what such activity is like from the inside.
Early nervous systems may have evolved less to relay sensory information than to coordinate the whole body into a single agent, as in the synchronized muscle contraction of cnidarians. On this view the first job of neurons was to hold together a multicellular self that could act as one unit.
Once an animal's own movements change what it senses, it must distinguish self-caused from world-caused input, for example through efference copies. This loop between acting and perceiving, which intensified in the Cambrian with mobile, eyed predators and prey, is a key driver of a unified point of view.
Octopuses evolved large brains and flexible behavior on a branch that split from ours more than 500 million years ago, with most of their neurons distributed in the arms. They show that complex minds evolved at least twice, along very different architectural lines. They also raise the question of whether a single body can host partly independent centers of control.
Experience has at least two separable strands: sensing or registering the world, and evaluating states as good or bad through feelings such as pain and pleasure. These strands can come apart. Arthropods and fish show wound-tending and trade-offs suggesting evaluative feeling, while sponges and plants sense without clear evidence of felt valence.