Selfhood seems to be an evolutionary advance on basic consciousness. Most animals demonsrate behaviours which indicate some level of consciousness in terms of awareness of their surroundings and response to stimuli. It's hard to tell with plants, admittedly.

The more intelligent an animal is, the more of sense of self its' behavours demonstrate. This is more notable in pack, herd or troop animals, where each individual tends to develop its' own personality and to build different relationships with ther individuals.

Therefore, we can see that consciousness and a sense of self are not unique to humans, and while currently not fully understood, there can be no doubt of its' existence as simple observation shows it to be fact.

The problem arises from the inability to determine a physical source or seat for this sense of self. Science has been concerned with material things for so long that it lacks, or has never developed, the tools to tackle the immaterial. Metaphysics and philosophy largely come down to matters of opinion, informed or otherwise.

Whether consciousness arises from the brain, or whether the brain is merely the connection point between the body and another, non-material, element of the living organism is the basis of the 'hard problem'.

It my be that one day a physicist will step far enough out of their culturally-defined box to consider investigating the quantum state(s) of the brain and perhaps find an answer there. Or the answer may be elsewhere.

Meanwhile, far too many people are reluctant to simply admit "We don't know".