The reason humans die physically is because they have fallen away from nature and violated its laws. To survive, humans must relearn the harmonic balance with nature. This is the next goal for humanity. All souls are equal at the spirit level. The spirit doesn't evolve, the personality of the soul does.
Humans have a group soul, but have also evolved to have an individual conscious.
Animals have only a group soul and reincarnate in groups. The races of humanity are actually personality clusters. Nations and cities also form their own clusters. Families have group souls. We are beings of light encased in physical bodies.