Remember that soul is a portion or unit of universal consciousness, or mind and spirit; they are synonymous. Life is spirit, and soul is that aspect of spirit inherent in each living entity. Soul, mind, and spirit are substantive in a form of energy that most likely works through gravity, although this is very speculative.
There are two ways that evolution can be thought of in terms of an active process. The first is on a large scale from the beginning, in the Darwinian sense, to the present, and the second on how it works through human spiritual evolution, although both follow the same initial path, and can be anticipated into the future.
In the beginning, the Idea, inherent in cosmic consciousness, came to work in combining the appropriate material to hold a tiny spark of itself given from within a star. This anatomically simple structure, under creative guidance and the laws of physics, could be built upon through time and the inertial energy within the spark. Soul came into being, probably in a ratio something like one part spirit to a hundred parts matter. Over time, as the entity evolved, the ratio began to change. Simple organisms soon reached a ratio of ten parts soul/mind to ninety parts matter. Plant life probably came into range within something like 20 to 50 percent, and animal life 50 to 80 percent. Evolution is the path to refinement of soul to a purer state. When human beings came upon the scene physical evolution became less involved, now the focus centered on the continued growth of the soul. This is the second way that spiritual evolution becomes active. The human entity now comes to represent probably 80 to 95 percent spirit, and less than 5 percent matter.
Once man came out of the cave, learned to make fire, and then the wheel, he had taken a separate route from that of the animal kingdom. That long path from the beginning is still in full operation, but through reproduction mankind is still an heir to his past heritage, and is born with his share of spiritual growth. But the human soul is still short of completion, and this is where Adam and Eve begin their journey for those of the Christian faith. Evolution now becomes dependent on ethics, values, morals, decisions, and choices. We have the means to fashion the future for the entire world.
But what of that last 5 percent of growth yet to be attained? Many already have reached that level as far back as Jesus Christ, or Gautama the Buddha, in this day and age maybe Gandhi, or Mother Teresa. Once souls reach 100 percent they no longer have any attachment to the physical plane. One might say they "bud" off from the system that gave them their beginning. But what do they do? Ah, their work is just beginning, far far away.