Evolution Theory simultaneously proposed by Charles Darwin and Alfred Russell Wallace. Both are credited with the theory. All life is based on nucleic acids and use a triplet code to produce proteins. All life has similar metabolic processes and is carbon-nitrogen based. This is because all life is related and is the result of change over time, evolution.
Species evolve into other types or die out.
Because species are related, even if the relationship is distant, we can apply what we know about a one species to another. This is what allows us to study cancer in sharks to develop cancer prevention in humans. This is what allows us to study genetic control of development in insects and use that knowledge to devise treatment for human disease. We can learn from one species and apply it to another because all life is related. What is debated and not totally clear is the mechanism of evolution.