Complex sentence: Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection; however, they lack key characteristics (such as cell structure) that are generally considered necessary to count as life.
Split into simpler sentences: Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection. However they lack key characteristics (such as cell structure) that are generally considered necessary to count as life.