Final answer:
Viruses and prions are not included in the tree of life because they are acellular and do not meet the criteria for living organisms, such as having cellular structure, metabolism, and the ability to reproduce independently.
Step-by-step explanation:
Viruses and prions are excluded from the tree of life primarily because they are not considered living organisms by the traditional scientific definition. Viruses are acellular and rely on a host's cellular machinery for reproduction, lacking self-sufficient metabolic processes and not made up of cells. Similarly, prions, which are infectious proteins without nucleic acids, defy conventional understanding of what constitutes a living organism.
The tree of life is a representation of the evolutionary relationships among various biological species, all known living organisms, which are cellular in nature. This is why viruses, with their parasitic lifestyle and lack of cellular structure and metabolism, do not fit within the tree. Although they have genetic material and can evolve, which are traits of living organisms, their inability to carry out life-sustaining processes independently excludes them.
Prions, on the other hand, are even simpler than viruses, as they consist solely of proteins and do not contain any genetic material. They are renowned for their role in diseases such as mad cow disease but are not placed on the tree of life due to their non-cellular nature.