The theory of evolution unifies all life on Earth by explaining its shared ancestry and the diversity of organisms we see today. It essentially states that all living things are connected through a common ancestor and have evolved over time through processes like natural selection.

Here's how it unifies life:

* Common Ancestry: The theory postulates that all life on Earth descended from a single, common ancestor. This means that all living organisms, from bacteria to humans, are related and share a common evolutionary history.

* Natural Selection: The driving force behind evolution is natural selection, where individuals with traits that make them better suited to their environment are more likely to survive and reproduce, passing those advantageous traits to their offspring. This process leads to gradual changes in populations over time, resulting in the diversity of life we see today.

* Shared Characteristics: The theory explains why organisms share certain characteristics, even if they are superficially very different. For example, the presence of DNA as the genetic material is common to all living things, a testament to their shared ancestry.

* Evolutionary Relationships: The theory provides a framework for understanding the relationships between different species, based on their shared evolutionary history. This information is used to create phylogenetic trees, which depict the evolutionary history of life.

In essence, the theory of evolution provides a unifying framework for understanding the history and diversity of life on Earth, connecting everything from bacteria to humans through their shared evolutionary past.