The concept you're describing is evolution. It's the idea that all living species have descended, with modifications, from common ancestors over long periods of time. This process is driven by natural selection, where individuals with traits better suited to their environment are more likely to survive and reproduce, passing those traits on to their offspring.

Here's a breakdown of the key points:

* Descent with Modification: Organisms inherit traits from their ancestors but with slight variations. These variations can be beneficial, neutral, or harmful in terms of survival and reproduction.

* Natural Selection: Individuals with advantageous traits are more likely to survive, reproduce, and pass those traits on, leading to a gradual shift in the characteristics of a population over time.

* Common Ancestry: All living organisms share a common ancestor, meaning we can trace back the evolutionary history of any species to a single point in time.

Evolution is a fundamental principle in biology and is supported by a vast amount of scientific evidence, including:

* Fossil Record: The sequence of fossils shows a gradual change in organisms over time, documenting the transition from one species to another.

* DNA and Genetics: Comparing DNA sequences across species reveals shared ancestry and evolutionary relationships.

* Biogeography: The distribution of species across the globe reflects their evolutionary history and the processes of migration, adaptation, and isolation.

* Direct Observation: Scientists have observed evolution happening in real-time, such as the development of antibiotic resistance in bacteria.

While the concept of evolution can be complex, it's a cornerstone of modern biology and helps us understand the diversity of life on Earth.