Darwin's theory of common descent states that all organisms, from the simplest bacteria to the most complex animals, share a common ancestor. This means that all life on Earth is related, and we can trace back the evolutionary history of all species to a single, ancestral organism.

This idea is supported by a wealth of evidence, including:

* Fossil records: Fossils show a clear pattern of change over time, with simpler organisms appearing earlier and more complex organisms appearing later.

* Comparative anatomy: Similarities in the skeletal structures of different animals, like the bones in a bat's wing and a human arm, suggest common ancestry.

* Molecular biology: The genetic code is nearly universal across all life forms, further indicating a shared ancestry.

It's important to note: Darwin's theory of common descent does not say that humans evolved from monkeys or apes. Instead, it states that humans and primates share a common ancestor that lived millions of years ago. Over time, different lineages evolved into the various species we see today.