Here's why:

* Common ancestor: This refers to an ancestral organism from which different species evolved. It's not necessarily a single organism, but a population of organisms that shared a common set of traits.

* Early form of an organism: This could refer to any organism in the past, but doesn't specifically imply a relationship to later forms.

Examples:

* The common ancestor of humans and chimpanzees is a primate species that lived millions of years ago.

* The common ancestor of all life on Earth is a single-celled organism that existed billions of years ago.

Let me know if you have any other questions about evolution!