Here's a breakdown:

* Common ancestor: This means that all life on Earth, from the smallest bacteria to the largest whales, can trace their ancestry back to a single, original life form.

* Descent with modification: Over vast periods of time, this ancestral life form diversified and evolved into the incredible variety of life we see today, with each new species inheriting traits from its ancestors, but also developing new ones through natural selection.

So, it's not just that organisms share similarities, but that those similarities stem from a shared evolutionary history.