Charles Darwin's observations of fossils that looked like ancient versions of living species led him to suggest that Earth was very old.

Here's why:

* Fossil Record: The fossil record showed a progression of life forms over time, with simpler organisms appearing in older rock layers and more complex organisms appearing in younger layers. This suggested a long history of life on Earth.

* Gradual Change: Darwin observed that fossils often resembled modern species, but with subtle differences. This suggested that life had changed gradually over time, a process he called "descent with modification."

* Extinction: Darwin also observed that many fossil species were extinct, implying that life forms could go extinct and new forms could arise. This again pointed towards a long and dynamic history of life on Earth.

These observations contradicted the prevailing view of a young Earth, which was based on religious interpretations of the Bible. Darwin's work helped lay the groundwork for our modern understanding of Earth's vast age and the slow process of evolution.