Biodiversity, a history in motion

Biodiversity changes over time

A biodiversity that has never been fixed

Biodiversity refers to the set of living species present on Earth at a given moment, as well as the genetic diversity within them. You might think this biodiversity is stable, but the history of life shows exactly the opposite: it has kept changing ever since the first organisms appeared, about 3.8 billion years ago.

Species that appear and disappear

Over time, species have appeared, and others have disappeared. It is estimated that more than 99% of the species that have ever existed on Earth are now extinct, such as the non-avian dinosaurs, which disappeared 66 million years ago. These mass disappearances, called extinctions, have sometimes been followed by periods of rapid diversification, with the appearance of many new species occupying the environments left vacant.

Evolution, an established scientific fact

The word evolution precisely refers to this gradual transformation of species across generations. It is not a mere hypothesis: it is a fact solidly established by numerous independent pieces of evidence (fossils, anatomical comparisons, genetics), which you'll discover in the next lesson.

A time scale not to be confused

It's essential to clearly distinguish two time scales:

on the scale of a human lifetime, the species around us seem fixed;
on the scale of millions of years, they continually transform.

It is this slowness, invisible on a human scale, that explains why evolution was long difficult to perceive directly, and why it relies on a body of indirect evidence rather than immediate observation.

Remember the key point