80. Fossils and Extinction

Fossils are the "remains" of organisms from millions of years ago, which are found in rocks. They provide essential evidence for evolution by showing how species have changed over vast periods of time.

1. How Fossils Form

There are three main ways that fossils are created:

• Gradual Replacement by Minerals: This is the most common method. Harder parts of the body, like bones, shells, and teeth, do not decay easily. Over time, they are replaced by minerals as they decay, turning into a rock-like substance in the exact shape of the original structure.

• Casts and Impressions: * Casts: Formed when an organism is buried in soft material like clay. The material hardens around the organism, which then decays, leaving a hollow space (a cast) of its shape.

  • Impressions: Occur when an organism leaves a mark, such as a footprint or a burrow, in a soft material that eventually hardens into rock.

• Preservation (No Decay): This happens when the conditions needed for decay are absent (no oxygen, moisture, or warmth). Examples include organisms trapped in amber (hardened tree sap), tar pits, glaciers (too cold), or peat bogs (too acidic).

2. The Incomplete Fossil Record

While fossils give us a glimpse into the past, the record is incomplete for several reasons:

• Soft-Bodied Organisms: Many early life forms were soft-bodied and decayed completely without leaving a trace.

• Geological Activity: Many fossils that did form have likely been destroyed by volcanic eruptions, earthquakes, or the movement of tectonic plates.

3. Extinction

Extinction occurs when there are no remaining individuals of a species left alive.

Main Causes of Extinction:

• Rapid Environmental Change: Such as the destruction of a habitat or global climate change.

• New Predators: A new species may arrive and hunt a population to extinction (including human hunting).

• New Diseases: A fast-spreading pathogen can wipe out an entire species.

• Competition: A new species may be more successful at competing for food or resources.

• Catastrophic Events: Single massive events, such as the asteroid strike that killed the dinosaurs 66 million years ago.

Summary Table: Types of Fossils