What Exactly Is a Fossil?

Fossils are one of the most fascinating clues we have to Earth’s deep past. They allow us to look back millions (sometimes hundreds of millions) of years to understand what life once looked like, how it evolved and how our planet has changed over time. But what exactly IS a fossil and why are they so important?

Perisphinctus Ammonite

What Is a Fossil?

A fossil is any preserved evidence of past life, usually found in rock. This evidence can be the remains of a once-living organism or traces of its activity. Most fossils are millions of years old, formed long before humans existed. Fossils are not always bones. In fact, many fossils don’t contain any original organic material at all. Instead, they are often mineral replacements or impressions that preserve the shape and structure of ancient life.

Amber with insect inside

What defines something as a fossil?

To be considered a fossil, it generally has these key features:

• It comes from a living organism

  This can be a plant, animal, or microorganism.

• It is preserved naturally

  Preservation can happen in rock, ice, amber (tree resin), tar or sediment.

• It is from the distant past

  Fossils are typically thousands to millions of years old, not recent remains.

  
  

Things like modern shells, recently buried bones or museum replicas (always remember to check the description as many are replicas) are not fossils, even if they look similar.

Different Types of Fossils

Fossils come in several forms, each telling a different story about the past:

Cast and Mould Fossil - Ammonite

Cast Fossils

Cast fossils form when an organism leaves an impression in sediment, called a mold, which later fills with minerals. This creates a three-dimensional replica of the original organism, preserving its shape in remarkable detail.

Body Fossil - Spinosaurus Tooth

Body Fossils

Body fossils are the actual remains of an organism, such as bones, teeth, shells or exoskeletons. These fossils often undergo mineralisation, where minerals replace organic material, preserving the structure of the organism for millions of years.

Petrified Fossil - Wood

Petrified Fossils

Petrified fossils occur when the organic material of plants or animals is gradually replaced by minerals, essentially turning them into stone. Petrified wood is a classic example, showing even microscopic details of the original plant tissue.

Carbon Fossil - Redwood Plant

Carbon Fossils

Carbon fossils form when organisms are buried under layers of sediment and heat and pressure drive off liquids and gases, leaving a thin carbon residue that outlines the organism. These are often seen in plant fossils or delicate creatures, capturing fine details of leaves, stems, or soft-bodied organisms.

How Do Fossils Form?

Fossilisation is rare. Most organisms decay or are eaten before they can be preserved. Fossils typically form when:

1. An organism dies and is rapidly buried by sediment (mud, sand or ash)

2. Oxygen is limited, slowing decay

3. Over time, pressure and minerals replace or surround the remains

4. The sediment hardens into rock, preserving the fossil

   
   

This process can take thousands to millions of years, its estimated around 0.1% of living things actually fossilise!

Knightia Fossil Fish

Fossils are essential to our understanding of life on Earth because they provide a window into the past. They allow us to reconstruct ancient ecosystems, understand how species evolved & went extinct plus track climate & environmental changes over millions of years. Fossils also reveal how continents and oceans shifted and show how life recovered after mass extinctions. Without them, much of Earth’s rich biological history would remain completely unknown.

Fossils provide the strongest physical evidence for evolution. Transitional fossils show how species gradually changed over time, linking ancient life forms to modern animals and plants. They help explain how complex life developed and adapted to changing environments.

Each fossil is a snapshot of a moment in deep time, a leaf that fell into mud, a fish that sank to the bottom of an ancient lake or a footprint left on soft sediment millions of years ago. Fossils connect us directly to Earth’s deep history, revealing how life began, survived, adapted and sometimes disappeared.

They remind us that our planet is constantly changing and that we are just a very small part of a much bigger story.