(7) Evidence of evolution of life from fossils and other sources

Human evolution is specifically dealt with in Part 8

Introduction

Know and understand that evidence for early forms of life comes from fossils.

Know and understand that fossils are the 'traces' or ‘remains’ of organisms from many years ago, which are found in rocks which we find dating back thousands, millions, and even billions of years ago (it is thought life began 3.4 to 3.8 billion years ago, age of the Earth is ~4.5 billion years).

If fossils are well preserved they provide a wealth of information of the structure of the organism.

Generally speaking, in layers of sedimentary rock, the deeper the layer, the older the rock, hence the older the fossils.

It is therefore possible to arrange fossils in a chronological order and observe the gradual changes in the form of an organism.

These changes in fossil structure provides good evidence for evolution as it shows how species change and develop over time from thousands to millions of years.

The fossil record shows the emergence of new species and indicates extinctions when a species disappears from the fossil record.

 

Fossils may be formed in various ways:

(i) Fossils form from the hard parts of animals that do not decay easily eg bones, shells or teeth, but may stay buried as the original material for quite some time, though eventually most become replaced by surrounding minerals (*).

So human bones might still be bones after a few thousand years, but dinosaur bones will be rock like after over 65 million years. The surrounding sediments might turn to a hard rock like limestone or sandstone, but the fossil structure can still be observed in its distinct detailed shape.

(*) Parts of the organism are very gradually replaced by other materials as they decay - eg mineralisation from surrounding sediments of sand or shale layers.

Fossils of sea creatures in sedimentary limestone rock.

Oxford University Museum of Natural History: This dinosaur skeleton fossil looks a fast mover and a carnivore!

 

(ii) Fossils can form from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent e.g.

Insects in solidified amber resin (no air/microbes can get in), amber is itself a clear yellow stone formed from fossilised resin from a tree. The sticky resin traps the insect. The preservation detail can be quite remarkable.

Glaciers or permafrost ground (too cold for decay microorganisms to function), preserves the bodies of animals - woolly mammoth in the permanent frosty ground of Siberia in Russia, an iron age man found high in an alpine pass.

More of such 'bodies' are likely to emerge with global warming!

Very acid peat bogs where the pH is too low for decay microbes to function. The calcium based mineral bones dissolve rapidly in the acid water but flesh and human clothing/animal coats can be preserved in a sort of 'mummified' state - see picture below of a 'bogman'.

 

(iii) Fossils occur as preserved traces of organisms by way of casts and impressions in sedimentary rock layers, eg footprints, burrows and rootlet traces.

An organism may get buried in soft material like clay or shale. The enclosing mineral material hardens around it so that when the organism decays a cast of it is left in the sedimentary rock formed.

You even find fossil footprints of dinosaurs or human beings left in soft sand or mud that became dry and buried under further layers of sand, eventually compressed into a sedimentary rock.

As erosion takes place, footprints can appear!

 

When arranged in chronological order, a series of fossils can show the gradual development changes in an organism - this is powerful evidence for evolution.

The evolution of the horse from the fossil record is illustrated on ...

https://en.wikipedia.org/wiki/Evolution_of_the_horse

Good diagram on the evolutionary record of the human skull (of importance to brain capacity and function) is on

https://www.bradshawfoundation.com/origins/short_story_of_human_evolution.php

 

BUT, be able to explain why there are gaps in the fossil record, including:

a) because fossils do not always form

b) because soft tissue decays

c) because many fossils are yet to be found

The fossil record is incomplete for several reasons e.g.

only a small proportion of organisms (creatures or plants) have by chance become fossilised, and same may be of an unsuitable structure to become fossilised,

many early forms of life were soft-bodied (very soft tissue), which means that they have left few traces behind,

and there are still lots other fossils have still to be discovered, in fact new species are being regularly discovered,

and many fossil traces have been destroyed by geological activity (volcanic, erosion etc.).

 

Know and understand that many early forms of life were soft-bodied, which means that they have left few traces behind.

What traces there were have been mainly destroyed by geological activity.

The fossil record is incomplete for other reasons e.g.

only a small proportion of organisms (creatures or plants) have by chance become fossilised,

and there are still lots other fossils have still to be discovered, in fact new species are being regularly discovered.

 

Know and understand that we can learn from fossils how much or how little different organisms have changed as life developed on Earth.

From fossils we can get some idea on what the animals and plants looked like e.g. general shape, skeletal structure and sometimes, though very rarely, detail of  internal organs.

Generally speaking, the deeper the layer of rock containing fossils, the older the fossils and this means we can follow the development and evolution of a species or the origin of new species by looking at similarities and differences, but its the gradual changes in the structure of plants and animals over millions of years that shows the evolutionary path of a species.

 

We know from the fossil record that many species don't exist today - they have become extinct.

Know and understand that extinction may be caused by:

changes to the environment over geological time - eg think of plate tectonic movement over millions of years from warm equatorial areas to cold arctic areas of the Earth's surface,

... species may adapt or change significantly over a long time ...

BUT, changes in environment-climate can be quite fast and species might not be able to adapt in time!

 

new predators - one species can consume another!, we humans have been responsible for many extinctions by 'over hunting'!

A classic example is the now extinct dodo bird became extinct on the small island of Mauritius. It was not only hunted (initially by Dutch sailors in the late 16th century), they also introduced dogs, pigs and rats, all of whom developed a taste for dodo eggs! no competition!!!

 

new diseases - eg an animal's immune system unable to cope with a new mutant bacteria or virus,

 

new, more successful, competitor for food invading a particular habitat,

 

a single catastrophic event, eg massive volcanic eruptions, collisions with asteroids (huge impact 65 million years ago may be responsible for the extinction of dinosaurs), onset of an ice age - through the cyclical nature of speciation - the evolution of a new species better able to cope with a rapid change in climate conditions.

 

Explaining how the anatomy of the pentadactyl limb provides scientists with evidence for evolution.

A pentadactyl limb is a hand or leg limb with five digits e.g. like your human hand or foot.

Pentadactyl digits are found in many species of animals ranging from mammals, amphibians, birds and reptiles.

Many pentadactyl limbs in these animals (especially in mammals) have a very similar bone structure, but not necessarily evolved and used for the same function e.g. examples of different functions of a pentadactyl limb ,,,

a human or monkey's hand is used for grasping,

a dolphin's fin is adapted for swimming

a mole's feet are adapted for digging,

a bat's 'hands and feet' supports the wings for flying

This suggests, that all these species exhibiting a pentadactyl limb, with similarity in bone structure, all evolved from some common species and this common ancestor had itself evolved to have pentadactyl limbs.

It is highly unlikely that so many different species could have independently evolved to have the same specific anatomical characteristic like a pentadactyl limb.

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key ideas and examples of evolution from the fossil record

Revision notes on fossil evidence for evolution and its significance in understanding the fossil record.

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Evidence of Evolution from Fossils

Fossils provide key evidence for how life has changed over millions of years, supporting Darwin’s theory of evolution by natural selection.

Fossils can show how species have gradually transformed, linking past organisms to modern-day species.

Types of Fossils

1. Body Fossils – Preserved remains of organisms (bones, teeth, shells).

2. Trace Fossils – Evidence of activity (footprints, burrows, coprolites).

3. Transitional Fossils – Fossils that show intermediate traits between ancestral and modern forms.

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Examples of Fossil Evidence for Evolution

1. Archaeopteryx (Transition between Reptiles and Birds)

• Found in Jurassic rocks (~150 million years ago).

• Had both reptilian features (teeth, claws, long tail) and avian features (feathers, wishbone).

• Supports theory that birds evolved from theropod dinosaurs.

2. Australopithecus (Early Hominins – Evolution of Humans)

• Fossils of Australopithecus afarensis (e.g., "Lucy") show traits linking ape-like ancestors to modern humans.

• Bipedal posture, reduced canines, and an increased brain size indicate gradual human evolution.

3. Horses (Evolution of Modern Horses)

• Fossil evidence shows evolution from small, multi-toed ancestors (e.g., Eohippus) to large, single-toed horses like Equus.

• Adaptations in limb structure support survival in open grasslands.

4. Trilobites (Ancient Marine Arthropods)

• Fossils from Paleozoic Era reveal evolutionary changes in eye structure and exoskeleton shape.

• Show gradual adaptations over millions of years.

5. Ammonites (Extinct Marine Mollusks)

• Used in relative dating of rocks due to their distinct evolutionary stages.

• Provide insights into marine biodiversity changes over time.

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Importance of the Fossil Record in Understanding Evolution

1. Chronological Evidence – Fossils track evolutionary changes over time, providing a timeline of species development.

2. Comparative Anatomy – Shows how organisms share common traits (homologous structures), indicating common ancestry.

3. Extinction Events – Fossils highlight mass extinctions that shaped biodiversity (e.g., dinosaurs' extinction led to mammalian dominance).

4. Environmental Adaptation – Fossils reveal how species adapted to changing climates and ecosystems.

5. Molecular Comparisons – Fossil evidence supports DNA studies that trace genetic relationships between species.

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The fossil record is a powerful tool for understanding the origins and adaptations of life over millions of years. It reinforces evolution by showing progressive changes, extinction events, and connections between ancient and modern species.

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Summary of learning objectives and key words or phrases

Be able to describe, discuss and explain evidence of evolution of life from fossils.

Know how fossils are formed, including ancient bacteria and animal skeletons, impressions of plants in rocks.

Know and explain why there are gaps in the fossil record.

Know, explain and describe that pentadactyl limbs are good anatomical evidence for animal evolution.

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