Evolution and extinction Key points to consider: How are fossils formed and what can we learn from the fossil record? How have organisms changed over time? Why can scientists not be certain about how life began on Earth? How and why have some organisms become extinct?

Exploring the fossil record Fossils provide evidence of how much (or how little) different organisms have changed since life developed on Earth.

Exploring the fossil record Fossils may be formed in various ways, including: from the hard parts of animals, e.g, bones, shells, exoskeletons, which do not decay easily; Fossil fish. As the fish decayed the flesh became replaced by minerals which then formed rock as preserved traces of animals or plants, for example, footprints, burrows or rootlet traces Footprints of Australopithecus, an ancester of humans, preserved in hardened volcanic ash from parts of animals and plants which have not decayed because it was too cold e.g. trapped in ice; there was too little oxygen, or toxic substances killed the bacteria that would cause decay when parts of the animal or plant are replaced by other materials (minerals) as they decay – these are the most common fossils; Fossilised fern fronds. As they decayed they became mineralised

CASTS OF ANIMALS OR PLANTS Formed when the hard parts of animals are replaced by minerals Human casts from Pompeii IMPRESSIONS These fossils are not actually animals or plants but traces they have left behind Ardley Quarry in Oxfordshire - Dinosaur tracks?

Some animals/plants do not decay after death but are preserved in ice ICE FOSSILS Some animals/plants do not decay after death but are preserved in ice mammoth fossil found Fossil evidence of the earliest eukaryotes –in Australia ** mention what ‘snowball earth’ means as it’s mentioned in the video - Snowball Earth describes a theory that for millions of years the Earth was entirely smothered in ice, stretching from the poles to the tropics. it's now thought that there may have been more than one of these global glaciations. They varied in duration and extent but during a full-on snowball event, life could only cling on in ice-free refuges, or where sunlight managed to penetrate through the ice to allow photosynthesis. rare fossils in Australia

Exploring the fossil record The fossil record is incomplete, because: • Most organisms were not fossilised when they died, as the right conditions for fossil formation are rare to find. • Soft body parts in particular may not fossilise – most early organisms were soft-bodied and left little trace of their existence. • Many fossils have not yet been discovered • Geological activity has worn away and destroyed rock in which fossils may have been found. Fossil of seed fern fossil formation Trilobites preserved in sedimentary rock

Fossils in a Rock Profile Layers of sedimentary rock are arranged in the order in which they were deposited, with the most recent layers nearer the surface. The interpretation of rock layers containing fossils allows us to arrange the fossils in chronological order (order of occurrence) Sedimentary layers can be disturbed by tectonic activity. Recent fossils are found in recent sediments Most recent sediments Numerous extinct species Fossil types differ in each sedimentary rock layer New fossil types mark changes in environment Oldest sediments Only primitive fossils are found in older sediments

What do fossils tell us?  Most of the organisms that survived millions of years ago are now extinct. Organisms have changed over time – we can find fossils of ancient fish, fossils of the same fish 10,000 years ago and the modern form of that same species now. The changes represent evolution. (Accidental changes to the appearance or physiology of organisms by (mutation) meant that they survived while others did not.) Evolution is still going on!

What do these animals have in common? Dodo Hairy Mammoth They are all EXTINCT Quagga T-rex Extinction may be caused by: 1. Changes to the environment 2. New predators 3. New diseases 4. New competitors 5. Catastrophies e.g. volcano/asteroid

û ü ü û û ü û ü ü True or false? The oldest rocks contain the youngest fossils. Animals and plants have changed over time and this is called evolution. Complete animals can become fossilised in ice. Fossilisation is very common. The fossil record is complete. Not everyone has the same view of the fossil record. We have found every fossil that exists on Earth. Extinction is the permanent loss of all the members of a species. A single catastrophic event can cause a species to become extinct. ü ü û û ü û ü ü

Can you answer these questions now? Key points to review: How are fossils formed and what can we learn from the fossil record? How have organisms changed over time? Why can scientists not be certain about how life began on Earth? How and why have some organisms become extinct?