Fossils and Evolution

Palaeontologists are scientists who study remains of ancient life and environments, and some look at fossils to find evidence for evolution. In the past however, people had very different ideas about fossils. Some people thought these mysterious objects were magical and told stories about them. Fossil ammonites were often called ‘snake stones’ because people believed they were snakes that had been magically turned to stone. These examples even have snake heads carved into them. We now know that ammonites were ancient sea creatures related to snails. They lived 240 – 66 million years ago, until they went extinct along with most dinosaurs. When they died, some ammonites were buried under layers of sediment on the sea floor. Over time, their shells were slowly replaced by minerals – ingredients of rocks – turning them into fossils. What do you think happened to the soft parts of their bodies? Why doesn’t everything become a fossil?

Some fossils can give palaeontologists clues about how old a layer of rock is. These fossils are called ‘index fossils’. The best index fossils are ones that are easy to recognise, found in many places and in large numbers, and are known to have lived in a specific time period. Trilobites, ancient sea creatures related to modern-day crabs, spiders and insects, are particularly useful index fossils. They are found all around the world, had quite large and hard bodies that left clear fossils, and are known to have become extinct at the end of the Palaeozoic period, about 252 million years ago. If a rock has trilobites in it, then palaeontologists know that this rock is from the Palaeozoic period. Would you recognise a trilobite in another piece of rock?

Mary Anning, one of the greatest fossil hunters ever to have lived, discovered many fossils in the early 1800s in the cliffs around Lyme Regis, Dorset – an area now called the Jurassic Coast. When she was just 12 years old, Mary and her brother discovered the head of a mysterious creature sticking out of a cliff. It looked like a crocodile, but they soon discovered that it was a fossil of an ancient reptile called an ichthyosaur (‘fish lizard’). Ichthyosaurs were sea reptiles that lived 250 – 90 million years ago. Although The Horniman’s ichthyosaur fossils are impressive, they are not the biggest. In 2018 an enormous fossilised ichthyosaur jaw bone was discovered. Palaeontologists believe it came from a giant ichthyosaur up to 26 metres long – one of the largest animals ever to have lived! People often think that ichthyosaurs were dinosaurs, but they were sea reptiles.

Imagine how you would feel if you discovered a giant fossilised footprint! These footprints are thought to be from an Iguanodon, a plant-eating dinosaur that roamed the earth in the early Cretaceous. Fossil footprints are an example of ‘trace fossils’. Trace fossils are not formed from an animal or plant itself (‘body fossils’); instead they are fossilised signs left by an animal or plant, like a footprint, teeth marks, a leaf print, or even vomit… or droppings! A fossilised poo is known as a coprolite and can tell palaeontologists a lot about a prehistoric animal’s diet! Place your hand in front of one of these giant fossil footprints and imagine the size of the dinosaur that made them. How would you feel if you came face to face with an Iguanodon?

This piece of sedimentary rock has imprints of tree ferns in it; these are trace fossils. Tree ferns first appeared more than 200 million years ago but there are still some species of tree fern around today – they are popular garden plants and some can be seen growing in the prehistoric garden in the Horniman Gardens! Tree ferns were the favourite food of some well-known plant-eating dinosaurs; an Iguanodon or a Diplodocus would have eaten about 500kg of plants each day! Compared with animal fossils, not many plant fossils have been discovered worldwide. Why do you think this is?

Is it a bird? A dinosaur? This is a cast of a famous fossil, the ‘Berlin Archaeopteryx’. The original was found by a farmer in Germany in 1874, who is said to have sold it to raise money to buy a cow! Only twelve Archaeopteryx body fossils have been found, all in Germany. First a fossil of a single feather was discovered in 1860, and the first (headless) fossil skeleton was found in 1861. Meaning ‘ancient wing’, Archaeopteryx confused and excited palaeontologists. It had jaws with sharp teeth like a dinosaur, but it also had feathers, a bird-like beak and a wishbone, like birds. They realised that this mix of dinosaur and bird features meant that Archaeopteryx is a transitional fossil, showing how birds evolved from dinosaurs. This discovery came soon after Charles Darwin published his book “The Origin of Species” about evolutionary theory in 1859, so there was great excitement worldwide.

Charles Darwin was a naturalist who wrote a theory of evolution by natural selection. One of the clues that helped him were birds from the Galapagos Islands in the Pacific Ocean. When he studied them, Darwin realised that the birds on each island had slightly different beak shapes; some stronger for cracking nuts, some thinner for picking insects out of trees. Darwin realised that the different birds had come from the same original bird species (‘ancestor’), some of which had flown to different islands where there were different types of food. Slowly, the beaks of these new groups of birds changed to better suit the food on each island. Over time, the beaks became so different that the birds on the different islands had changed (‘evolved’) into separate species. Looking at the birds in this display case, can you work out what kind of foods their beaks are suited to?

One of the key ingredients for evolution is ‘variation’. Think of us humans – we have different eye colour, hair type or height, and yet we are all the same species, Homo sapiens. These differences are known as variations and are found in all living things. If a living thing has a feature (‘trait’) that helps it to survive, then it will live longer and is more likely to reproduce. When it has young, they will ‘inherit’ the useful trait – it will be passed on to them. As more and more of the group (‘population’) have the trait, over time they evolve into a new, different species. This display case shows some examples of natural variation. Look at the sea snails. Can you spot the differences in their shells? Why might having a pointier shell help a sea snail to survive?

Do these look similar? This impressive jaw belonged to a tiger shark, named after the dark vertical stripes on its body. It is the fourth largest shark species alive in the oceans today, but it is dwarfed by the extinct Megalodon shark, which is thought to be the largest predatory fish that ever lived. Megalodon means ‘giant tooth’ and this fossil tooth shows us just how impressive this enormous predator would have been. By comparing fossils with creatures still alive today, palaeontologists have been able to find connections between extinct and living species. This is important evidence for evolution, showing how creatures have evolved from ancestors living millions of years ago and also gives important clues about how ancient creatures lived.

Evolution can lead to some weird and wonderful adaptations. Animals that can blend in with their habitat are less likely to be spotted and eaten by predators. Those surviving animals can then reproduce, passing their camouflaged patterns to their young. In this way, an adaptation like camouflage can become more and more impressive, as with this leaf butterfly. With its colourful wings open, this butterfly is easy to see, but as soon as it closes its wings, it looks like a leaf on a twig! Can you think of another example of an adaptation that helps an animal to survive?

Flight has evolved at least four times, in insects, pterosaurs, birds and bats – but these groups of animals are not closely related. Known as ‘convergent evolution’, sometimes a trait evolves in very different species on different branches of the evolutionary ‘tree’. Bats and birds are a good example. Bat wings have skin while bird wings have feathers. Bats’ wing bones reach the wing tip, but birds have short wing bones and long flight feathers. Despite this, bats and birds can both fly. Some animals have evolved to glide rather than fly, such as flying squirrels and flying fish. Perhaps one day they will evolve the ability to truly fly. Do you think this is possible? Which other animals might learn to do this? Which animal would you like to learn to do this?

Because islands are isolated (cut off), interesting adaptations have appeared or been lost there through evolution. An example of this are flightless birds like the dodo, which evolved on Mauritius. There were no large predators on the island, so birds did not need to fly; as dodos put more energy into growing larger bodies, their wings became smaller. In the late 1500s, Dutch sailors colonised Mauritius and by the 18th century, the dodo was extinct. The exact date is unknown because nobody wrote down the last dodo sighting. Dodos could not adapt quickly enough to the new threats that the sailors brought, from hunting, to the egg-eating rats, pigs and dogs that arrived on their ships. Being flightless was the dodo’s downfall, but only because news species arrived that it was not adapted to deal with. Can you think of another example of a flightless bird that still alive today?

There have been several mass extinctions in history, and most of the species preserved as fossils are long extinct. Could it happen again? Humans and nature have had an interesting relationship through history. People use plants and animals for food, medicine, clothing, building materials and much more, however this can damage habitats and the species living there, even causing them to go extinct. This carved elephant tusk comes from the Democratic Republic of the Congo in Central Africa and is a good example of the threat. At first sight it might look beautiful, but in reality many elephants are harmed or killed for their tusks. How does this make you feel? What if elephants become extinct? Would it matter? What can we do to help?

These fragments come from layers of rock known as ‘coal measures’. Coal measures were formed over millions of years under extreme heat and pressure, from the remains of ancient swamp plants that lived in the Carboniferous Period, 327 – 299 million years ago. Along with oil and gas, coal is known as a ‘fossil fuel’. Fossil fuels are not actual fossils, but were formed from the remains of ancient plants and animals, which gives them their name. For centuries, people have used fossil fuels to power machines and vehicles, create heat and electricity, and make useful materials like plastics, paints and pesticides, but we now know that fossil fuels are running out and that they release harmful gases which cause climate change. What fuels are there to power vehicles or generate electricity that do not run out (‘renewable’) and cause less pollution?

Although fossils only give us a small amount of evidence, the fossils that have been discovered so far tell us that modern humans, Homo sapiens, evolved about 300,000 years ago in Africa. Our large, well-developed brain has allowed our species to develop and evolve in incredible ways. We have cleared forests, developed languages and cultures, built cities, invented air travel, computers and much more. But what is the impact of human development on nature? Will there be as many species in 100 years as there are today? What will fossils of the future look like?