Fossils are evidence of ancient life forms or ancient habitats which have been preserved by natural processes. They can be the actual remains of a once living thing, such as bones or seeds, or even traces of past events such as dinosaur footprints, or the ripple marks on a prehistoric shore. Geologists can tell the age of a fossil through a variety of radiometric dating techniques. The breakdown of radioactive isotopes of certain elements, such as carbon, uranium and potassium takes place at a known rate, so the age of a rock or mineral containing these isotopes can be calculated.

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History of paleontology

People have been fascinated by fossils for thousands of years, and as long ago as ancient Greek times were correctly interpreting them as the remains of long dead creatures. Palaeontology began to be formalised and treated with scientific rigour from the 17th century onwards. At this time, people started to calculate the age of the Earth and get to grips with the fact that the extinction of a whole species was not only possible, but had occurred many times already. The publication of Darwin's 'On the Origin of Species' in the mid-19th century gave new impetus to palaeontology, as patterns and trends in evolution and extinction were eagerly sought and studied. Modern palaeontologists have an array of tools and processes at their fingertips, from sophisticated dating techniques to electron microscopes and medical scanners.

Fossil types

Body fossils are the preserved remains of the actual body parts of an animal or plant such as a skeleton or a pollen grain. Trace fossils are the remains of ancient activity, such as the burrow left by a worm or a stone tool made by a prehistoric person. Some fossils preserve original features in exquisite detail, while others are much cruder remnants.

How fossils are formed

  1. A dead dinosaur lies partially in a stream. Fossilisation only happens in the rarest of cases, when a plant or animal dies in the right circumstances. Animal corpses are usually eaten by something, or bacteria rots them away before fossilisation can occur, and even hard parts like bones and shells are eventually destroyed through erosion and corrosion. The trick to becoming a fossil is to die in a location where your body - or bits of it - are protected from scavengers and the elements. This means getting buried in sand, soil or mud and the best place for that is on the seabed or a river bed.
  2. The bones of a dinosaur buried beneath a hill. Only in very rare cases do the soft parts of animals - the flesh, skin and internal organs - become fossils. Even when buried under mud or soil, decay still takes place, though lack of oxygen does slow it down. If a skeleton is dug up at this stage, it will still be made of bone. Remains like these that haven't truly fossilised yet are sometimes called 'sub-fossils'.
  3. The bones of a dinosaur get buried more deeply and begin to change chemically as layers of sediment accumulate on top of them. As more time passes, sub-fossils become buried deeper and deeper. What was mud or sand becomes compressed on its way to becoming rock. But even safely sealed away underground, time doesn't stand still. Chemicals and minerals percolate through the sediment and the original bone or shell gradually recrystallizes. In extreme cases, the entire thing can dissolve away, leaving a hollow where it once was. If palaeontolgists find a hollow like this, they can pour liquid rubber in to make a fossil cast, or put it in a medical scanner to see what the original looked like.
  4. The bones of a dinosaur are buried deep enough and long enough to fossilise. In other cases, minerals from the rocks gradually impregnate the bone, shell or wood, changing its chemical composition and making it capable of surviving for as long as - or sometimes longer than - the rock enclosing it. In cases where the original has dissolved away, the minerals can gradually fill the hollow to create a natural cast of the original. So sometimes a fossil doesn't contain anything of the original creature except its shape. Even that shape can take a battering! If the rocks are distorted and squeezed by geological forces, then the fossils within them will be too.
  5. The fossil bones of a dinosaur appear in the soil as the rock enclosing them erodes away. Even rocks have a finite lifespan. Eventually the rock enclosing a fossil is eroded away, and the fossil is revealed on the surface of the ground. With luck, a sharp-eyed fossil collector will spot and excavate it. Otherwise the elements will continue to batter it, until it - along with the rocks around it - is reduced once more to sand, silt or mud.

History of life on Earth

The history of life on Earth began about 3.8 billion years ago, during the Archean era, initially with single-celled prokaryotic cells, such as bacteria. Multicellular life evolved over a billion years later and it's only in the last 570 million years that the kind of life forms we are familiar with began to evolve, starting with arthropods, followed by fish 530 million years ago (Ma), land plants 475Ma and forests 385Ma. Mammals didn't evolve until 200Ma and our own species, Homo sapiens, only 200,000 years ago. So humans have been around for a mere 0.004% of the Earth's history.

A timeline of Earth's history from 4.6 billion years ago to the
evolution of the Dinosaurs 225 million years ago.

The Tree of Life

The Tree of Life illustrates how different species arise from previous species via descent with modification, and that all of life is connected. The diagram below shows the relationship between the major biological groups. The centre represents the last universal ancestor of all life on earth, the outer branches the major biological groups.

A phylogenetic tree of life, showing the relationship between the major biological groups.