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Read more: “Instant Expert: Fossils“

Fossils are not just petrified body parts such as bones, teeth and shells. They also include ancient bacteria and the detailed remains of tissues such as feathers, fur and skin. Some are mere impressions – footprints of a dinosaur, for example, or casts of ferns and traces of burrows. Others are not particularly old: 5300-year-old Ötzi the Ice Man is a fossil of sorts, as are Iron Age bodies preserved in European peat bogs and the 500-year-old freeze-dried Inca mummies of highland Peru. Even flies and frogs preserved in amber are fossils. Although only a tiny percentage of everything that has ever lived is fossilised, fossils are our most reliable guide to the organisms and environments of the past

The oldest remains

It is unlikely that the very first life on Earth became fossilised, but finding the oldest fossils that do exist could help solve the twin mysteries of when and how life began. This is a huge challenge, not least because the oldest life forms probably didn’t resemble anything alive today. As a consequence, candidates for the title of first fossil are inevitably controversial, with sceptics suggesting they are merely strange geological formations rather than once-living organisms.

See diagram: “How to make a fossil”

At present, the top contender is a collection of 3.4-billion-year-old microscopic blobs found in Strelley Pool, Western Australia, by David Wacey of the University of Western Australia and colleagues (). Several lines of evidence suggest that these are biological rather than geological in origin. Since life on Earth is cellular, the oldest fossils ought to have regular cellular structures, and these blobs seem to fit the bill. Most are ovoid or spherical in shape, have hollow centres surrounded by what look like cell walls of a uniform thickness, and they fall into the size range of known microbial organisms.

The blobs also show indications of biological behaviour, says the team. They are confined to a layer of rock comprising black sandstone rich in pyrite, suggesting the blobs have a preferred habitat. Some have what look like split cell walls, with the contents extruded, just as might be seen in a deteriorating cellular organism. The walls themselves are rich in carbon, nitrogen and sulphur, suggesting that this early life had a metabolism based on sulphur. This finding is key, the team argue, because 3.4 billion years ago the world was hot and nearly oxygen-free, and volcanic activity would have led to an abundance of sulphur.

Ediacaran fauna

The earliest fossils of recognisable multicellular organisms formed about 585 million years ago – long after the oldest fossils. Named after the Ediacaran Hills in Australia where they were first identified, these animals were oxygen-dependent, so could only have evolved after photosynthesising microbes had transformed the early, sulphur-rich atmosphere into one rich in oxygen.

Ediacaran fossils have been found on all continents except Antarctica, suggesting they inhabited oceans worldwide. Because these animals were soft-bodied, their fossilisation is something of a puzzle. Fine-grained sediments and volcanic ash seem to be key; these sediments are held together by the sticky secretions of microbial mats, which may have cushioned the delicate bodies. The fossils range from a few millimetres across to metres in length and are so unusual in form that their identification as animals is sometimes debated. One, Charnia, could be loosely described as a fried egg with a feather or fern-like appendage. Another, Dicksonia constata, looks like a squashed, quilted bag. Others look more familiar, resembling sea pens, primitive jellyfish and sea squirts. What is indisputable is that they were highly complex, multicelled creatures.

The demise of the Ediacaran fauna came at the start of the Cambrian period, around 542 million years ago. No one knows why they disappeared. Temperature fluctuations and changes in ocean and atmospheric chemistry have been suggested, as has the break-up of the supercontinents and the evolution of predatory organisms.

The Cambrian explosion

The strange world of the Ediacarans was followed by something even more bizarre. About 505 million years ago there was an explosion in biological diversity called the Cambrian. Fossils from this time are found in the Burgess Shale of the Canadian Rockies. This is a dark, almost black, mudstone that formed on the ocean floor and the fossils are found in films that can best be seen by tilting the rocks in the light. Exactly how they were preserved is a mystery. All the fossils are flattened and some seem to have been broken up by churning of the sediments.

Many Burgess Shale species used filter feeding to extract organic matter from the muddy sea floor or from the ocean waters, but about 10 per cent were predators of weird design. One of these, Anomalocaris, was up to 70 centimetres long. It was originally thought to be three separate organisms: its arm or feeding appendage was taken to be a shrimp-like creature, the mouth was classified as a jellyfish and the body as a sponge. This misinterpretation arose partly because no one expected to find such a large creature and partly because Anomalocaris had both hard and soft body parts, which preserved differently and often became separated. The truth was revealed when a specimen with an unmistakable connection between arm and mouth was found.

Another famous Burgess Shale specimen, Hallucigenia, is so peculiar that it is unclear where the head is or even which way up it goes. Named for its “bizarre and dream-like quality” by Simon Conway Morris from the University of Cambridge, it is believed to be related to arthropods – crabs, spiders and the like. Originally, Conway Morris reconstructed the creature as a sort of worm that walked on seven pairs of jointed spines and grasped food with a set of seven large tentacles and three pairs of smaller ones, all on its back. These days, most people flip the creature over, so that it walks on paired tentacles and is protected by spiny armour. Either way Hallucigenia is a very odd animal.