Unearthed: Life Recorded in Fossils

The Natural History Museum

The first evidence for life on Earth suggests it began around 3.5 billion years ago. Fossils at the Natural History Museum trace the evolution of life from small and humble beginnings through revolutions in shape, size and habitat right up to fossils that can still yield DNA.

These red-grey rocks may not look like much, but they contain the world’s oldest fossils.

The Apex Chert, from Western Australia, is estimated to be 3.465 billion years old.

That they contain traces of simple organisms means that life has been around for more than two thirds of Earth’s 4.6-billion-year history.

The fossils themselves are less than one-hundredth of a millimetre long; mere filaments that bear a resemblance to modern cyanobacteria.

Their appearance, as well as their chemical composition, point to them being some of the earliest life forms recorded, although some do still doubt their origin, contending that the shapes could be formed by inorganic processes.

The Apex Chert specimens in the Museum’s collection are a valuable scientific resource for this debate.

The fossils themselves are less than one-hundredth of a millimetre long; mere filaments that bear a resemblance to modern cyanobacteria.

Their appearance, as well as their chemical composition, point to them being some of the earliest life forms recorded, although some do still doubt their origin, contending that the shapes could be formed by inorganic processes.

The Apex Chert specimens in the Museum’s collection are a valuable scientific resource for this debate.

From the humble beginnings of bacteria and single-celled organisms, life evolved slowly – until the Cambrian explosion.

This period, starting around 540 million years ago, saw a sudden diversification in body types and myriad new forms appeared.

One of the major developments during this period was not what the creatures looked like, but how they saw.

One group of creatures with remarkable early eyes were the trilobites – a successful type of creature that evolved elaborate forms but eventually went extinct.

The eyes of this Erbenochile trilobite that lived 400 million years ago are arranged in unique cylinder shapes.

About 500 lenses cover each eye, with a ledge on top to shield the eyes from the glare of the sun in the shallow water Erbenochile lived in.

The eyes would have given the creature 360 degree vision – perfect for finding prey and avoiding predators.

Several mass extinctions have occurred throughout Earth’s history, where many types of organisms have gone extinct around the world within a relatively short time.

The winners and losers of these catastrophic events can sometimes be surprising. Nautiloids, like Cenoceras shown here, lived in seas of the Jurassic and Cretaceous periods alongside the superficially similar ammonites.

Both were tentacled creatures inhabiting spiral-chambered shells, but whereas species of nautiloids still swim today in the Indo-Pacific seas, every species of ammonite went extinct in the mass extinction that also killed the dinosaurs.

At first this might seem counterintuitive: nautiloids were far less abundant, had fewer species and showed less diversity in shell shape than ammonites. However, research has shown that they fed on different prey.

The ammonites’ prey collapsed during the extinction event, while the nautiloids were able to keep feeding, allowing them to survive until today.

The success of some creatures means we can use them for more than just studying the biology of the organisms themselves.

Gryphaea are such a common fossil in Jurassic rocks throughout the UK that they have been given a name in folklore, ‘devil’s toenails’, and even used in folk medicine.

But their commonality gave British palaeontologist Arthur Trueman and subsequent generations of geologists something better than an unpalatable concoction: they were a perfect case study of evolution.

It was possible to trace the change in form of Gryphaea through different species up the rock record and to conclude that increasingly broader and flatter shells helped the organism stay more stable in the mud they lay on, resisting sinking.

Common creatures not only chart evolution, but also helped one man make the very first chart – or map – of British geology.

William Smith (1769-1839) was a canal engineer who came up with a unique insight that allowed him to link layers of rocks across the country, and distinguish ones that were only superficially similar but could be millions of years apart in time.

He used the fossils of sedimentary rock layers as a ‘signature’, including these ammonites from the Blue Lias Formation found in Somerset.

These correlations allowed Smith to create and publish the first geological map of England and Wales in 1815.

Another famous fossil collector, many of whose finds can be found in the Museum, is Mary Anning. Anning made most of her finds around Lyme Regis on the Dorset coast, an area famous for ammonite fossils, but her discoveries were much more visually impressive.

In 1811 she uncovered the five-metre long skeleton of an ancient sea reptile that came to be known as an ichthyosaur (‘fish lizard’).

While impressive, this ichthyosaur fossil, as well as many others she uncovered, also exemplify the idea of ‘convergent evolution’ – where two totally separate groups of organisms evolve similar lifestyles.

In this case, the reptilian ichthyosaur’s streamlined body, fins and tail allow it similar swimming prowess to the modern mammalian dolphin.

A lot of the fossils in the Natural History Museum's collection come from the sea, where the fossilisation process occurs more readily.

However, this small fossil documents one of the most important revolutions in the history of life on Earth, 400 million years ago.

Although some bacteria and lichens may have got their first, no organism successfully colonised land in the way plants did.

This Cooksonia specimen is only a few centimetres tall and has no leaves, but was uniquely adapted for the challenges of dry land.

It had a vascular system that allowed it to transfer water from the soil through the stem.

It also had a waxy cuticle to prevent water escaping, punctuated with stomata to allow for the exchange of gases - all features found in modern plants.

Eventually, animals joined plants on the brave new world of land. From our point of view, the migration of vertebrates – animals with backbones – was crucial as it eventually led to the evolution of humans.

We know this probably started with fish evolving into amphibians, but exactly how this happened is still being uncovered.

One clue comes from fish like this Eusthenopteron.

Although fully aquatic, this 380-million-year-old fish had lobe fins – paired pelvic and pectoral fins joined to the body with a single bone – thought to be the precursors to the limbs of land dwellers.

If it took a fish to colonise land, then it took a dinosaur to colonise the skies.

Although insects had been doing it for millennia, the first vertebrates to fly were likely creatures like this Archaeopteryx.

Widely considered the first birds, they also had many features left over from their time as land-dwelling dinosaurs, including teeth, a bony tails and claws on the hands.

Only about a dozen decent specimens of Archaeopteryx exist, and all come from one quarry in Germany that provides exceptional preservation.

The Natural History Museum’s specimen came via local doctor Karl Häberlein, who acquired the specimen as part payment for a medical bill by one of the quarrymen.

He later sold it along with the rest of his fossil collection to provide a dowry for his daughter.

Evolution has helped animals develop new adaptations and colonise revolutionary new environments, from the land to the sky. However, sometimes advantages come not in being bigger and stronger, but by shrinking in size.

Island dwarfism is a phenomenon where large animals evolve smaller body forms when isolated on islands.

This helps conserve scarce resources, but is also possible because there are fewer predators in these places, meaning being big has less of an advantage.

Pioneering palaeontologist Dorothea Bate (1878-1951), who spent 50 years working at the Natural History Museum, discovered several examples of island dwarfism that evolved on many Mediterranean islands during the Pleistocene epoch (2.6 million – 11,700 years ago).

These included pygmy deer, elephants and this miniature ‘mouse goat’, adults of which reached only 50 centimetres at the shoulder.

At the same time as some mammals were shrinking on islands in the Mediterranean, others were becoming extremely large. This example was found in Ilford, near London, in 1863.

It is a steppe mammoth that lived around 200,000 years ago, and was one of the largest mammoth species to ever exist, reaching up to four metres tall with tusks nearly three metres long.

Given how recently mammoths lived, compared to most creatures on this list, and their excellent preservation, in some cases it has been possible to extract samples of DNA.

While scientists have used this to learn more about how different groups were related to each other, and to modern elephants, it has also led to the tantalising possibility of bringing mammoths back from extinction.

Perhaps the museum of the future will have a real mammoth on display next to impressive skulls like this one?

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