Reconstruction of the Cambrian ocean, with trilobites and early vertebrates

The Cambrian explosion was a moment of incredible diversification of early life, but it may have been a very rapid moment © Katrina Kenny

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The Cambrian explosion was far shorter than we thought

The Cambrian explosion is one of the most important intervals in the history of life.

Now, new research published in the Proceedings of the National Academy of Sciences (USA) is revealing that this explosion was far shorter than many experts had thought.

What was the Cambrian explosion?

The Cambrian explosion happened more than 500 million years ago. It was when most of the major animal groups started to appear in the fossil record, a time of rapid expansion of different forms of life on Earth.

While there has been a lot of research into exactly when the Cambrian explosion kicked off, little has been done to nail down when this burst of evolution ended.

Evolution during the Cambrain

Dr Greg Edgecombe, Merit Researcher at the Museum and co-author of the study, has been using the diversification of trilobites to uncover precisely how long the Cambrian explosion went on for.

His work shows that this burst of evolution may have only occurred for around 20 million years - actually very brief in the grand scheme of Earth's history.

The age of the trilobites

Trilobites are extinct marine animals with a curious fossil record.

One of the earliest known groups of arthropods (the phylum which includes all modern-day insects, arachnids and crustaceans), trilobites were wildly successful in the ancient oceans, with some 20,000 species spread over ten orders. They're also known from their extensive fossil record. 

But their evolution is something of a mystery. At one point during the Cambrian Period, trilobites suddenly went from being soft-bodied animals to developing hard shells. And they were rapidly found in shallow seas all around the world.

Fossi trilobite with eyes on the ends of stalks

Trilobites were an incredibly diverse group of animals, with tens of thousands of known species  © Trustees of the National History Museum, London

Greg explains, 'There is a certain point at about 521 million years ago when you start to get trilobite body fossils in different parts of the world.

'So within a window of a few million years, you get trilobites with hard exoskeletons from Spain and Morocco to Siberia, China and Australia.'

What happened to allow this sudden shift has long been debated. It is thought that something changed in the chemistry of the oceans at around this point in time, allowing soft-bodied animals such as early trilobites and echinoderms to suddenly be able to build solid biomineralised shells, but that isn't the whole story.

'Is the fossil record literally accurate?' asks Greg. 'Did a trilobite first evolve a mineralised exoskeleton in Siberia or North Africa, and within a few million years rapidly disperse around the world, or did the last common ancestor live before trilobites started to become fossils? That is what we wanted to test.'

What happened during the Cambrian explosion?

The Cambrian explosion, considered a crucial event in the evolution of animals, was in full burst by 518 million years ago.

'It's when most of the few dozen major animal body plans first appear in the fossil record and diversify,' explains Greg. 'But it is actually more than that.

'It is a time of body plan innovation and of diversification, but it is also an ecological phenomenon as life responded to changing environmental conditions.'

Fossil triolbite with spikes down its back

Evolving many different forms, their characteristics can be used to track the rate of evolution during the Cambrian explosion © Trustees of the National History Museum, London

There is no one aspect that can be held up as the cause of this period of rapid evolution, but it is best viewed more as a swirling together of factors.

It was around this time that massive amounts of nutrients that had eroded from continental rocks on land were washed into the oceans, providing the calcium and phosphorus needed to build skeletons and hard shells. Animals start burrowing into the sediment, aerating beneath the seafloor and stirring these nutrients up.

This helped plankton take off in a big way, which in turn formed the basis of ever more complex food webs, ushering in the first major predators. 

'You start getting bite marks appearing in the fossil record and the crushing of shells by other organisms,' says Greg. 'In some fossils you even get the gut contents, so we can look inside the bellies of these extinct animals to see what they were eating. This gives us a better understanding of which animals were eating each other.'

These environmental changes were combined with changes in the development of animals, where new genes that regulate body patterning and segmentation appear.   

Solving Darwin's Cambrian puzzle

Many trilobites from the Cambrian have been preserved as fossils, so they are useful animals to study in the quest to understand what exactly was going on.

Fossil trilobiote with massive eyes

The trilobite fossils have shown that there was a spark of rapid evolution early on, rather than a prolonged period of innovation © Trustees of the National History Museum, London

Greg and his colleagues have been looking at the different branches of the trilobite evolutionary tree that suddenly appear during the Cambrian. They used methods originally developed for building and dating evolutionary trees using DNA sequence data to track how rapidly anatomical features of these animals changed shape over time.

This has helped them to figure out how quickly animals were evolving during this period, while also giving clues as to when the last common ancestor to all the trilobites lived.

Greg's team discovered that this ancestor likely lived not long before the sudden divergence seen in the fossil record. This suggests that the rate of evolution throughout the majority of the Cambrian was surprisingly stable.

'There was a very short bust of accelerated evolution at the start, then it flat-lined for the rest of the Cambrian,' says Greg.

It means that rather than there being a long and protracted evolutionary explosion throughout the period, it was more of a quick spurt at the start, during which all the major animal body plans came into being. 

The results finally settle an evolutionary quandary that plagued even Darwin himself, as it questioned his idea that evolution occurred gradually over time. It shows that startlingly fast evolutionary rates can indeed happen, as it did during the start of the Cambrian.