The Cambrian explosion
Hello everyone, welcome to the twelth episode of ‘What We Don’t Know’, a podcast that explores the boundaries of human knowledge, investigating the unanswered questions and theories that unravel them at the frontiers of science. During this podcast I hope to get you interested in new areas of science, maths and technology, teaching you about existing concepts and igniting a curiosity for the things we have yet to know.
We often use the phrase ‘it’s the end of an era’ to signify some great change in our lives, like leaving school forever. But actual eras are far, far longer than our brains can comprehend, usually lasting several hundred million years, with dramatic, global ecological changes as their finale. Around 541 million years ago, there was such an ending. As the Neoproterozoic era came to a close, so did the Proterozoic eon, and nearly 3.5 billion years of bacterial rule. Suddenly the prokaryotic mats were breaking apart and the slow, soft-bodied organisms that characterised the late Neoproterozoic were dying. Following this mass extinction, the new Cambrian period brought stunning increases in the diversity and complexity of life. These increases are called the Cambrian explosion. But what caused such a striking shift?
This episode I will start with the fundamentals and work our way to the theorised explanations for the Cambrian explosion. I’ll explain how evolution works, summarise the great history of life on Earth, and outline the methods that scientists use to examine this history. Then I’ll draw our attention to the border between Ediacaran and Cambrian periods. We will see what was so significant about the evolutionary changes there, before assessing some of the most plausible reasons why the Cambrian explosion happened, and why it happened then of all times.
First: evolution. Any organism’s physical characteristics are determined by their genetics, environmental influences, or a mixture of both. For example, a person can be born with black hair (determined genetically) but after bleaching and dyeing it purple, that feature is a result of lived actions. Dyed hair is an exception. Most of the information describing a living creature can be found in its genetic code, i.e. its DNA, a complete copy of which is in nearly every cell. Importantly, this DNA is hereditary. This means that it gets passed down from an organism to its offspring. But because of the way that DNA is passed down, and sometimes transferred without reproduction between organisms like bacteria, every individual organism has a slightly different DNA sequence.
During natural selection, organisms whose characteristics are more suited to their environment will live longer and have more offspring, passing on their genetic information. Organisms less suited will eventually die out from the gene pool. For example, in a population of rabbits where some have white fur, others have brown fur, and the ground is covered in snow, white-fur rabbits will be selected for. After multiple generations, all the rabbits will have white fur. An environmental feature which favours some organisms in a population is called a selection pressure.
As generations come and go, the characteristics of a population will change slightly until they are distinctly different from those of the original group, maybe even becoming a new species. This process is evolution.
Another important concept to understand is that of geologic time. Geologic time is organised into units called eons, eras, periods, epochs, and ages, with decreasing duration, and these units are established using records in rock strata, i.e. stacked layers of sedimentary rocks. By analysing where in the strata different fossils are found, we can reconstruct geologic time intervals. Evolutionary history can be correlated with the fossil record. Absolute dates can then be quantified numerically using radiometric dating, which measures the presence of radioactive elements or their decay products in order to calculate the age of rocks and minerals.
Every living thing on this Earth evolved from the first prokaryotes 3.5 billion years ago (give or take a couple hundred million). You may have heard of Hades, Greek god of the Underworld. The first geologic eon is named after him: the Hadean eon, dominated by molten rock, volcanic fury and frequent asteroid impacts. This began about 4.6 billion years ago with the Earth’s formation, and ended 600 million years later with the Archean eon, named after the Greek word for ‘origin’. Here, continents formed and life began. There was liquid water, but no free oxygen in the atmosphere, and for 1.5 billion years life remained very simple, existing mainly as stromatolites. Stromatolites are layers of photosynthesising microorganisms (like cyanobacteria) which stick to each other and to rocky materials to sculpt ‘microbial mats’. Their photosynthesis released oxygen as waste, but most of this oxygen was stored in iron at the seafloor.
Then 2.5 billion years ago, atmospheric free-oxygen levels rapidly increased, killing most of the anaerobic life on Earth in the Great Oxidation Event, but allowing the development of more complex Eukarya, with membrane-bound organelles, in the Proterozoic eon. Anaerobic refers to cellular respiration without oxygen (as opposed to aerobic with oxygen). For more on Eukarya’s evolution, check my earlier podcast episode on the Origins of the Nucleus.
During the Proterozoic, many exciting evolutionary events happened, such as the division of the ancestors of modern plants, fungi and animals, the first multicellular life, the first sponges and comb jellies. Bilateral symmetry evolved. In the final period, the Ediacaran period, of the final era of the Proterozoic eon, we find the earliest fossils of complex multicellular life, including the oldest animals in the fossil record. Chordates - animals with a primitive backbone - appeared.
541 million years ago, the Proterozoic eon gave way to the Phanerozoic eon, which we are currently in. 541 million years ago, there was an explosion of complex life. Many new body types appeared. Ecological relationships rapidly became more complex. This was the Cambrian explosion.
Since then, the earth has witnessed startling developments in the sophistication of its life. Picture a tree, growing for around 3.5 billion years, but growing incredibly slowly, so that every new ring of its trunk is hardly different from the last. The Precambrian trunk of life, formed from crowds of microbes. Then, all of a sudden, it explodes outwards into a tangle of branches, each splitting again and again: vertebrates, cephalopods, bony fish; insects with woody stems, amphibians, the oldest fossilized tree; reptiles, dinosaurs, cynodonts, mammals. Monotremes like the duck-billed platypus evolve, then flowering plants emerge 50 million years later. Large sections of the tree die when there is another mass extinction. But it recovers quickly enough. After 3.5 billion years of slow growth, the tree’s crown diversifies within 500 million years.
Of course, it was only 5.5 million years ago that the oldest human ancestor, Orrorin tugenensis, walked on two legs, 2.5 million years ago that hominids started regularly using stone tools, and 195 thousand years ago that Homo sapiens appeared. When you are confronted with the vastness of earth’s history, that tiny slice, those 195 thousand years of human development and society and violence and art, it seems very small.
But this episode isn’t intended to give you existential dread. I want to focus on the moment where the trunk became the tree, where life truly blossomed. The Cambrian explosion.
Just before the Ediacaran period, the earth endured two major glacial events, each lasting around several million years, which would have cooled the planet and laid ice across huge stretches of ocean. The tectonic fracturing of the supercontinent Rodinia created shallow seas, and at the end of the Ediacaran, the landmass Gonwana began to form, likely introducing wealths of nutrients to the oceans.
Unfortunately, there are few fossils from the Ediacaran period. This is because hard-shelled animals, which are easier to fossilize, had not yet populated earth. Most Ediacaran biota (the life forms of the time) were tubular and frond-shaped, often immobile, organisms, which represent the earliest known complex multicellular organisms. Microbial mats were widespread. Ediacaran multicellular organisms are notoriously difficult to categorise, because their fossils do not look like any modern animal. Resembling tubes, discs, bags or quilted mattresses, some suggest they were lichens, algae, protists called foraminifera, colonies of microbes or fungi, or strange intermediaries between plants and animals. Most biota were very small, millimeters or centimeters long, with feathery forms. There were no predators. Instead, animals blindly wandered across the microbial mats, grazing. An alien world; a simple lifestyle.
The Cambrian explosion produced arthropods, invertebrate animals with an exoskeleton, a segmented body and paired jointed appendages, which make up 84% of known animals and include insects, centipedes and lobsters in their ranks. Echinoderms, a phylum of marine invertebrates including starfish and sea urchins, also developed. As did mollusks, worms and chordates. Note that specific examples of animal groups, like insects and starfish, did not evolve until much later (the Cambrian period held their ancestors). Actual Cambrian arthropods included the famous trilobite, which looks a bit like a mix between a woodlouse and a horseshoe crab, and always reminds me of a roomba.
I want to make the distinction between Ediacaran and Cambrian periods clear. The Ediacaran world was alien, full of organisms that died when the Cambrian began. Ecological relationships, and the species which formed them, were simple. Cambrian animals had legs, compound eyes, sophisticated coordination, hard mineralised shells. They adapted to predator-prey relationships with sharp teeth and jaws. With the rise of predation, the large, sedentary animals of the Ediacaran were disadvantaged, and soft-bodied Ediacaran fauna disappeared from the fossil record within a few millennia. The surviving fauna had to change their behaviour. Trails left in fossilised microbial mats by grazing animals changed from haphazard and overlapping to neater and tighter, with occasional sharp changes of direction which suggest evasion of predators. This change in grazing style may have been essential in the fragmentation of the microbial mat during the early Cambrian. As it broke apart, animals could burrow in the seabed for the first time, gaining access to more nutrients and refuge. Predation also forced animals upwards, into open water. Life was no longer contained to the two dimensional plane of the sea floor.
An explosion of life occurred. But as we know, evolution only happens when there is a selection pressure to favour certain characteristics. Such significant evolutionary changes over the Ediacaran-Cambrian border must indicate significant environmental changes then, right?
Now we’re going to explore one of the biggest factors attributed to the Cambrian explosion: rising oxygen levels.
First, oxygen concentrations. Single-celled organisms dominated the pre-Cambrian planet in environments lacking oxygen, using carbon dioxide, molecules containing sulphur or iron minerals as oxidising agents to produce energy. Using oxygen as an oxidising agent is much more effective. Aerobic respiration produces more energy than anaerobic respiration. If you recall from earlier in the episode, big evolutionary events happen when oxygen levels rise, such as during the Great Oxygenation Event. If, 541 million years ago, oxygen levels rose, they may have passed a certain threshold to enable more energy-intensive processes in animals, such as the formation and use of muscles, nervous systems, mineralised shells, exoskeletons and teeth. More oxygen may have enabled animals to metabolise more effectively, evolving more complex bodies and behaviour.
Researchers (unfortunately) cannot directly measure oxygen levels over 500 million years ago. So how can we find evidence for change? For metals like iron and molybdenum in sediment rocks, solubility depends on the amount of oxygen present, so the amount and type of metals present reflects ancient oxygen levels. Many researchers have studied ancient oceanic sediments and found that oxygen concentrations rose to approach today’s sea-surface concentrations at the start of the Cambrian.
On the other hand, when Erik Sperling, of Stanford University, analysed a database of 4,700 iron measurements taken from rocks around the world, from the Ediacaran and Cambrian periods, there was no statistically significant increase in the proportion of oxygenated to oxygen-deficient water. Donald Canfield, at the University of Southern Denmark, found that oxygen levels were high enough to support simple animals, like sponges, hundreds of millions of years before they appeared. Of course, Cambrian animals would have needed much more oxygen than simple sponges, but Canfield’s discovery implies that oxygen could have been sufficiently abundant before it was made use of. For further disputation, Timothy Lyons, of the University of California, worked with trace metals to suggest that pre-Cambrian increases in oxygen were usually short, temporary peaks.
That being said, more recently a study by Tianchen He, of the University of Leeds, suggested that rapid series of oxygen spikes may have been responsible for increased diversity. He’s team investigated a shallow Siberian seabed with a detailed carbon and sulphur geological record, as well as fossil record, around the time of the Cambrian explosion. Within 10 million years, this sea experienced five separate oxygen spikes. Each burst was accompanied by a rise in biodiversity. Each dip by a rise in extinction rates. Perhaps the volatility of rapid fluctuations, as well as rising oxygen levels themselves, played an important role in the Cambrian explosion.
I’ve mentioned some flaws with the theory of rising oxygen levels. This is not to say that oxygen levels were not responsible - they almost certainly were. But the Cambrian explosion was likely driven by a complex interplay of other factors too. For example, rising oceanic calcium concentrations, habitat changes, and ecological relationships.
Skeletons and shells are made of calcium carbonate. A team of researchers at Edinburgh University analysed fossils in Siberian limestone rocks, and found that hard-bodied organisms only populated environments with enough calcium carbonate. As calcium ion levels rose in the early Cambrian, animals may have developed more sophisticated calcium-dependent body parts. During the early Cambrian, shallow seas moved back and forth across the land, eroding surface rock, uncovering fresh rock, which reacted with air and water to release mineral ions like calcium, iron, potassium and silica into the oceans. Animals needed to avoid an imbalance of ions. So they used the extra ions to make structural supports, protective shells, and predatory weapons, in a process called biomineralization.
I mentioned earlier that the supercontinent Rodinia broke apart during the early Cambrian. Tectonic activity was likely important in releasing nutrients and minerals. Glacial events and shifting sea levels may have created habitats viable for accelerated evolution. Some scientists believe that broad continental shelves, i.e. areas of continental land submerged under relatively shallow water, with lagoons and pools, may have provided enough living space for a wider variety of more advanced animals to coexist.
Speaking of animal coexistence, ecological relationships - namely predator-prey dynamics - may have further accelerated the Cambrian explosion of life. If you are a slow, soft-bodied animal of the late-Ediacaran, happily grazing and absorbing nutrients through your skin, your species has little reason to change its body or behaviour. Changes in climate and environmental geochemistry do act as selection pressures. But they act very slowly. This is clear in the 3.5 billion years of Precambrian history. But then everything aligns - oxygen levels, mineral levels, water depth - and earth passes a threshold, a set of criteria for animals to develop carnivory. Now you, that simple Ediacaran animal, are being hunted. Small environmental changes may have triggered an evolutionary arms-race between species, as prey adapted to evade predators, predators adapted in response, and prey was forced to adapt again. Hence an explosion in biodiversity.
This episode, I introduced you to the Cambrian explosion. I explained how evolution works and how we determine geologic time scales using stratigraphy. Then I gave a speed-run of all of earth’s history, through the Hadean, Archean, Proterozoic and Phanerozoic eons. After highlighting the enormity of life’s past, we focused on the transition from the Ediacaran to the Cambrian period 541 million years ago, and asked why the rapid escalation of biodiversity there was so significant. It was because of how quickly animals developed, and how many new groups formed. Ultimately, the intrigue around the Cambrian explosion is: what caused it? We examined likely contributing factors: rising oxygen concentrations, rising oceanic calcium concentrations, changes in the available living space for animals, and developments in ecological relationships.
In reality, not all the developments commonly attributed to the Cambrian explosion happened in the early Cambrian period. There’s fossil evidence of complex multicellular animals and hard mineral shells in the Ediacaran, and even animals as early as the Cryogenian period. Even so, the fossil record around the Ediacaran-Cambrian border indicates milestones worthy of interest.
Geologists, paleontologists, and evolutionary biologists are constantly producing evidence that supports and diminishes the importance of the Cambrian explosion’s potential causes. But they were likely all important. Life is complex, as is its history, and evolutionary developments are usually produced by a multitude of interacting factors. And that’s before one considers the relevancy of feedback loops in cascading development.
Considering how long ago the Cambrian explosion happened, one might question if we can ever truly know why it happened. Clues are hard to find, and even harder to piece together.
But this is science, and as scientists do, we’ll continue to search for the truth, or partial truths, at least; partial answers to another of life’s great unsolved questions.
Thank you for listening.
References: