If the origin of life on Earth is a mystery wrapped inside a black box, then so is the great explosion of life that happened during the Cambrian period, beginning 540 million years ago, when carbon went wild.
Nearly four billion years after Earth’s formation, life was still mostly bacteria, plankton, and algae. Complex, multi-celled organisms developed only 600 million years before today, during the Precambrian period, just as the planet emerged from a long period of intense glaciation (Ice Age). A mini-explosion of life swiftly followed, including the appearance of soft-shelled tubular and frond-shaped organisms, all of which lived in the sea. Although it is not clear to scientists what caused this mini-explosion, it is probably not a coincidence that it occurred as a massive supercontinent called Rodinia began to break up, likely causing temperatures on the seafloor to warm.
Whatever the reason, it was just the opening act to the Main Show.
Early on, geologists recognized that the sudden appearance of complex animals with mineralized skeletons in the fossil record of the Cambrian period represented a ‘explosion’ of life. Further research revealed it to be a biological eruption of extraordinary diversity and quantity (“radiation” is the scientific term). All major animal body plans and most of the major animal groups that we know today (i.e., every extant phylum) appeared during this unprecedented event. To many scientists, it was the most important evolutionary transformation in the history of life on Earth, which is why it is sometimes called the “biological Big Bang.” Of course, this event didn’t take place overnight – ten million years is more accurate – but in geological terms it was still a blink-of-the-eye. And it was never repeated again on this scale.
The creatures that came into existence during the Cambrian Period were relatively small, widely dispersed and entirely aquatic. Thanks to their reproduction in school books, the exotic shapes of these creatures are familiar to us, including brachiopods, with their clam-like shells, trilobites, which were armored arthropods, early mollusks and beautiful echinoderms, known today as starfish. Despite their proliferation, however, many Cambrian creatures eventually went extinct, including the exotic Opabinia, which had five eyes and a nose like a fire hose, and Wiwaxia, an armored slug with two rows of upright scales.
Along with all this biological diversity came a radical new ecological development: predation. The fossil record clearly shows that some creatures were hunters and some were prey – a development that had profound evolutionary consequences for life from this point forward. Ecosystems became much more complex as a result and many animals moved (or were chased) into a variety of new marine habitats. Soon, Cambrian seas teemed with animal life of various sizes, shapes, and ecologies; some lived on the sea floor, while others swam around in the water. By the end of the period, a few animals had also made revolutionary (and temporary) first forays onto land, soon to be followed by plants, changing life on Earth profoundly once again.
All of this was a great worry to Charles Darwin, who fretted that his theory of evolution, which postulated a steady, gradual process of change over the eons, would be attacked by religious critics who believed that such an explosion of life was indisputable evidence of God’s hand at work – a belief that persists to this day. Darwin assumed the answer to his concern lay in the sketchy Precambrian fossil record (soft-shelled creatures make poor fossils). He hoped new discoveries would eventually support his theory – which has more or less happened. Meanwhile, a theory of rapid, or ‘punctuated,’ evolution was put forward in the 1970s by biologists as an alternative to Darwin’s ‘gradualism’ thesis. This theory argues that evolution can happen in bursts when conditions are right. Others argue that the Cambrian Explosion was just too huge to be explained this way – and the debate goes on.
Why did life explode like that? Some scientists point their finger at a rise in oxygen levels that started around 700 million years ago, which might have provided ‘fuel’ for an evolutionary explosion. Others believe that a biological extinction event just prior to the start of the Cambrian opened up ecological niches for new creatures (the way that mammals filled the big niche left by the sudden extinction of dinosaurs 65 million years ago). Others point at the ‘stitching together’ of the supercontinent Gondwana at this time (today’s South America, Africa, Antarctica, and Australia) as the likely stimulant. Then there was the so-called ‘carbon anomaly’ at the Precambrian-Cambrian boundary, in which the normal ratio of carbon isotopes in the carbon cycle were dramatically upset by something, possibly a result of the earlier extinction event. Other researchers say the animals themselves were responsible. One of the evolutionary consequences of predator-prey behavior, for example, might have been the development of shells and bony skeletons for protection. Maybe creatures were forced into ‘marginal’ ecological niches where they had to adapt to survive, creating new body types where none existed previously. Maybe it was something else. Maybe it was all of the above.
It’s certainly a fascinating mystery still.
Here’s a fossil of a trilobite:
One thing is clear: life is a force that will not be denied.
Four billion years ago, against every conceivable odd, life came into being where no life existed previously. Chemistry yielded biology, and once life gained a perch it tenaciously clung on, enduring billions of years of environmental stress. Seas boiled and froze; land flooded, rose, sank, and rose again. Oxygen levels – essential to life – were dangerously low for much of Earth’s history, an issue that was only resolved in favor of existence by the miracle of photosynthesis – an invention of life. Life perpetuating life. Undaunted by circumstance, biology pushed forward, urged on by evolution, overcoming whatever physical challenge or toxic condition chemistry could throw at it. Life endured because it had one overriding purpose: to keep living. To keep going; to adapt, change, respond – whatever you want to call it – and not stop doing so. Then suddenly, 540 million years ago, the conditions became optimal for a massive bloom of life.
In my unscientific opinion, the real reason why the Cambrian Explosion happened is this: life finds a way. We can debate the triggers, whether it was a flood of oxygen, the arrival of predators, or the great mash-up of continents, but the result was the same – life found a way to take off. There’s nothing particularly mysterious about it, I think, nor do we need to resort to divine explanations. What happened in the Cambrian Period wasn’t magical, inexplicable, or miraculous. Life finds a way, that’s all. Roughly 500 million years later, for example, the greatest biological extinction event in Earth’s history took place, called the Paleocene-Eocene Thermal Maximum (PETM), when global temperatures spiked 6 degrees Celsius over a period of only 20,000 years, apparently as a result of a sharp rise in greenhouse gases. Life on Earth crashed and might have been extinguished – except it didn’t. Life hung on, rebounding steadily over time, growing into the modern profusion we know today, refusing to be denied.
Life is a force is be reckoned with – and this gives me hope. Knocked to the mat, life gets up to fight again. It may not look the same, it may have changed in some fundamental way, but it will find a way to continue.
Human beings, on the other hand, may be another story. That’s because species come and go. By one estimate, there have been 100 million species on the planet since Cambrian times, though I suspect that does not include the myriad of bacteria and other critters that live in the soil. The best guess (mathematically) for the quantity of species alive today is 9 million, according to a recent study. The average life span of any one species is five million years. So, our odds of continuing much longer maybe aren’t so great. We’re just one species in 100 million, and we’re about five million years old. And look at what we’ve done to the place!
Whatever happens to us, it seems certain that life will continue with or without us. Always has, always will. We might be denied – by our own hand too – but the force that created us will carry on – and I find that an encouraging thought.
And carbon has been there from the start.
Here’s a drawing of the Cambrian Explosion: