The reconstructions of Cambrian animals were so strange that at scientific conferences they aroused widespread mirth: Opabinia (an arthropod) had five eyes on stalks and a trunk, ribbed like a vacuum cleaner pipe, with a grasping jaw at the end. The name Hallucigenia speaks for itself; this animal resembled something one would usually only see under the influence of LSD.
If, with our geological hammer in hand, we examine rock layers from the oldest to the youngest, we will notice an amazing phenomenon: in the earliest layers, with the naked eye, we will not see any signs of life; no fossils. However, from a certain moment, on a line in time as sharp as a knife, incredibly rich forms of life appear. This is the Cambrian world, a geological period that began 541 million years ago and lasted just over 55 million years. The demarcation between the Cambrian and the previous period, devoid of any signs of life, is so marked that this phenomenon acquired the description ‘the Cambrian explosion’.
Family excavations
“Camping in the Canadian Rockies is a relatively simple affair if one is accustomed to going about with saddle and pack animals for conveyance,” wrote the American palaeontologist Charles Walcott for National Geographic in 1911. Two years earlier he had discovered an extraordinary palaeontological site in those mountains: the Burgess Shale. The uniqueness of this place rested not only on the fact that the hard shells or exoskeletons of organisms were preserved there, but also entire imprints of their soft parts. There were also the imprints of animals that didn’t have any hard skeleton at all.
Today, the Burgess Shale sits at a height of 2500m above sea level, but 505 million years ago, during the Cambrian Period, it was sunk 100m below the surface of the ocean near the equator. So, the Canadian Rockies have preserved the traces of an ancient undersea life.
For several years Walcott returned to the Burgess Shale with his wife, Helen, and their children, Sidney, Stuart and Helen, in order to look for the characteristic trilobites and other fossils. Sidney, for example, discovered the imprint of a predatory arthropod, a dozen or so centimetres long which, in honour of the young boy, was later named Sidneyia inexpectans.
Thanks to the National Geographic article, there was quite a noise about the Burgess Shale, but this fame quickly passed and the Shale was forgotten about until, half a century later, a certain young Italian happened upon them.
This is not a hallucination
One of the most important palaeontological discoveries of the 20th century took place in… a museum. In 1962, the young Italian biologist Alberto Simonetta was looking again through fossils from the Burgess Shale. Among the numerous specimens, he came across the remains of animals that aren’t found anywhere else. Wiwaxia looked like half an artichoke with a double mohawk of feathers. The miniscule Marrella could have been a monster from a sci-fi horror film, with backward-curving horns and more than 20 pairs of branched legs. Anomalocaris was a predatory arthropod a metre long with a series of fins, eyes on stalks and two menacing, 20-centimetre arms for catching prey.
A little while later, independently of Simonetta, a British man, Harry Whittington, began to study the Burgess Shale. His discoveries and reconstructions were so strange that at scientific conferences they caused widespread mirth: Opabinia (an arthropod) had five eyes on stalks and a trunk, ribbed like a vacuum cleaner pipe, with a grasping jaw at the end. The name Hallucigenia speaks for itself; this animal resembled something one would usually only see under the influence of LSD; it crawled across the seabed on numerous, flexible, clawed legs, with symmetrically pointed sharp spikes.
This entire undersea menagerie was surprising not just for the richness of the wild forms, but also for the suddenness of their appearance. How had the rapid appearance of complex organisms on Earth happened? The key to solving this riddle must be sought in the period immediately preceding the Cambrian explosion.
A lost world
Today we know that life on Earth began at least 3.5 billion years ago, shortly after the planet had been bombarded by meteors and the temperature had fallen enough to allow seas and oceans to appear. However, for the majority of the time since birth, life was single-celled, simple organisms, whose remains have survived to today in the form of stromatolites, i.e. multilayer sheets of cyanobacteria and other bacteria. It was long thought that no other fossils had survived from that time; no fragments of larger animals or plants, imprints or shells. That’s why this era is known as the Proterozoic era, i.e. the era of hidden life.
However, at the very end of the 19th century in Newfoundland, in layers directly preceding the Cambrian Period, mysterious traces were found, made up of concentric cavities or protrusions, somewhat reminiscent of fried eggs or cartoon animal eyes. One of the first Canadian palaeontologists, Elkanah Billings, suggested that these could be animal fossils. This conflicted with the prevailing dogma which said that fossils from before the Cambrian Period were impossible, and so the scientific world rejected his interpretation. When identical fossils were discovered in Namibia in the 1930s, they responded in the opposite way: although they agreed that the find (today called Aspidella) was the traces of animals, they said that it must therefore come from the Cambrian Period. As you can see, sometimes science also follows the principle that if the facts do not match the theory, it’s bad luck on the facts!
In time, they started to come across ever greater numbers of species that undoubtedly came from before the Cambrian Period. In the 1950s, the 15-year-old British girl Tina Negus discovered a beautiful, fern-like fossil, later called Charnia (from the Charnwood forest in England where it was discovered). The teenager was interested in geology and was well aware of the unusual nature of her discovery, in which she had tried, without success, to interest her teachers. It was recognized in the end, but credited not to her, but to a boy – a student called Roger Mason, in whose honour one species was named Charnia masoni. Later there was an absolute avalanche of discoveries, including some in southern Australia in the Ediacara Hills. This is why this geological period and the fauna related to it is today called the Ediacaran Period.
Thanks to these discoveries, we know that the situation changed approximately 570 million years ago, several tens of millions of years before the Cambrian explosion. Fantastical shapes appear in the fossils of the Ediacaran period: ribbed, symmetrical, beautiful, delicate imprints called Dickinsonia; Kimberella, which looks like a pair of castanets separated by a soft lip; and Spriggina, who resembles the later trilobites with its ribs. So, although we traditionally count the Ediacaran Period as part of the Proterozoic Eon, there is no shadow of a doubt that we are looking at the imprints of multi-cellular organisms. The common feature of all of these creatures was most probably that they led a settled lifestyle; we can therefore assume that they were not predators.
All these remains, however, have another fascinating common feature: they bear no similarities to anything that appeared later on Earth. Organisms from the Ediacaran Period were unique. Even today we cannot be sure how they really looked and to which contemporary animals they are related. Probably they lived in an environment dominated by micro-organisms, which covered the bottom of the seas and oceans with large swathes of microbiological mats. And we know that this world ended irreversibly with the rise of the Cambrian Period. Microbiological mats stopped being so widespread, and the delicate, fantastic forms of the Ediacaran organisms also disappeared. Their place was taken by an array of segmented bodies, eyes on stalks, branched legs, shells, gills, claws and spines. Predators hunting other animals had appeared.
What caused the end of the Ediacaran Period? There are two main hypotheses. Above all, just as with the five great extinctions of the Phanerozoic Eon (during which the dinosaurs, among others, died out – everything except birds), this too could have been caused by some natural catastrophe; e.g. a gigantic meteor strike or a period of exceptional geological activity.
There is, however, another explanation.
The element of the predators
From the very outset, life changed the face of the Earth. It led to the creation of sedimentary rocks and changed the chemical composition of the oceans and the atmosphere. Since its inception, one of the most important results of life is the production of oxygen. Photosynthesising organisms convert carbon dioxide into carbohydrate and give off oxygen. The production of this element by the early living organisms led to the first ecological catastrophe on Earth – an oxygen catastrophe that took place around 2 to 2.5 billion years ago. Masses of organisms that were sensitive to oxygen were wiped out. However, this dramatic event allowed animals to develop. They don’t take their energy from photosynthesis, but from oxidization. The world of animals therefore needed oxygen in order to exist.
Predators – animals that hunt other animals – are particularly fond of oxygen. A predator has to be on the move constantly in search of its victims. It must have efficient musculature, sensory organs, and a complex nervous system that enables it to process stimuli from its surroundings. So it needs legs, fins, gills and eyes, all of which we first find traces of in the Cambrian Period. What’s more, the evolution of the hunter demands a response from the hunted, who must learn to observe its environment and protect itself or run away.
For the majority of the Proterozoic Eon, the concentration of oxygen in the atmosphere was 10 to 100 times lower than today. This therefore prompts the fundamental questions: When did oxygen appear on Earth in quantities similar to those today? When did the concentration of oxygen get high enough for animals? And for predators? It turns out that there are minerals whose composition reflects the amount of oxygen dissolved in water. Thanks to these studies, we know today that 520 million years ago, at the start of the Cambrian Period, the concentration of oxygen in the seas and oceans was more or less the same as today. Earlier, however, it was gradually increasing, creating local environments where predators could develop.
Yet this explanation isn’t entirely satisfactory either. Did predators really only appear on Earth in the Cambrian Period? Maybe they existed beforehand, but there are simply no surviving traces of them?
A clock in genes
The DNA of all living organisms can be presented as an unimaginably long series of four letters, A, G, C and T, signifying the four building ‘blocks’ from which our DNA is built. Some of them perform a very important function – they contain the information used by every living cell. Some of this information is so important that any mutation causes instant death. Other mutations can be beneficial.
But many letters don’t have any significance and can mutate without consequences. These mutations – neither beneficial nor harmful and therefore neutral – appear by chance and establish themselves in future generations, and the pace at which they appear can be constant for millions of years. Related organisms, such as dogs and cats, humans and chimpanzees, and even a human and an arthropod, will always differ from each other by a certain number of neutral mutations, this number being higher the longer it is since they last had a common ancestor. This is, in fact, the famous ‘molecular clock’.
The question of when the common ancestor of all animals with bilateral symmetry lived (their time came in the Cambrian Period) has long fascinated scientists. The results of molecular analysis were surprising; showing unequivocally that this mysterious protoplast existed a good 100 million years before the start of the Cambrian Period. So, in the times when the Earth’s fauna was dominated by the mysterious Ediacaran organisms, there was an organism alive somewhere whose descendants included both trilobites and Hallucigenia (and humans, because we also have bodies with bilateral symmetry).
So why haven’t we yet found traces of it? There’s no good explanation. Some think that the Ediacaran fauna are related to the ancestor of the animals from the Cambrian explosion. However, our wonderful archive, in the form of fossil records in rock formations and geological layers, is very incomplete, particularly if we are talking about the earliest times. Very few layers have survived from the most interesting period just before the Cambrian explosion, where potentially we might find some fossils. Moreover, the amount of remains would be a tiny fraction of all the organisms living on the Earth; hundreds of thousands of years, if not millions, might pass from the appearance of some species to the extraordinary event that is the fossilization of one of them in the form of an imprint. Rare or small organisms – and this is what the ancestors of all animals were like – had a much smaller chance of preservation.
Like weed on a pond
There is yet another reason, which can be illustrated nicely by an old riddle. If pondweed on the surface of a pond doubles its surface area every day and, after 40 days, covers the whole pond, on which day is exactly half of the surface covered? The intuitive answer, the 20th day, is wrong; half of the pond is only covered on the 39th day, as each doubling only takes one day. This is what we call an exponential growth rate. From the perspective of someone observing the pond, the change was almost instant: for many days nothing obvious was happening and even four days before the end the pondweed covered less than 10% of the surface; after which suddenly the pond surface disappeared under its weedy coat.
If the spread of the first animals and predators looked similar, this process must have happened tens, if not hundreds of millions of years before the organisms reached sufficient numbers to land up in the fossil record. So, the process of the emergence of the Cambrian Period fauna probably started 100 million years before the Cambrian Period itself. As one of the researchers put it, the Cambrian explosion was neither an explosion, nor Cambrian.
Looking from a distance, we usually see sudden starts to new geological periods. But if one examines the matter more carefully, it may be that the seeds of the new world existed long before the old world was finished. They were developing in the background, unseen, and the explosion was only the last step as they took advantage of the opportunity.
Translated from the Polish by Annie Jaroszewicz
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