The Economist What caused the Cambrian explosion? The other Big Bang In the fourth of our series of articles on scientific mysteries we ask why, a mere 542m years ago, animal life suddenly took off Aug 29th 2015 IN THE 1800s, when early geologists started to work out the order in which things had happened in Earth’s history, they quickly assembled a rough chronology. Though their successors had to wait until the invention of radio-isotopic dating, a century or more later, to find out how old the rocks these pioneers were looking at actually were, 19th-century geologists were able to discover those rocks’ ages relative to one other.
The chronology they created was based on the ever-changing species of fossils within the rocks. These fossils are mainly animals, mainly marine and mainly ones with hard parts such as shells, which are easily preserved. The method worked well, but only up to a point. Below the strata they dubbed the Cambrian, fossils vanished. Unknown, lifeless aeons stretched into the past. It was as if an almost biblical act of creation had populated the Earth overnight, after an indeterminate period of its being a desert. That period, it is now known, lasted for 4 billion years: some 90% of the planet’s history. It was not, as discoveries made in the second half of the 20th century have shown, quite as deserted as was once believed. But the Cambrian’s beginning marked such a radical shift in the planet’s biology that the term “Cambrian explosion” is often used to describe it—for, in the course of perhaps 20m years, the world’s fauna diversified from simple beginnings into something resembling its modern, complex variety. Annelids, arthropods, brachiopods, echinoderms, molluscs and backboneless ancestors of the vertebrates all made their first appearances then. No animals yet lived on land, but the seas were suddenly teeming with them. What has never been clear is why. Shell game Part of the mystery is that it took so long for the explosion to detonate. Fossils unknown to the Victorians show bacterial life goes back at least 3.5 billion years, while chemical traces in rocks 3.8 billion years old are interpreted by some as signs that biology was at work even then. About 2 billion years ago bacteria were joined by more complicated (but still single-celled) creatures called eukaryotes. Then, some time before 760m years ago, some of these eukaryotes linked up to form tiny sponges. Those were the first animals, though not very complicated ones. Signs of more complex creatures appear 632m years ago in rocks from the Doushantuo formation in China, just after the beginning of a period called the Ediacaran (see chart). These early fossils are interpreted by some (though not all) palaeontologists as eggs or early-stage embryos. What they would have grown into is obscure, for the Doushantuo contains no adult animals. But creatures as much as a metre across that may or may not be jellyfish, sea pens and worms of some description (see pictures), and which are known as the Ediacaran fauna, appear and become abundant from about 575m years ago. None, though, had shells, so they did not preserve easily—one reason why early fossil hunters missed them. A jellyfish? Then, suddenly, animals started putting on armour. The fossil record of these armoured creatures, known as the small shelly fauna, marks the beginning of the Cambrian period, 542m years ago. Crucially, although the small-shelly-fauna fossils are mainly disarticulated bits and pieces, those bits and pieces come from different types of animal, and are made of a variety of materials. They are probably, therefore, the result of parallel evolution rather than of a single innovation which bestowed upon its descendants the power to take over the world. Where the creatures of the small shelly fauna led, others rapidly followed. Echinoderms (a group represented today by starfish and sea urchins) were there from 540m years ago. Brachiopods (lampshells) appeared 530m years ago and the Cambrian’s most iconic fossils, the trilobites, date from 521m years ago. In a geological eye-blink, the explosion had happened. Once more, though, things are not straightforward. The diversity of form of the first known trilobites, for instance, suggests they had been evolving for millions of years before they all, simultaneously, acquired coats of armour. And some people claim that an Ediacaran fossil called Arkarua is an echinoderm, though it lacks the shelly exoskeleton characteristic of the group. Moreover, certain strata with particularly good preservation, such as the Burgess shale in western Canada, show that the explosion was not just connected with the evolution of hard body parts. Soft-bodied creatures diversified as well. One explanation for the Cambrian explosion is that something changed in the physical environment. Perhaps it was a rise in oxygen levels, which allowed the respiratory requirements of larger animals to be accommodated. Perhaps it was more calcium in the sea, providing material for the construction of shells—for these are often made of calcium carbonate, calcium phosphate or both. Evidence for either of these possibilities is scarce, but there may be a link to a phenomenon dubbed “Snowball Earth”. This was a period of huge, intermittent ice ages, known formally as the Cryogenian, when glaciers sometimes stretched close to the equator. When these glaciers melted, huge areas of rock were suddenly exposed to weathering, and their mineral contents dumped into the oceans. This might have raised calcium levels (for rocks are full of calcium) or oxygen levels (by pouring nutrients into the sea, and encouraging the growth of photosynthetic algae) or both. Unfortunately for this explanation, the last of these giant ice ages ended 635m years ago. They might thus have played some part in the evolution of Ediacaran animals, but invoking them to explain the Cambrian explosion is a stretch. Alternative explanations focus on biology—specifically, the idea that the animals crossed some sort of evolutionary threshold at the beginning of the Cambrian. Their history of increasing complexity certainly makes this plausible. Sponges are mere agglomerations of cells, but jellyfish and sea pens belong to a group called diploblasts, which have much more structure. Diploblasts develop from specialised embryos that have two cellular “germ layers” , the endoderm and the ectoderm. Their adults have proper tissues (for example, nervous systems) and regular, radially symmetrical body shapes. The most complex animals of all, though, are triploblasts. Their embryos have a third germ layer, the mesoderm, between endoderm and ectoderm, and they have many more sorts of tissues than diploblasts, and also discrete organs. In addition, they are bilaterally symmetrical, at least when they are young. And it is the triploblasts that do most of the exploding in the Cambrian. Bilateral symmetry is advanced as an explanation of the Cambrian explosion because it encourages animals to have a front and a rear. That means they can move purposefully in a particular direction, which radially symmetrical animals find hard. When that happens, sense organs and the nerves associated with them tend to accumulate at the front, where they are most useful. This process, called cephalisation, encourages bilateral animals to evolve brains, in order to interpret and integrate the signals from the sense organs. And bilateral animals also have linear guts, with a mouth and an anus. That is a much more efficient arrangement than the diploblastic one of expelling the undigestible parts of food items back out of the mouth. A who-knows-what On this view, bilateralism was an advance akin to the evolution of wings. It opened previously unavailable opportunities, one of which was active hunting that relied on directed movement and a cephalised nervous system. That would explain the sudden rise of armoured skeletons in different sorts of animal, as a response to being hunted. Bilateralism alone, then, might have triggered the explosion—but for the inconvenient fact that some Ediacaran fossils (such as the one pictured here) are bilaterally symmetrical. Transitional benefits There is, however, one other thought—that the Cambrian explosion is not the fundamental mystery it seems to be. The true mystery, rather, is the Ediacaran, whose animals really did appear out of nowhere, and then vanished for reasons unknown before the Cambrian got going. The fossil record is full of sudden cast changes like this. They are known as mass extinctions. The most famous is the disappearance of the dinosaurs (and a lot of other reptiles) at the end of the Mesozoic era. In due course, these were replaced by a hitherto unimportant group, the mammals. Much of the supposed uniqueness of the Cambrian explosion is a hangover from the 19th-century belief that there were no Precambrian animals. But, as the Ediacaran fauna shows, that is not true. In a mass extinction the board of life is cleared of many pieces by an external event, such as the asteroid strike that did for the dinosaurs. It then takes several million years for replacements to evolve from whatever is left. Those remnants might have been (as Mesozoic mammals were) insignificant in the previous regime. In the case of the Ediacaran, given how long ago it was and how few of its rocks are available for inspection, they might thus remain almost undetected by palaeontologists. Arkarua, known from a single site in Australia, may be an example of one of these cryptic animals-in-waiting. It is true that no evidence of an extinction-causing event has been found in rocks that straddle the Ediacaran and Cambrian periods, but several later mass extinctions have no known cause, either. And, if such an extinction did happen, it is perfectly plausible that some unidentified Ediacaran triploblasts went on to play the role of the Mesozoic mammals by emerging from obscurity when the competition had been eliminated. In the chaotic aftermath of the extinction, they would have been able to multiply, diversify and drive each other’s evolution (by such means as hunting one another) in ways that foreshadowed those of the modern world, red in tooth and claw. -- -- Centroids: The Center of the Radical Centrist Community <[email protected]> Google Group: http://groups.google.com/group/RadicalCentrism Radical Centrism website and blog: http://RadicalCentrism.org --- You received this message because you are subscribed to the Google Groups "Centroids: The Center of the Radical Centrist Community" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
