Life’s a Blast: The Cambrian Explosion
The Cambrian period, roughly 541 to 485 million years ago, was a definitive time in the development of life on earth. Finally, after 3 billion of years of microbial history, single cells assembled into functional groups, and multicellular animals appeared. Animals wasted little time, geologically speaking, in evolving structural support and protective gear: cuticles, skeletons, and shells appeared over a period of only 5 million years. The seemingly rapid evolution of early animals has prompted paleontologists to dub this event “the Cambrian explosion” of life, and this time is notable for the rapid evolution of diverse phyla, including the major lineages of organisms that still dominate the planet. Most relevant to our story is the first appearance of the phylum Arthropoda, the armor-plated lineage from which the insects would eventually emerge.
With the evolution of hard parts in animals, the earth’s geology was forever changed. Those hard parts fossilize well, and while fossils of Precambrian soft-bodied organisms are rare, the remains of hard-bodied Cambrian critters left comparatively abundant fossils. So from the Cambrian onward, the earth’s rock layers literally preserve impressions, pressed snapshots of past life.
These rock layers have been accurately dated using radioactive isotope distributions, but those from the Cambrian onward can be easily identified by the kinds of fossils found in them. The signature fossil group of that period, no doubt, is the trilobite. These small creatures had a segmented, three-lobed hard skeleton, hence the name: they are trilobed. Trilobites are some of the first common examples of the larger animal group into which insects are also assigned: the arthropods. Insects, spiders, lobsters, shrimp, millipedes, centipedes, scorpions, and trilobites all share the basic arthropod anatomical innovations: a hard, segmented external skeleton and several jointed legs. We will learn more about that later. For the moment, I just want you to recognize that those skeletons, being hard structural parts, fossilized very well. Trilobites were abundant enough in ancient shallow seas that their skeletons were frequently covered by sediments. Nowadays, you can take a walk in the prairies of Wyoming’s Bighorn Basin and see that the rocky landscape is studded with trilobite fossils. It’s a sure sign that the area was once under ocean water, and the rocks are of Paleozoic age. However, you won’t find trilobites or any other animal fossils in rocks that are 3 billion years old. They went extinct by around 252 million years ago. So you won’t find them in rock layers from the middle Mesozoic, mixed with dinosaur bones. And you certainly won’t find trilobites in places like Hawaii and San Ramon, Costa Rica, because those landscapes were formed by volcanic activity over the past few million years. They have no rocks dating back to the Paleozoic.
FIGURE 2.1. The fossilized molted exoskeleton of a Cambrian trilobite, Elrathia kingii, a common species in the Wheeler Formation of Millard County, Utah.
But in the quarries of the continental United States there are lots of Cambrian-aged rocks, and trilobite fossils are fairly common. You don’t need to be a paleontologist to see one. Walk into any rock shop in North America and you will probably find a bin full of trilobite fossils, most likely with a sign declaring them to be the oldest of animals. So common are the trilobites that you can buy a trilobite fossil for a couple of bucks and carry it around in your pocket, if you wish. In Ohio, Pennsylvania, and Wisconsin, they are official state fossils.
Skips in the Fossil Record
As much as we can learn by examining fossils, it is important to remember that they seldom tell the entire story: the fossil record is never as complete as we would wish. Things only fossilize under certain sets of conditions. Shallow marine communities with frequent sedimentation produce fossils comparatively well, so the record of Cambrian animals is not so bad. Modern insect communities are highly diverse in tropical forests, but the recent fossil record captures little of that diversity. Many creatures are consumed entirely or decompose rapidly when they die, so there may be no fossil record at all for important groups. It’s a bit similar to a family photo album. Maybe when you were born your parents bought a camera and took lots of pictures, but over the years they took photographs sporadically, and sometimes they got busy and forgot to take pictures at all. Very few of us have a complete photo record of our entire life. Fossils are just like that. Sometimes you get very clear pictures of the past, while at other times there are big gaps, and you need to notice what they are. One example from the Cambrian should suffice to make the point. A microscopic fossil of a tardigrade, a cute little animal that looks rather like a miniature teddy bear has been found in Sweden’s Cambrian sediments. Tardigrades, also known as water bears, still exist today—we have discovered them in water samples from bromeliad tanks in Ecuadorian forests—but no intervening fossils have been found. The fact that they were found in two places, the Cambrian shallow marine communities and the wet forests of the modern world, doesn’t mean that water bears evolved twice. It does show us that these animals evolved as early as the Cambrian and have persisted since, although there is no fossil record of it.
Rocks over time do not show a record of gradual steady change, and they certainly do not show any rapid progression toward humans. What the layers do show is a record of distinct times when communities of life emerged, and remained stable for millions or tens of millions of years. The reason we’ve divided the ages of life into these various times is not because we just wanted to divide it up. It’s because the layers themselves present distinct communities of life, and the interfaces of layers show sudden rapid change, often after tens of millions of years of stasis. When times are good, life doesn’t evolve simply because it has the capacity to change. Instead, when times are pleasant, species and communities of life tend to get into an equilibrium mode, where they’re well adapted to existing environmental conditions and move along comfortably for long periods of time. But the record of life is punctuated by occasional dramatic events that really give life a jolt: glaciers, continental drift, comets, asteroids, and the like. Occasionally, communities of life have suffered catastrophic mass extinction. But each time change occurs new species quickly evolve to fill the empty niches until a new equilibrium of life is established. This is what biologists call “punctuated equilibrium,” a term that was coined by the paleontologists Niles Eldredge and Stephen Jay Gould.
Setting the Stage for Arthropods: What Ignited the Cambrian Explosion?
One of the most striking observations about the history of life is the significant fact that life on this planet remained single-celled for roughly three billion years. Why did it take so agonizingly long for multicellular animals to evolve? A simple answer has been suggested: oxygen.
For 3 billion years, ancient bacteria bubbled away, making oxygen and binding carbon dioxide into calcium carbonate and carbon-based sediments. They may have been releasing oxygen, but for a long time the oxygen content of the atmosphere did not change much. Before oxygen could accumulate in the air, the free oxygen reacted with iron and other substances in the earth’s crust and oceans. It was locked up into sedimentary rocks, banded iron formations, and minerals for millions and billions of years. On the other hand, massive amounts of available carbon from excess carbon dioxide in the atmosphere were being drawn down and locked up into limestone deposits. During that period of mineral formation, a fortuitous balance was set between the sun and the earth. The ancient sun was cooler, but the thick carbon dioxide–rich atmosphere of old earth provided a warming blanket in the form of a greenhouse effect. Over time, as excess carbon dioxide was drawn down out of the earth’s atmosphere by living processes, the sun was growing warmer, so the earth remained comfortable for life. But eventually, around 2.3 billion years ago, the carbon dioxide level dropped too low, the earth entered a catastrophic ice age, and life experienced the first of what would be many punctuating events.
After billions of years of floating in balmy seas full of tasty amino acids, global glaciers overwhelmed the oceans, and most of earth’s ancient microbial life was presu
mably exterminated. Survivors probably included any bacteria lucky enough to be at comfortable interfaces where volcanic vents met and intermingled with frigid waters. Today, these kinds of bacteria thrive in Antarctica and in Yellowstone’s steaming volcanic vents and sulfurous hot springs. It was a tough situation for life, to be sure, but one that isolated cells into unique microenvironments and allowed genetic lines time to drift apart; natural selection insured that the remaining cells were indeed real survivors.
The global ice age, which may have lasted for millions of years, is indicated in the earth’s rocks by banded iron layers; these layers formed when iron accumulated in the oceans then precipitated into sediments. The rocks are capped with a calcium carbonate layer, indicating that the ice age ended abruptly with a period of global warming when continental minerals were washed into the seas, stimulating a worldwide flush of bacterial growth. Oxygen was ejected back into the atmosphere, and the world teemed once again with eukaryotic cells.
We might suppose that such a near-death experience might have been the necessary jolt to set life along a more complex pathway. But that does not seem to be the case. Bacterial cells resumed their old pattern of floating around for tens of millions of years. Then, about 850 million years ago, continental drift brought the land masses into an unfavorable configuration near the equator, and again the earth was cast into a planetary deep freeze. Glacial ice approached the equator, and the chill lasted for millions of years. Finally, the ice was broken, and life enjoyed a brief reprieve. But this time a cycle was established, and between 850 and 590 million years ago the earth experienced not just one but at least four global ice ages. The most recent of these, the great Varanger ice age, lasted 20 million years, from 610 to 590 million years ago; scientists have dubbed it the time of the “snowball earth.”
As the last snowball earth came to an end and the last global glaciers retreated toward the poles, life reassembled into multicellular clusters. Soon after that, abundant early animals appeared in the “Cambrian explosion.” What finally stimulated such dramatic changes in life forms, after billions of years of single-cellular domination? What happened most notably is that atmospheric oxygen levels finally rose to levels approximating our modern atmosphere. Potential oxygen toxicity drove cells into clusters for safety, but at the same time an energetic system existed to motivate animal life: aerobic respiration. Animals live more complex and energetic lives than bacteria because oxygen forced them to do it, and oxygen enabled them to do it. This process of oxygen levels affecting the evolution of life, the history of changes in oxygen levels, and the geological evidence for all this is thoroughly covered in Nick Lane’s popular and entertaining book, Oxygen, The Molecule That Made the World. So, there’s no need for me to repeat it all here.
After the last snowball earth, another very important event happened: continental drift accelerated and the Precambrian continents were dramatically reconfigured. The continental drift rate during the Cambrian has been estimated to be about ten times faster than the average rate since then. This was important for two reasons. First, it brought continental land masses back near the poles in a surprisingly rapid fashion. This stabilized the planet by reestablishing the cycle of weather that keeps ice ages more moderate. Some land masses have been near a pole, one way or the other, since that time, keeping modern earth out of the severe “snowball” phase. Second, for the Cambrian animals it was a bonanza, because rising sea levels and rapidly drifting continents meant lots of shorelines and more shallow marine communities with abundant mineral sediments. The earth was spinning faster during the Cambrian and the moon was closer, so tidal forces on shallow marine communities caused rapid pulses of nutrient flow. The time was ripe for rapid evolution of animal life.
Skeletons in the Cambrian Closet
Some of the earliest Cambrian animal action took place in shallow marine sediments. Among the oldest Cambrian fossils are “trace fossils” that do not show the actual animal, but animal tracks. These are abundant fossilized burrows, presumed to be caused by ancient marine worms. The oldest Cambrian animal to be given a name is Trichophycus pedum, based entirely on fossil burrows. It precedes the appearance of any hard-shell fossils. Having only the tunnels, we don’t know for sure if Trichophycus was one species of animal, or several with similar habits. Nevertheless, we can deduce a surprising number of things about them. Since they lived in bottom sediments, we assume they fed on accumulating organic sediments from bacterial life. The tunnels are long, narrow, and directional, so we know they could burrow through the sediment. This implies that they had front and back ends, a mouth and anus, and a digestive system. Since they moved and made tunnels, they must have had muscles and therefore an opposable cuticle system, some method of circulation, and of course, respiration. Most likely Trichophycus was a segmented wormlike creature, similar to the annelid worms, and insects presumably have inherited their segmentation from such ancestors.
The next layer of the Cambrian rocks is the first to contain hard part remnants of animals: tiny shells, spines, and small hard pieces that could be traces of the earliest external skeletons and are difficult to assign to more modern groups with any certainty. This layer is called simply the “small shelly fossils” or the “early shelly fossils.” While not much is known about these ancient animals, they do teach us something important. You may recall that the ancient cyanobacteria secreted calcium carbonate to form stromatolites. The evolution of external shells was a similar process. The early shelly animals built portable hard parts simply by secreting waste products that solidified. As animals evolved predatory habits, the aspect of shelliness would have immediate benefits. Aside from a protective covering, hard parts form the basis for skeletal systems, providing the opposable parts for musculature. So the evolution of shells was a step toward the evolution of more complex skeletons, musculature, and ultimately faster locomotion.
In the next layers of Cambrian rocks, layers 529 million years old and more recent, we begin to find the fossils of the so-called Cambrian macrofauna. These are the first fossils of animals with full skeletons and distinct limbs, which became more abundant as the Cambrian elapsed. For the most part, they are fossils of trilobites, and other arthropods. Examples of other recognizable groups (other phyla), such as sponges, corals, mollusks, and annelid worms also are present. There were also a bunch of weird and wonderful animals, unlike anything modern, that lived for a while then disappeared.
Often you will hear that most of the modern animal phyla appeared in the Cambrian period. This means only that we see the first examples of arthropods, annelids, mollusks, echinoderms, and chordates, the ancestors from which modern groups can trace their lineages. It certainly doesn’t mean that animal groups burst on the scene with anything like the species diversity that exists in modern animal phyla. It simply means that during the Cambrian the first arthropod species appeared, the first mollusk, the first echinoderm, the first annelid, the first chordate, and that each of these groups developed the basic body plans which characterize the modern animal phyla we see today.
Rock Stars of the Cambrian Seas
The real success story of animal diversity is the arthropods, as exemplified by the Cambrian trilobites. Found in the oldest Cambrian layers with the first Cambrian macrofauna, trilobites lived in the oceans until the end of the Paleozoic era, diversifying over a span of nearly three hundred million years. We have discovered nearly twenty thousand species of trilobites, most of which lived in the Cambrian and Early Ordovician. By the Late Cambrian, trilobite diversity peaked with more than six thousand species classified into eight hundred genera and seventy different trilobite family groups. Most of the trilobites dwelled on or burrowed into the shallow bottom sediments. Some large bottom-dwelling species appear to have had immature forms that were planktonic. But some trilobites could swim, and other small species appear to have been planktonic as well, moving about with the currents and tides. The trilobites may be long gone, but all modern arthropods have inherited som
e similar aspects of their body form: namely, a hardened external skeleton and several multijointed legs.
Let’s consider the evolution of skeletons, because if anything, the Cambrian explosion was a proliferation of hard parts, an explosion of skeletal diversity. Much of what’s been said about Cambrian animals in the popular press has focused on the weirdness of these animals’ skeletal forms. The Cambrian menagerie included strange creatures like Hallucigenia, which was so spiny and leggy that for years we didn’t know which side was the bottom or the top, or which was the head or the tail. But let’s not get distracted by the weirdness of subsequent modifications. When skeletons first evolved, there were only a couple possible approaches. You could build your skeleton on the outside, supporting and protecting your soft growing cells on the inside. Or, you could build a skeleton on the inside, purely for support. Basically, you could have either an external skeleton (trilobite style) or an internal skeleton (fish style). Some animals with internal skeletons might also mimic the arthropod anatomy by adding some exterior body armor: armored fishes, plated dinosaurs, modern-day armadillos, and King Arthur’s knights. But fundamentally, skeletons come in two styles: outside and inside. Both skeletal styles provide the necessary structural support for muscle attachment and locomotion, a key aspect of what it means to be an animal.
The advantage to having an external skeleton should be immediately obvious: it provides protection as well as support. It’s the same reason why we mostly wear shoes. The disadvantages are more subtle: an outer skeleton places some limits on sensory systems, as well as limiting growth. Arthropods may not have an outer skin to feel things as we do, but they compensated by covering the skeleton with sensory spines. Growth is more challenging for an arthropod; it’s tough to keep growing when you live inside a suit of armor. This required arthropods to evolve metabolic pathways allowing them to periodically molt an old skeleton and regrow a new one. It’s an adequate solution, but it does mean that they all have times when they are temporarily soft-bodied and vulnerable, like a soft-shelled crab. No doubt that tender stage is the most vulnerable to predators. We vertebrates, with our tedious internal skeletons, have only one real advantage over the arthropods. We do not need to molt or regrow new skeletons. Our growth period can be continuous, without such interruptions. Of course, we have the very serious disadvantage that our tender, tasty outside is constantly exposed to predators and the environment. So to protect their exposed bodies, vertebrates have compensated with scales, slime, armor plates, feathers, fur, and Levi’s denim. We protect our tasty flesh by encasing ourselves in the hard metallic shells of massive fossil-burning automobiles.
Planet of the Bugs: Evolution and the Rise of Insects Page 4