Planet of the Bugs: Evolution and the Rise of Insects (6 page)

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 clus
ters. 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
Tri
chophycus 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 some 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.

I present to you a simple proposition: when it comes to animal form, an external skeleton is better. It’s certainly more likely to develop in the first place, because it’s better to shunt toxic excretory byproducts to the outside rather than storing them on the inside, and it’s obviously far more likely to succeed because of immediate defen
sive advantages. Just look at the vast diversity of trilobite species that lived at the end of the Cambrian period versus our little
Pikaia
relative hiding in the sediments. Then also consider our modern world, where all vertebrate species combined present only a small fraction of seemingly astronomical arthropod diversity. Success might be measured in various ways. We are ever so proud of our oversized contemplative brains. But with those brains we must ponder the sheer improbability of our existence and our constant vulnerability through the ages. The humble insects cannot contemplate their own measure of success: the numerical dominance of arthropod species through all ages of animal life. If this were to all play out again on another planet, it seems to me highly improbable that soft-bodied creatures with internal skeletons would develop first or become successful over the long run. Hard-covered creatures with external skeletons would almost certainly hold the advantage over time, in most contests of soft versus hard-shelled players.

Looking back on the earliest Cambrian trace fossils, those worm burrows, we know that even those simple animals could tunnel in sediments. We don’t suppose that they had any legs yet, because there are no fossil footprints. So how did they move? We must assume they had muscles arranged in body segments, allowing them to contract segments and wiggle their bodies, as with modern earthworms. Segmentation is a common body form, but it is an ancient one as well. All modern insects are segmented animals; hence the name “insect,” which means “in sections.” But that characteristic is not unique to insects. It is an inherited trait from earlier ancestors. All arthropods, including the trilobites, were segmented creatures, and so were simpler creatures, such as the annelid worms. That ancient burrowing creature
Trichophycus pedum
was probably also segmented, precisely because it could tunnel, but did not have apparent legs.

The origins of segmentation clearly reside in the earliest multicellular animals. Just as single cells became multicellular aggregations by building duplicates of themselves, early multicellular creatures became segmental by building duplicates of their cellular arrangements and linking them in a chain. Segmentation is an excellent trait for an animal, not just because the components are easiest to build and link together, but also because the shock is less when you lose a part to an accident. If you have seen the science-fiction movie
The Core
, you may
recall the following example: explorers used a multisegmented craft to travel deep into the earth, losing parts along the way but surviving. Trilobites and insects can lose body parts and survive more easily than we can.

It’s no mistake that the earliest skeletal parts were on the outside. As I already mentioned, it makes more sense to shunt waste products to the exterior than to pile them up on the inside. If an ancient wormlike creature were evolving a hard outer skeleton, it is only logical that the skeleton would form in segmental plates, just as in all arthropods. They already had segments with muscles. Body flexibility could only be maintained if the segmental parts remained flexible, with membranes at the edges. Any attempt at a fully hardened exterior would be useless and maladaptive, because a completely hardened creature could not move at all.