Planet of the Bugs: Evolution and the Rise of Insects (5 page)

By now, the implicit human-centrist bias in some of that history I just recounted should be obvious. By noting the times of the proliferation of mammals, reptiles, amphibians, and fish, we are merely observing some tenuous history of events leading to the origin of the human species. How ridiculously unlikely it seems that we should be here at all, and how those labels for the geological periods distract from the real pattern of life’s diversity. I’ll be picking apart this human-centrist mythology bit by bit as we proceed. For now, it will suffice to examine the Cambrian.

The Cambrian period has generally been called the “age of invertebrates.” That’s certainly not because anyone sought to glorify our invertebrate ancestry. It’s simply an observation that we didn’t initially see any of our vertebrate ancestors in fossils from Cambrian layers. Notice that we didn’t call it the “age of arthropods” or the “age of trilobites,” either of which would be apt. Calling it the “age of invertebrates” is a bit like calling it the “age of no humans.” The name subtly derides the success of arthropods by noting the absence of vertebrae rather than touting the evolution of exoskeletons. But subsequently, we did discover our likely vertebrate ancestor in Cambrian times, and what a humbling event that was. A small creature called
Pikaia
was discovered in the 515-million-year-old Burgess Shale fossils of Canada.
Pikaia
was a mere one-and-a-half-inch-long, wormlike creature that burrowed in bottom sediments. She was soft-bodied but did have an internal supporting structure: a primitive notochord, the ancestral structure of a vertebral column.
Pikaia
is now regarded as the most likely common ancestor of fish, amphibians, reptiles, dinosaurs, birds,
and mammals. But she was such a modest ancestor that no one lobbied for a renaming of the Cambrian as the “age of
Pikaia
.”

When viewed from a great temporal distance and with a nonhuman eye, the history of life appears differently. It’s certainly clear that there was no inevitable or rapid progression toward humans. A nonhuman space traveler might rewrite our biological history far more succinctly. The first three billion years or so might simply be called the “age of bacteria.” The time from the Cambrian to the present (the last half-billion years or so), the time of multicellular animal life, could simply be called the “age of arthropods.” Since the onset of animal complexity, the arthropods have been the singular successful group, both in diversity and abundance. The rise of insect diversity was ancient enough that the last three hundred million years or so could be dubbed the “age of insects.” The last ten thousand years, during which human civilizations have arisen, is but a miniscule and insignificant blip compared to the vast spans during which first bacteria, then arthropods, then insects in particular, have dominated this planet’s landscapes.

The other remarkable thing about the Cambrian is simply that
we
survived it at all, and by “we” I mean not just humans but the entire lineage of vertebrate animals. There it was,
Pikaia
, our humble wormlike ancestor, tunneling along in bottom sediments. It is quite rare in Cambrian fossils and perhaps was never a very abundant creature, even in those days. In the waters above, along cruised animals like
Anomalocaris
, a three-foot-long, nightmarish predatory arthropod with long, spiny feeding appendages.
Anomalocaris
paddled along in the Cambrian seas, picking off whatever small animals it could catch—no doubt feasting on lots of trilobites. From time to time,
Anomalocaris
doubtless swooped down to pick off a tender
Pikaia
for dinner. Luckily for us humans, most of the abundant Cambrian arthropods, trilobites, also fed in the bottom sediments, peacefully, along with
Pikaia
; however, there is evidence that some of the Cambrian trilobites may have been predatory. There are fossils of trilobite tracks intersecting with worm burrows and resting, which suggests that some trilobites may have preyed on soft worms in the sediments. Trilobite body forms and mouthpart styles are certainly diverse enough to suggest that they had evolved a comparable variety of feeding habits. But still, if Cambrian trilobites had become extensively predatory, then it’s exceedingly unlikely that
we
would be here to piece together this story.

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 Paleo
zoic 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 fos
sils, 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 vari
ous 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 presumably 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.