Planet of the Bugs: Evolution and the Rise of Insects (27 page)

The cursorial hypothesis gets around these problems by proposing that birds learned how to fly from the ground up. Although hotly debated, this idea remains close to my heart for a simple reason: it
asserts the fundamental importance of insect feeding as a driving mechanism in bird evolution. The cursorial hypothesis suggests that birds started out by running along the ground chasing insects. It further suggests that these animals first evolved large feathers on the forelimbs not for flight, but as improved insect-catching devices. The idea of ancient feathery dinosaurs rapidly running along through the forests, using their front legs as fly swatters, is not far-fetched. We have already come to grips with the notion that many small ground-dwelling Triassic dinosaurs were omnivorous carnivores and therefore highly insectivorous. And birds clearly evolved from these very same dinosaurs, little ones, like
Ornitholestes
, that would most likely have fed extensively upon bugs. It makes perfect sense to suppose that the earliest protobirds would also have eaten insects, and that strong selection would have favored any behaviors that improved their ability to rapidly catch insect prey. The main drawback to this hypothesis is evidence suggesting that
Archaeopteryx
spent little or no time on the ground: their claws show very little wear. But the two ideas need not be mutually exclusive. While
Archaeopteryx
may have been a flier living in the treetops, its immediate ancestors might have been ground-dwelling insect chasers.
¹³

Whatever you think of this wing-swatter idea, the history of the birds’ emergence is intertwined and codependent with the insects’ history and success. The first birds could move into the air, and what better reason than to gobble up the highly nutritious swarms that were everywhere around them? What the birds lacked in size, they made up in numbers, evolving into more than eight thousand modern species, the majority of them insectivorous.
¹⁴

One might suppose that once the feathery dinosaurs burst into the skies, they spelled trouble for the insects and hunted some of them to extinction; however, there’s not much evidence to support this. One group did disappear about the same time that the early birds arose—the titan insects—but they were overly large and noisy. They might have been all too easily hunted. Other groups appear not to have been impacted and actually became more diverse. We can safely assume that, from the first, birds were powerful selective forces which drove insect evolution in several ways. Day-active feeders, they probably influenced the evolution of many insects’ night activity. And as visu
ally searching predators, they helped stimulate the evolution of crypsis, aposematic coloration, and mimicry. So when dinosaurs finally feathered their way into the air, they didn’t hinder insect success. Bugs had been flying for 150 million years by the time birds finally came along; by then they were pretty good at it.

As birds diversified, insects were probably quick to exploit them. A new group of parasitic insects, the biting and chewing lice, were likely early bird colonists, though they’re not to be confused with their more common cousins, the blood-sucking mammal lice, which evolved much later. Both the biting and the blood-sucking lice (order Phthiraptera) originated first as chewing lice that themselves probably evolved from bark lice feeding in the nests of early birds. Since the chewing lice have such a strong affinity for living among feathers—so strong that we often call them bird lice—we can’t rule out the possibility that even earlier, ground-dwelling, feathered dinosaurs might have already had lice prior to the birds’ emergence. However they came to be there, the bird lice grew very successful in that miniature feathery forest: today, there are more than 1,200 modern species.
¹⁵
The potential fauna of parasitic lice was limited by the available vertebrate hosts; they could never attain the levels of hyperdiversity displayed by the wasps, which exploited the extreme species richness of other insects. Despite this constraint, the origin of the parasitic lice nevertheless remains another page in the saga of how insects successfully colonized our planet.

During the Jurassic years, Laurasia and Gondwana continued to drift apart, creating more coastlines, changing continental climates from dry to wet, and transforming arid Triassic deserts into moist forests, whose towering redwood-like conifers soared above even the tallest
Diplodocus
. Although a multitude of new dinosaur species stalked through fern meadows and clamored among ginkgo branches in the real Jurassic Park, most of the period’s diversification occurred at a microscopic level, among the bugs, beetles, wasps, and flies. As the Jurassic drew to a close, legions of parasitoid wasps drilled into dead trees for beetle larvae, while termites chewed delicately amidst the ground litter, and tenacious lice nestled among the feathers of small dinosaurs. But insect and plant evolution in the Mesozoic forests was
about to become vastly more visible. During the dawn of the Cretaceous, a profusion of colors erupted as the forests bloomed with the hues and scents of the first flowers. The Cretaceous period also saw the first bees, flower flies, and butterflies. Nearly three hundred million years had passed since plants colonized the land. Why did the world take so long to blossom?

9

Cretaceous Bloom and Doom

The evolution of the flower in all its complexity of form, color, and scent has gone hand in hand with the evolution of pollinating insects.

V. H. HEYWOOD,
Flowering Plants of the World

Perfectly reasonable scientists, who pride themselves on their caution when dealing with their own specialty, indulge on the wildest flights of fancy when it comes to cracking the mystery of the Cretaceous killer.

ROBERT T. BAKKER,
The Dinosaur Heresies

The king is gone, but he’s not forgotten.

NEIL YOUNG, “My My, Hey Hey”

As I write these lines it is a frigid February in Wyoming. The landscape is about as cold and barren as can possibly be, and yet I am thinking of flowers. Valentine’s Day will not let me forget them. The television networks, radio stations, newspapers, and Internet all remind us to buy flowers for the ones we love, and for a few short days, in the bleakest of winter, the stores are saturated with bundles of roses and a lush variety of other blossoms in dozens of colors.

Unless we have allergies, flowers generally make us feel good. We plant them in our yards and gardens around our homes and workplaces. We culture them in small containers and bring them indoors. We like their riotous colors and the extravagant forms of their petals, and so we create floral art, drapery, and wallpaper; grace our fabrics with floral images; and swaddle ourselves in flowery clothing and jewelry. Sometimes we even tattoo flowers onto ourselves. We especially like their smell. We harvest or duplicate their scents for aromatherapy and incorporate them into perfumes, soaps, oils, lotions, shampoos, deodorants, and candles.

Perhaps our attraction to flowers might be another example of the
phenomenon that Edward O. Wilson has termed biophilia, the innate love that humans have for nature and for other living things. Because we have evolved over millions of years in nature, maybe we possess a genetically programmed yearning to surround ourselves with its more pleasant aspects—flowers being one of its best-loved items. But we seem to like flowers more than we do other living creatures. We certainly don’t feel quite the same way about fungi, salamanders, frogs, spiders, or snakes, all of which can be colorful and interesting to look at. Some stink bugs have very pleasant odors, nicer indeed than some flowers, but we don’t give them to our wives or girlfriends on Valentine’s Day. Perhaps at a very basic level flowers attract us with the same characteristics that lure insects to them. Their vivid colors and curious shapes allow us to spot them from a long distance, just like a bee. At close range we react positively to their odors and sweet nectars, just like a butterfly. Flowers, bees, and butterflies are so commonplace today, it is easy to forget that the earth was not always filled with them.

Rift, Shift, and Dinosaurs Adrift

We tend to hear more about the Cretaceous’s catastrophic finale and not much about its long years. The period ended spectacularly with the colossal asteroid impact that (presumably) extinguished the
Tyrannosaurus rex
and its kind, and that (finally) allowed mammals to emerge from the carnivorous dinosaurs’ long shadow. We will hear more about that later, but for now let’s focus on what things were like during the Cretaceous, which lasted about seventy-nine million years—a long time, even by cosmological standards.

With the end of the Jurassic and the onset of the Early Cretaceous, about 130 to 145 million years ago, the composition of this planet’s big animal communities noticeably changed. The
Apatosaurus
,
Diplodocus
, and their relatives faded to extinction, and the Cretaceous forests became filled with new communities of large herbivores, most notably the duck-billed hadrosaurs and the horned and frilled
Triceratops
and its multipronged relatives. Meanwhile, the southern supercontinent of Gondwana was violently fragmented, wrenching apart startled dinosaur herds. The land mass now known to us as South America
began to split away from what is now Africa. With each massive earthquake, rifts formed from the south and north and ocean water rushed in, until eventually these areas were completely separated by a water gap and a narrow, infant South Atlantic Ocean was formed. The South Atlantic sea floor has continued to grow and widen since then, gradually but relentlessly pushing South America and Africa further and further apart, but it was in the Early Cretaceous that South America became a large island continent, at first just narrowly separated from Africa and already widely separated from North America (the Central American land bridge did not join South and North America together until just a few million years ago).

If we were to view the Cretaceous continents from outer space, we could recognize at least the shapes of modern areas like South America, Africa, India, Antarctica, and Australia, but their positions were stunningly different than they are today. South America and Africa were separated by a narrow waterway that resembled a winding channel rather than a modern ocean; Antarctica was located further to the north, much closer to South America and Africa; and Australia, tilted on its side, was still very close to Antarctica, divided by only a thin gap. In the Early Cretaceous these southern continents had mild climates, and animals and plants were dispersed along the southern corridor from South America, across the northern coast of Antarctica, to Australia in the east. South America was not the Cretaceous’s only distinct island continent. The subcontinent of India had already fragmented from the eastern coast of Africa, and over the period a fully isolated India drifted north and east across what we now call the Indian Ocean.

Although the shapes of Cretaceous India and South America might have looked familiar, their terrains would have been unrecognizable. Most notably, they were relatively flat; the modern mountain ranges that we know as the Andes and the Himalayas had not yet formed.
¹
Moreover, in the Early Cretaceous the Amazon and Ganges rivers did not exist. More ancient rivers drained across the lowland forests as duck-billed dinosaurs browsed in the misty twilight. Yet as strange as this scene might seem to us now, it also had a growing familiarity: in the Cretaceous forests the flowering plants first evolved and rapidly proliferated, and myriad flower-associated insect communities developed.

Dance of the Sugar Plum Fairies: The Coevolution of Insects and Flowers

The flowering plants, the blossom and fruit-producing organisms known to botanists as angiosperms, may have first evolved in the Jurassic period or earlier, but they were initially rare woody shrubs restricted to wet forest habitats. We have fossil flower pollen dating to the Early Cretaceous, 134 million years ago, and fossil leaves and flowers dating to 124 million years ago, and we know that by 120 million years ago the first angiosperms, including such recognizable species as water lilies and magnolias, quickly radiated and diversified. By the Middle Cretaceous (and on to the present day) angiosperms had become the dominant plant species.

The early angiosperms’ method of reproduction is similar to that of the Carboniferous Gnetales, which predated the flowering plants by at least 160 million years and resembled the coniferous cycads, primitive seed plants with stout woody trunks, topped by crowns of stiff evergreen leaves, that had dominated the dinosaur-ridden forests since the Triassic. The Gnetales used pollen to reproduce and had two forms: some had pollen-producing structures and others pollen-collecting structures.
²
Their female reproductive parts don’t look like anything we would call a flower, but they function in the same way—so although the gnetaleans are not classified as angiosperms, they might reasonably be considered the most ancient flowers. More importantly, it appears that the gnetaleans were insect-pollinated. Modern species, such as
Ephedra antisyphilitica
, are known to produce sticky droplets of pollination fluid at the tip of their flowering structure. This fluid grabs microscopic pollen grains but also attracts insect visitors with its sweetness.

Sweet nectar and nutritious pollen allowed flowers—and insects—to overrun the planet. Plants produce them in sacrificial abundance, enough to feed ravenous hordes of flies, beetles, wasps, and moths, all of whom, in turn, scatter the protein-packed pollen that sticks to their hairs. The nutritional benefit to the insects is obvious, but how did the plants profit from this mutualism? Plants are rooted in place; they cannot get up and trot away in search of mates. Until the Cretaceous, their distribution was limited mostly by the constraints of wind pollination. But with the insects’ assistance—and thanks to the energetics
of insect flight—plants at this time could spread their genetic material over long distances. Now they could exist as widely dispersed populations, scattered in forests with little wind movement.