A New History of Life (48 page)

Plants need carbon dioxide. Yet the history of carbon dioxide through the billions of years on Earth has been one of short-term rises and falls that in fact are only minor variations in a much longer-term trend—the long-term reduction in this gas. With this long decline, our planet is gradually cooled, especially over the last 40 million years. Yet it is far more than the change in temperature that affected the evolution of plants during the Cenozoic era. Perhaps even more important has been an evolutionary formation of a more efficient form of photosynthesis, called C4 photosynthesis, which in many plants supplanted the more archaic mechanism, named C3 (the 3 and 4 in these terms is derived from different chemical changes taking place as sunlight and carbon dioxide are combined to form living plant cells and tissue). C4 photosynthesis, in fact, has shown an extraordinarily rapid rise in importance in terms of the number of plants using one over the other.

Plants that use the C3 pathway leave a different carbon isotope signature than those that use C4. Not only do the plants show this signature, which can be measured when any tissue from the plant is analyzed using a mass spectrometer specialized in looking at living tissue, but any animal eating those plants will leave a trace of it as well. Thus we know from the fossil record whether given herbivorous species ate C3 or C4 plants (or even a combination of the two).

We have two lines of evidence demonstrating when C4 plants first arose. The first is the molecular clock. By comparing the genomes of C4 to C3 plants, geneticists deduced that the differences were large
enough that the C4 mechanism could not have arisen less than 25 million years ago (or any earlier than 32 million years ago as well). However, the fossil record yields a quite different answer to the question of when the C4 photosynthetic method first appeared, for the first fossils of C4 plants are only 12 to 13 million years of age.

The evolution to the C4 pathway was not a breakthrough that was then passed down to ever-greater numbers of plant species. In fact, it may have separately evolved more than forty times in the past, by that many separate lineages of plants. The eventual C4 plants are fire and desiccation-resistant plants adapted to heat and dry climates.

The most important C4 plants are grasses because of the dominance of the grass diet to so many kinds of herbivores, large grazing mammals as well as many kinds of birds, including the ubiquitous geese now found on most urban lawns near bodies of water. The reduction in carbon dioxide, especially over the last 20 million years, greatly abetted the expansion of C4 grasslands.
¹¹
Most grasses cannot live on forest floors, where the cooler, shadier conditions do not favor their growth.

Deforestation, however, creates a more open habitat, and therefore one that is far better for grasses. While the main idea has long been that the long-term drop in carbon dioxide sparked the evolution to dominance of C4 grasses, an alternative and newer idea is that a change in forest cover of the planet was as important as or perhaps even more important than a drop in carbon dioxide levels. But what would have caused radical reduction in forestation? The answer seems to be forest fires.

A dramatically underappreciated aspect of a planet with plants is the effect of forest fires. Fire, of course, is affected by oxygen levels. In times of higher oxygen, especially during the Carboniferous period of around 320 to 300 million years ago, forest fires may have been ongoing. A view from space during this interval would have shown an atmosphere darkly smudged and thick with smoke, so that there would have been a world-covering global haze that would have made a clear sunny day a rarity. But such smoke covering much of the continents itself would have had a highly significant effect on global
temperatures, because much of the smoke from a forest fire can be light in color when viewed from above. The global haze and smoke would have reflected more sunlight back into space than would otherwise have happened, thus changing the albedo (the degree of reflectivity of the sun’s rays hitting the planet).

All of this would have created a chain of events radically changing not only global climate but also the entire history of life from that point onward. The rise of oxygen concentration and its prolonged high for more than 30 percent of all the Carboniferous period would have caused more forest fires. As noted above, this caused global temperature to drop, setting off a chain of events ending in one of the most prolonged polar glaciations in all of Earth’s history. Although it was not global in extent like the snowballs, it was nearly as long as some of them. That time of ice may have lasted more than 50 million years, a time interval coincident with some of the most important of all events in Earth history, including the conquest of land by animals, the evolution of new and advanced (for the time) land plants that were capable of colonizing upland regions of the continents previously uninhabitable by plants, and the first appearance of some of the most important of all vertebrate groups—including the earliest reptiles, and soon after the ancestors of the mammals. But there is another aspect of fire that would’ve affected the history of plant life, and therefore the history of life in general.

New studies on Amazon basin fires have demonstrated that wildfires can greatly influence climate, and not just in the tropics. David Beerling, in his book
The Emerald Planet
, noted that during April 1988, smoke from fires may have inhibited cloud formation over parts of North America—which in turn affected rainfall patterns. This interval of time, in fact, was one of severe drought—and resulted in one of the driest months of the twentieth century. This spring drought followed some of the most extensive wildfires ever, two of them present in North America during July of 1988, a year when gigantic areas around Yellowstone National Park extensively burned. Beerling invokes a new way of understanding the spread of C4 grasslands—very positive feedback system may have been put in place.
¹²

Positive feedbacks are those that increase environmental change within one particular direction. In our world today, the warming atmosphere causes ever more of the Arctic ice pack to melt, so that there is an ever-smaller percentage of highly reflective white ice in the northern hemisphere. The white, ice-covered oceans reflect sunlight back into space, but when the ice melts and is replaced by dark-colored, open water, the oceans absorb much more heat—and the seas warm. As the seas warm, more ice is melted and the cycle continues. The positive feedback is that the warming causes more warming.

David Beerling suggested that there is a positive feedback in forest fires causing ever more forest fires. The fires change the climate, causing more drought, which makes ever greater areas susceptible to burning, causing a greater extent of fire damage. And so the cycle goes—burning causes more burning.

We enter a time when global temperatures are rapidly rising. The eventual effects this will have on the planet is not entirely unknown. Less predicable is the effect a new, warmed, high-sea-level world will have on human industry, population, and civilizations.

The history of life as often first taught to us as children is broken down thusly: fish began in what we call the Age of Fish; some crawled ashore to start the Age of Amphibians, which then began what was once called the Age of Reptiles or sometimes the Age of Dinosaurs. Things finished off with an Age of Mammals. It is not hard to see why this has become the common knowledge: humans like to pigeonhole things, and a succession of “ages” is pigeonholing at its best. But among the many, many problems with this account is one of many other truths: there are no pigeons at all in this succession. Let us change that here and consider what we might call an Age of Birds.
¹

The evolution of birds is a major topic of research.
²
It has been a controversial area of research as well, with two major schools of “belief”—one, that birds evolved from a nondinosaur diapsid, something akin to one of the many reptilian-like forms that gave rise to the dinosaurs themselves, or two, that dinosaurs were the direct ancestors of birds. This school even invokes the methodology of cladistics to reinforce the claim that what we call birds are in fact dinosaurs, just highly modified.
³

A host of fossils have shown that not only did many smaller bipedal, carnivorous dinosaurs resemble birds in the way they laid their eggs but that these eggs also looked like the eggs of birds. Even more striking has been the new discoveries that many dinosaurs both before and after the first appearance of
Archaeopteryx
even showed evidence of winglike arms, with feathers, suggesting a second attempt by dinosaurs to gain the ability to fly. The question was whether or not this famous fossil was even a dinosaur.
⁴

The dispute goes back to about 1996, when paleobiologist Alan Feduccia investigated the then newly discovered fossil of what he interpreted to be an intriguing bird that lived about 135 million years ago, just after
Archaeopteryx
. The bird,
Liaoningornis
, did not look
like a dinosaur bird at all.
⁵
It had massive flight muscles attached to a breastbone similar to modern birds. Yet it was it was found alongside fossils of ancient birds not unlike
Archaeopteryx
. How could such advanced evolution have taken place so quickly? Instead, Feduccia concluded, birds may have been already very widespread by the time that
Archaeopteryx
first appeared in the time interval roughly from 140 to 135 million years ago, and that by that time they were already occupying a variety of habitats. While more “advanced” than
Archaeopteryx
, they were still very primitive by modern bird standards. So where are they? Feduccia believes that most of them died out with the dinosaurs, about 65 million years ago, and that the ancestors of all today’s birds evolved later, between 65 and 53 million years ago, independently of the dinosaurs. This is the so-called big bang theory of birds.
⁶
Feduccia and his colleagues view any similarity between birds and dinosaurs as simply due to convergent evolution, where natural selection independently produces similar morphologies.

This school of thought has the modern birds appearing late—either coincident with the 65-million-year-ago K-T extinction—or some tens of millions of years later. This is certainly not mainstream understanding about bird evolution anymore.
⁷
Within the last decade a large number and variety of birds have been found in Cretaceous rocks ranging from 130 to 115 million years ago, most from China. Some of these fossils show that a great diversity of birds with long, bony tails preceded the evolution of birds with the familiar short, bony tail.
⁸
However the dinosaur-to-birds theory was further supported by the discovery of two species of feathered dinosaurs in China, dating from between 145 million and 125 million years ago, followed by younger, early Cretaceous birds.

In fact, a great deal of scientific interest has gone into feather research: why did they evolve in the first place (in terms of function), and how did the wing feathers necessary to allow flight come about in the first place? Much of this research involves the concept of exaptation—where a particular adaptation is coopted to do something else. We all know the value of feathers in down vests and sleeping bags. Clearly feathers are good for insulating and staying warm, but
the feathers used for warmth are far different from those used and necessary for a bird to fly. Feathers are rarely preserved, and like so much else in paleontology, working out their origin, first appearance, and use involved a fossil record of scant help. Yet as so often in the last several decades, fossils from China have come to the rescue. In this case, exquisite dinosaur fossils that do preserve feathers,
⁹
and sometimes (and not just from China) even soft parts.
¹⁰
Yet as is also so often, no evidence dealing with bird evolution ever gains acceptance without clamor of dissent and noisy opposition.
¹¹
The evolution of flight (not just gliding), a major innovation that was successfully undertaken by arthropods, reptiles, dinosaurs (the bird version), and mammals, has been and remains a fertile topic of study.
¹²

At present, over 120 avian species are known from the Mesozoic, from all continents except mainland Africa.
¹³
Despite this new information, controversy surrounds several aspects of avian evolution, including the timing of the origin and diversification of modern birds (Neornithes).
¹⁴

The birds found in the oldest time intervals of the Cretaceous (which is divided into an Early Cretaceous, which began at about 145 million years ago and ended around 100 million years ago, and was succeeded from 100 to 65 million years ago by the Late Cretaceous). Birds of the Early Cretaceous must have rapidly evolved into a wide range of shapes and sizes. Some were crow sized, with strong beaks, such as
Confuciusornis
, a form that also possessed enormous claws in its wings. Others from this time, such as
Sapeornis
, had very long and narrow wings like those of a seagull. There were also smaller birds, such as the sparrow-sized
Eoenantiornis
and
Iberomesornis
. Yet for all their improvements in flying, these early Cretaceous birds still had toothed jaws similar to those of
Archaeopteryx
. But the variety of skulls, wings, and feet indicate that these Early Cretaceous birds had already specialized into a variety of different lifestyles, including seed feeders, fish eaters, insect feeders, sap eaters, and meat eaters. Their wings and rib cages suggest that soon after
Archaeopteryx
, birds evolved flying abilities not very different from modern birds.

For all the improvements of the Early Cretaceous birds, one remaining archaic feature of these early birds was their teeth. All modern birds have horny beaks, of a spectrum of morphologies that are adaptations for the many kinds of feeding that modern birds undertake. But when did the toothless birds first appear? This remains a contentious question—perhaps just answered in the cold wastes of the Antarctic Peninsula.