A New History of Life (29 page)

The transition from aquatic tetrapods such as
Ichthyostega
or, more probably,
Pederpes
, passed through the
Tiktaalik
grade of fish organization and involved changes in the wrists, ankle, backbone, and other portions of the axial skeleton that facilitate breathing and locomotion. Rib cages are important to house lungs, while the demands of supporting a heavy body in air, as compared to the near flotation of the same body in water, required extensive changes to the shoulder girdle, pelvic region, and the soft tissues that integrated them. The first forms that had made all of these changes can be thought of as the first terrestrial amphibians. Yet a great radiation of new amphibian species, which would be expected soon after the evolution of a respiratory system that could breathe air, not water, and limbs that could move a heavy body across land, did not occur until 340 to 330 million years ago. But when it did finally take off, it did so in spectacular fashion, and by the end of the Mississippian period (some 318 million years ago) there were numerous amphibians from localities all over the world.

The evidence at hand suggests that the evolution of the amphibian grade of organization, essentially a fish that came on land, may have taken place twice, or even three times, the first being some 400 million years ago as evidenced by the Valentia footprints as well as the
Tiktaalik
fossil discovery, and the second some 360 million years ago, and the last some 350 million years ago.
Ichthyostega
, long thought to mark the appearance of the first land vertebrate, may
have been far more fish-like than first thought, and the fact that it lost its gills is not evidence of a fully terrestrial habitat. In fact, we now know that over a hundred different kinds of modern fish use air breathing (as well as gills) of some sort. Air breathing has evolved independently in as many as sixty-eight of these extant fish, showing how readily this adaptation can take place.
Ichthyostega
may not even have been on the line leading to the rest of tetrapod lineages, but one that was evolving back into a fully aquatic lifestyle, forced off the land by its primitive lungs and the dropping oxygen levels of the Late Devonian.

Artist rendition of
Tiktaalik
, created for the Animal Planet program
Animal Armageddon
. (Art by Alfonse de la Torre in conjunction with Peter Ward, used with permission from Digital Ranch Productions, Rob Kirk)

It has long been assumed that the first amphibians were freshwater forms, and indeed this has been a major question in the history of life: was the route to land through freshwater first, or did some organisms evolve directly from salt water to air? However, new research has shown that early lobe-finned fish and lungfish—the immediate ancestors of the first tetrapods—were most often marine forms. Similarly, paleontologist Michel Laurin has noted several classical Carboniferous-aged localities that have yielded early amphibians and that have long been considered to represent freshwater deposits may in fact have been either marine or near marine deposits, such as intertidal or lagoonal environments. However, it seems equally sure that the famous
Tiktaalik
and some early amphibians such as
Ichthyostega
and
Acanthostega
have been interpreted as freshwater forms. It is thus likely that these first amphibians and near amphibians inhabited a wide variety of environments: salt water, freshwater, and terrestrial environments in the Late Paleozoic. This brings up an interesting point. Modern amphibians are intolerant of salt water; their skin, which takes in oxygen when immersed in water, cannot deal with the salt. This must be a trait evolved much later in their history.

In summary, colonization of the land came in two steps, each corresponding with a time of high oxygen. The time in between, the time of the Devonian mass extinction through the so-called Romer’s gap, had little animal life on land. Thus Romer’s gap should be expanded in concept to include arthropods as well as chordates.
⁹
It
finally ended in the Carboniferous period (split in two in America, where we call it the Mississippian and Pennsylvanian periods), when oxygen levels rose in spectacular fashion, and in the last intervals of Carboniferous and then continuing into the successive Permian period, when the oxygen levels finally topped out at nearly 32 to 35 percent, creating a unique interval in Earth history. A time of giants.

A staple of Hollywood in the immediate post–World War II interval, the dawn of the nuclear age, was the “giant-creature-produced-by-A-bomb-radiation” movie. Sometimes these monsters were examples of some kind of giant extinct life, often thawed out of some 70-million-year-old glacier. More often they were a familiar insect, scorpion, or spider of giant size. While easy to dismiss as “unscientific,” these movie monsters do let us pose a legitimate question about the maximum size that can be obtained by any given animal body plan. Since large size is often a protection against predation, it seems that most animals grow as large as they can. What ultimately limits the size of animals? In the case of terrestrial arthropods (spiders, scorpions, millipedes, centipedes, and insects among a few other more minor groups) it is clear that two aspects of the arthropod body plan limited and still limit them from attaining large mammal-like size.

One of these is the exoskeleton. Because of scaling properties and strength of the material called chitin, the hard-part material that makes up most of the arthropod exoskeleton, a giant ant, spider, scorpion, or mantis of even human size would collapse, its walking legs snapping. The second aspect of arthropod design that limits size is respiration. Insects, spiders, and scorpions appear to be limited in size by the degree to which oxygen can diffuse into the innermost regions of their body. Today, no insect is bigger than about six inches in body length. In the past, however, much larger forms than this did exist, during the interval of the highest oxygen in Earth history.

While the various specialists modeling past atmospheric composition differ in values, their respective models suggest for past time intervals that there is unanimous agreement that oxygen reached extraordinarily
high values in the time interval from about 320 to 260 million years ago, with maximal values occurring near the end of this interval. The Carboniferous period (again in North America subdivided into the Mississippian and Pennsylvanian periods) and the first half of the subsequent Permian period were the times of high oxygen, and the biota of the world at that time has left clear evidence of the high oxygen. Insects from the time present the best evidence.

The Carboniferous oxygen high (and much else as well) was well described by Nick Lane in his 2002 book
Oxygen
.
¹
In a chapter titled “The Bolsover Dragonfly,” Lane wrote about a fossil dragonfly discovered in 1979 that had a wingspan of some twenty inches. An even larger form, with a thirty-inch wingspan, is also known from fossils of this Carboniferous time, a beast aptly named
Meganeura
, yet another dragonfly. It was not only the wings that were large. The bodies of these giants were also proportionally larger, with a width of as much as an inch and a length of nearly a foot. This is about seagull size, and while seagulls are never linked in any sentence with the word “giant,” an insect with a twenty-inch wingspan was indeed a veritable giant. In comparison, today’s dragonflies may reach four inches in wingspan, but more commonly are smaller. Other giants of the time included mayflies with nineteen-inch wingspans, a spider with eighteen-inch legs, and two-yard-long (or longer) millipedes and scorpions. A three-foot-long scorpion could weigh fifty pounds, and would be a formidable predator of all land animals, including the amphibians. But, as we will see, the amphibians also evolved some giant species of their own.

In the case of insects, it is the nature and efficiency of the insect respiratory system in extracting oxygen and getting it into the most interior recesses of its body that dictates maximum size. All insects use a system of fine tubes, called trachea. Air actively ventilated into the tubes where it then diffused into the tissues. Air is pulled into the canals either by rhythmically expanding and contracting the abdominal region or by using the flapping of wings to create air currents around the tracheal opening. The tracheal system is thus made more efficient in either case. Flying insects achieve the highest metabolic rates of any animal, and experimental evidence shows that increasing
oxygen to higher levels enables dragonflies to produce even higher metabolic rates. These studies showed that dragonflies are both metabolically and probably size limited as well by our current 21 percent oxygen levels.

Whether or not oxygen levels control arthropod size has been contentious. The best evidence that it does comes from studies of amphipods, small marine arthropods that are widely distributed in our world’s oceans and lakes. Gauthier Chapelle and Lloyd Peck examined two thousand specimens from a wide variety of habitats and discovered that bodies of water with higher dissolved oxygen content had larger amphipods. More direct experiments were conducted by Robert Dudley of Arizona State University, who grew fruit flies in elevated oxygen conditions and discovered that each successive generation was larger than the preceding when raised at 23 percent oxygen. In insects, at least, higher oxygen very quickly promotes larger size.
²

It was not only higher oxygen that allowed the existence of giant dragonflies. The actual air pressure is presumed to have been higher as well. Oxygen partial pressures rose, but not at the expense of other gases. The total gas pressure was higher than today, and the larger number of gas molecules in the atmosphere would have given more lift to the giants. There was clearly more oxygen in the air than now. The question is why.

Earlier we saw that oxygen levels are affected mainly by burial rates of reduced carbon and sulfur-bearing minerals like fool’s gold (pyrite). When a great deal of organic matter is buried, oxygen levels go up. If this is true, it must mean that the Carboniferous period, the time of the Earth’s highest oxygen content, must have been a time of rapid burial of large volumes of carbon and pyrite, and the evidence from the stratigraphic record confirms that this indeed happened—through the formation of coal deposits.

We are looking at a long interval of time: 70 million years, longer than the time between the last dinosaurs and the present day, in the 330–260-million-years-old time of high oxygen. It turns out that 90 percent of the Earth’s coal deposits are found in rocks of that interval. The rate of coal burial was much higher than any other time in Earth
history—six hundred times higher, in fact, according to Nick Lane in his book
Oxygen
. But the term “coal burial” is pretty inaccurate. Coal is the remains of ancient wood, and thus we see a time when enormous quantities of fallen wood were rapidly buried and only later through heat and pressure turned to coal. The Carboniferous period was the time of forest burial on a spectacular scale.

The burial of organic material during the Carboniferous was not restricted to land plants. There is much carbon in the oceans tied up in phyto- and zooplankton, the oceanic equivalents of the terrestrial forests, and here too large amounts of organic-rich sediments accumulated on sea bottoms. The ultimate cause of this unique buildup of carbon, leading to the unique maximum of oxygen levels, was the coincidence of several geological and biological events that culminated in the vast carbon deposit accumulations. First, the continents of the time coalesced into one single large continent by the closing of an ancient Atlantic Ocean. As Europe collided with North America and South America with Africa, a gigantic linear mountain chain arose along the suturing of these continental blocks.

On either side of this mountain chain great floodplains arose, and the configuration of the mountains also produced a wet climate over much of the Earth. Newly evolved trees colonized the vast swamps and their adjoining drying land areas that came into being. Many of these trees would appear fantastic to us in their strangeness, and one of their strangest traits was a very shallow root system. They grew tall and fell over quite easily. And there are lots of falling trees in our world, but nowhere near the accumulation of carbon. More was at work than a swampy world ideal for plant growth.

The forests that came into being some 375 million years ago were composed of the first true trees that used lignin and cellulose for skeletal support. Lignin is a very tough substance, and today it is broken down by a variety of bacteria. But even after nearly 400 million years, the bacteria that do this job take their own sweet time.
³
A fallen tree takes many years to “rot,” and some of the harder woods, those with more lignin than the so-called soft woods like cedar and pine, take longer yet.

Decomposition of trees is accomplished by oxidation of much of the tree’s carbon, so even if the end product is eventually buried, very little reduced carbon makes it into the geological record. Back in the Carboniferous, many or perhaps all of the bacteria that decompose wood were not yet present,
⁴
with the key to this the seeming inability of microbes to break down the main structural component of wood, the material lignin. Trees would fall and not decompose back then. Eventually sediment would cover the undecomposed trees, and reduced carbon was buried in the process. With all of these trees (and the plankton in the seas) producing oxygen through photosynthesis, and very little of this new oxygen being used to decompose the rapidly growing and falling forests, oxygen levels began to rise.