A New History of Life (27 page)

Plants have an exquisite signaling system, allowing fully grown and mature leaves to communicate with leaves just undergoing first growth and development. The larger leaves inform the smaller about the optimal number of stomata to produce for the environmental conditions they are all living in. If we go back in time to observe the levels of carbon dioxide in the atmosphere when land plants first began their evolutionary rise, over 400 million years ago, we see a period of extremely high carbon dioxide levels—and thus an extremely warm
planet. So warm, in fact, that heat itself may have been a major brake on plant evolution and ecological success. The same stomata that let carbon dioxide in also allow the removal of water from the inside of the plant—and it is this process that actually cools the plant.

A little desiccation cools a plant, but a lot kills it, and as in so much, success comes from balance. In a very hot climate, a lot of cooling is needed. But in a high-CO
₂
atmosphere, a plant needs very few stomata to handle its carbon dioxide needs. Yet the same number of stomata necessary for “ingestion” or carbon dioxide into the body of the plant might be too few to allow cooling—especially if the stomata are located on a large, flat surface—like a leaf. In such a case, a large leaf with few stomata will cause overheating to the point of death. This is the newest view of why it took so long for leaves to evolve. The genetic tool kit necessary to make them was in place. But the atmosphere had so much CO
₂
in it that plants did not dare build leaves.

The new early twenty-first-century work of David Beerling and others suggests that it took a drop in carbon dioxide before leaves could be viable at all. Before this time any leaf would be a death sentence for the plant. Thus it was that it was only after 40 million years following the first appearance of
Cooksonia
that leaves as well as better internal plumbing systems within the plant (including new and deeper boring roots) first appeared. This latter, the ability to send roots to ever-greater depths, had two advantages for plants. First, deeper roots provided more stability. Second, deeper rooting gave greater access to both soil nutrients and water. The first plants have extremely shallow rooting systems. But once leaves evolved, roots also began to change and evolve to go ever deeper into the soil.

By the Devonian period, we see the evidence of roots that extended downward for as much as three feet. The new, deeper roots vastly increased the weathering of rocks beneath these early plants. As more plants lived in the soil, more and more of them died, adding organic material to the soil. At the same time, ever-deeper penetration by roots vastly increased both mechanical and chemical weathering of rocks beneath. This had important consequences for the makeup of the atmosphere as well as the temperature of the Earth.

We have seen that perhaps the most important driver removing carbon dioxide from the atmosphere is the weathering of silicate rocks, the granites, and sedimentary and metamorphic rocks with a granite-like chemical composition, a rock type rich in the element silicon. The reaction of chemically weathered silicate rocks on land is such that molecules of carbon dioxide are removed from the atmosphere. This is called the biotic enhancement of weathering, and it would have been taking place as soon as tree-rich forests began covering the land, about 380 to 360 million years ago. As roots went deeper into silicate rocks beneath, the granite and compositionally granite-like rocks of the continents began to weather much more quickly than the time before forests, and this caused carbon dioxide levels to plunge, and plunge quickly.

The lowering carbon dioxide levels allowed ice to appear on the continents, first only at the highest latitudes, but eventually at ever-lower latitudes. But the juggernaut of evolution favored taller trees, and with taller trees came deeper roots. Plants became taller, roots went deeper, and the planet became ever colder. The evolution of land plants with their ever-deeper rooting in fact plunged the planet into one of the longest-running ice ages ever in Earth history, one that began in the Carboniferous period. But before this happened, the world would have been warm, lush, and rich in plant-friendly levels of carbon dioxide. In short, the continents, newly green with vascular plants, would have been like a gigantic, stocked, but customer-free grocery store. Free food, if only you can get into the store. Or in this case, out of the sea and onto land—to stay.

The major problem facing any would-be terrestrial animal colonist was water loss. All living cells require liquid within them, and living in water does not provide any sort of desiccation problem. But living on land requires a tough coat to hold water in. The problem is that solutions that allow a reduction in surface desiccation are antagonistic to the needs of a respiratory membrane. So here we are twixt the
devil and the deep blue sea: build an external coating that resists desiccation, an advantage, but at the same time risk death from suffocation. The alternative was to evolve a surface respiratory structure that allowed the diffusion of oxygen into the body, but caused increased risk of desiccation through this same structure. This dilemma had to be overcome by any land conqueror, and it was apparently so difficult that only a very small number of animal, plant, and protozoan phyla ever accomplished the move from water to land. Some of the largest and most important of current marine phyla certainly never made it: there are no terrestrial sponges, cnidarians, brachiopods, bryozoans, or echinoderms among many others, for instance.

The oldest fossil land animals all appear to have been small arthropods resembling modern-day spiders, scorpions, mites, isopods, and very primitive insects. It is unclear which of these quite different arthropod groups was first, but being first did not last long, as all of these groups are found in the fossil record in ancient deposits. Identifying these first land animals has necessarily relied on a fossil record that is notoriously inaccurate when it comes to small terrestrial arthropods. All of these groups have very weakly calcified exoskeletons, and thus are rarely preserved as fossils. By the Late Silurian or Early Devonian time intervals, however, or around 400 million years ago, the rise of land plants also brought ashore the vanguards of the animal invasion, and it is clear that multiple lines of arthropods independently evolved respiratory systems capable of dealing with air.

The respiratory systems in today’s scorpions and spiders provide a key to understanding their successful transition from marine animals to successful terrestrial animals. Of all structures required to make this crucial jump, none was more important than respiratory structures. It also seems apparent that the earliest lungs used by the pioneering arthropods would have been transitional structures nowhere near as efficient as in later species. But in a very high oxygen atmosphere, air can diffuse across the body wall of very small land animals—and the first land animals all seemed to be small, as well as taking in oxygen by even their primitive lung structures.

Of the phyla that made it onto land, which included many kinds of arthropods, as well as mollusks, annelids, and chordates (along with some very small animals such as nematodes), the arthropods were preevolved to succeed, for their all-encompassing skeletal box was already fashioned to provide protection from desiccation. But they still had to overcome the problem of respiration. As we have seen, the outer skeleton of arthropods required the evolution of extensive and large gills on most segments to ensure survival in the low-oxygen Cambrian world where most arthropod higher taxa are first seen in the fossil record. But such external gills will not work in air. The solution among the first terrestrial arthropods, spiders, and scorpions was to produce a new kind of respiratory structure called a book lung, named after the resemblance of the inner parts of this lung to the pages of a book.

A series of flat plates within the body have blood flowing between the leaves. Air enters the book lungs through a series of openings in the carapace. This is a passive lung in that there is no current of air “inhaled” into these lungs. And because of this, they are dependent on some minimum oxygen content.

It is well known that some very small spiders are blown by winds at high altitudes and have been dubbed “aerial plankton.” This would seemingly argue that the book lung system in spiders is capable of extracting sufficient oxygen in low-O
₂
environments. But these spiders are invariably very small in size, so small that an appreciable fraction of their respiratory needs may be satisfied by passive diffusion across the body. Larger-bodied spiders are dependent on the book lungs.

Book gills may be more efficient at garnering oxygen than the insect respiratory system, which is composed of tubelike trachea. Like spiders and scorpions, the insect system is passive in that there is little or no pumping, although recent studies on insects suggest that some slight pumping may indeed be occurring, but at very low pressures. The book lung system of the arachnids has a much higher surface area than does the insect system, and thus should work at lower atmospheric oxygen concentrations.

The “when” of this first colonization of land is hampered by the small size and poorly fossilizable nature of the earliest scorpions and
spiders. Present-day scorpions are more mineralized than spiders, and not surprisingly have a better fossil record. The earliest evidence of animal fragments is from late Silurian rocks in Wales, about 420 million years in age, near the end of the Silurian period—and a time when oxygen had already reached very high levels, the highest that had up to that time ever been evolved on Earth. These early fossils are rare and of low diversity, but identifications have been made: most of the material seems to have come from fossil millipedes.

A far richer assemblage is known from the famous Rhynie Chert of Scotland, which has been dated at 410 million years in age. This deposit has furnished fossils of very early plants, as well as the fossils of small arthropods. Most of these arthropods appear to be related to modern-day mites and springtails, which both eat plant debris and refuse, and thus would have been well adapted to living in the new land communities composed mainly of small, primitive plants. Mites are related to spiders. Springtails, however, are insects, presumably the most ancient of this largest of animal groups now on Earth today. It might be expected that once evolved, insects diversified into the most abundant and diverse of terrestrial animal life in our time. However, this was not the case, and in fact just the opposite appears to be true.

According to paleoentomologists, insects remained rare and marginal members of the land fauna until nearly the end of the Mississippian period, some 330 million years ago—when oxygen levels had reached modern-day levels, and in fact were on their way up to record levels, which climaxed in the Late Pennsylvanian period of some 310 million years ago. Insect flight also occurred well after the first appearance of the group, with undoubted flying insects occurring commonly in the record some 330 million years ago. Soon after this first development the insects undertook a fantastic evolutionary surge of new species, mainly flying forms. This was a classic adaptive radiation, where a new morphological breakthrough allows colonization of new ecological niches. But that radiation also took place at the oxygen high, and was surely in no small way aided and abetted by the high levels of atmospheric oxygen.

Insects were also not the first animals on land. That accolade may go to scorpions. In mid-Silurian time, some 430 million years ago, a lineage of proto-scorpions with water gills crawled out of the freshwater swamps and lakes that they were adapted to and moved onto and then about on land, perhaps scavenging on dead animals such as fish washed up onto beaches. Their gill regions remained wet, and the very high surface area of these gills may have allowed respiration of sorts. They certainly did not have functional lungs, only semi-serviceable gills.

Here is the timetable as we now know it: scorpions onto land about 430 million years ago (MA), but of a kind that may have been still tied to water for reproduction and perhaps even respiration; followed by millipedes at 420 MA, and insects at 410 MA. But common insects did not appear until 330 MA. How does this history relate to the atmospheric oxygen curve?

The newest estimates of atmospheric oxygen levels at this time indicate that a high oxygen peak occurred at about 410 million years ago, followed by a rapid fall, with a rise again from very low levels (12 percent) at the end of the Devonian to the highest levels in Earth history by somewhere in the Permian when it exceeded 30 percent (compared to 21 percent today). The Rhynie Chert, which yielded the first abundant insect-arachnid fauna, is right at the oxygen maximum in the Devonian. Insects are then rare in the record (according to paleontologists who study insect diversity) until the rise to near 20 percent in the Mississippian-Pennsylvanian, the time interval from 330 to 310 million years ago—the time of the diversification of winged insects.

The conquest of land by various vertebrate groups was seemingly enabled by a rise in atmospheric oxygen levels during the Ordovician-Silurian time interval. Had that not happened, it is possible that the history and kind of animals that did colonize land might have been much different—or it might never have happened at all; animals might never have colonized land. We also know that following this colonization, animals became seemingly rare, during the subsequent time of
low
oxygen.

There are three possibilities for this observed pattern of fossil abundances and diversity. First, this seeming pause in the colonization of land is not real at all; it is simply an artifact of a very poor fossil record for the time interval from 400 to about 370 million years ago. Second, the “pause” is real; because of very low oxygen there were indeed very few arthropods and especially insects on land. But the few that survived were able to diversify into a wave of new forms when oxygen again rose, some 30 million years later. Third, the first waves of attackers coming from the sea as part of the invasion of land were wiped out in the oxygen fall. Yes, here and there a few survivors held out. But the second wave was just that—coming from new stocks of invaders, again swarming onto the land under a curtain of oxygen. The colonization of land by animals (arthropods, and as we shall see, vertebrates as well) thus took place in two distinct waves: one from 430 to 410 million years ago, the other from 370 onward.