The Greening of the Earth
from the book, "Our Fascinating Earth"
The Greening of the Earth
In the mid—1930s three young girls were playing in a small stream in Valley Grove, West Virginia. They were looking for something of interest, like a crawdad (crayfish), some polliwogs (tadpoles), or a horsehair snake (actually a worm, but looking for a snake seemed more fearless). If no creatures showed up they would settle for an unusual rock.
One of the girls picked up a piece of shale. The blackened imprint of a fernlike leaf clearly showed on the rock's surface; in fact it looked like part of the rock itself. Noticing that the leaf on the rock looked very similar to those on the nearby locust trees, the girls decided the imprint was made by a leaf that had fallen from one of the trees. Searching the area, they found several other rocks that displayed similar marks. Each girl took home a sample to illustrate the type of trees that grow in West Virginia. Their assumption that they had found samples of West Virginia flora was correct, but little did they realize that these trees had grown there almost 300 million years ago.
One of the girls stored the leaf impression in a drawer full of things too interesting to discard. About 15 years later she and her geologist husband, during a visit to her West Virginia home, sorted through some of her keepsakes. Her husband instantly recognized the fossil leaf imprint, so the search began for the locust trees that shaded the brook where the girls had innocently discovered a treasure of fossil leaves.
For three summer seasons her husband and several other geologists excavated this quarry. They collected several hundred specimens for the museum and laboratories of the university at which he was employed. The original discovery site yielded all its specimens during the first summer, but upstream less than a hundred yards away the scientists found an even richer quarry; some of the slabs they removed contained impressions of entire branches. Miners from the coalfields of the Ohio Valley also contributed samples that they had unearthed in their work.
The fossils were leaves from several types of trees that formed the vast coal deposits of West Virginia. This area had been a part of the great, lush swamplands that produced peat, later to be converted into coal. The leaves had fallen from the overhanging branches into the mud below, where they were buried and, over time, nothing remained but a thin film of carbon marking the shape of the original leaf. The fossil—bearing shale disappeared into a large nearby hill, carrying fossils up to the point of disappearance. If this hill could be sliced away cleanly to expose the top of the shale layer, the fossils thus uncovered would probably number in the hundreds of thousands.
The great swamp forests grew at a time when the earth was covered with vegetation. It would never again be as green. The lush greens were only occasionally interrupted by the subtle brown of dead and decaying plants. Nowhere was the color of flowering plants to be found, for they had not yet evolved. The greening of the earth was well advanced, but where, when, and with what type of plant life did it all begin?
The evolution of life took place over periods of time so great that they stagger humans' everyday time sense. It helps to imagine the history of life on earth as compressed into a 24—hour day. The first microscopic organisms originate at midnight and evolve as the day advances. Not until about 6 p.m. (three—fourths of the day already gone by) does life in the oceans become abundant. By 8 p.m. plants invade the land, and at 9 p.m. the great coal forests flourish. Dinosaurs take over the land at 10 p.m., modern flowering plants appear at about 10:30 p.m., and finally the recorded history of modern humans begins at a quarter of a second before midnight.
Three billion years ago the land was desolate and dismal, but no living things were around to lament the barren terrain. No life existed on terrestrial earth; the only sounds were the wind rushing through rock crevices and water cascading across the land, accented by explosions from erupting volcanoes that were abundant on the primeval earth. Life did exist in the sea; in fact most scientists believe that all life originated in the sea. The misty beginnings of plant life took place well over 21/2 billion years ago, unceremoniously, with pond scum. Probably these earliest plants were one—celled algae that slowly evolved into complex, multicelled plants.
Gradually, about 420 million years ago, the first pioneering plants moved out of the swampy areas and began a life on the barren world of naked rocks unsoftened by a green mantle. No scientist can say which plants made the first hesitant steps on land. They were undoubtedly borderline forms that developed from waterweeds in coastal swamps and still lived a half—water, half—land existence. Compared with today's complex and varied plant life, this early vegetation would seem insignificant. The plants lacked true roots and leaves and, deprived of continuous contact with moisture, died quickly. Their foothold on land was nevertheless secure; it was a staggering event for the history of the earth.
By the mid—Devonian, 375 million years ago, a great surge of vegetation swept the earth, producing a landscape that resembled the swampy Florida Everglades. Land plants, very scarce before Devonian times, diversified greatly within 30 million years. A sampling of a mid—Devonian landscape can be seen in an exposure of rocks near Gilboa, New York. It includes petrified stumps of trees 3 feet in diameter, estimated at over 40 feet tall, and slender 100—foot ancestors of the modern club mosses, which are less than 12 inches high. These are the fossil remains of the earliest known forest that ever clothed the land.
But many things were missing from this dawn forest. It was still an almost soundless world, for animal life on land was a rarity. There was no buzzing of insects, singing of birds, roar or bellow of mammals, not even the hiss of a snake. Animal life included a few mites and several forerunners of insects and spiders that had crawled ashore. This was the vanguard of a host that would soon populate the earth.
The world of 300 million years ago was springtime in the history of plants, an unbelievable flourishing and harvest of plenty. In this, the Carboniferous Period, warm tropical swamplands prevailed over much of the world. Dense jungles housed giant ancestors of modern horsetail, some growing almost 130 feet tall, surrounded by 40—foot fernlike trees. With plentiful swamps and forests, crawling and flying insects diversified greatly. Without flowering plants, however, there was little use for bees. The land was still the many shades of green.
Scientists refer to this era as the age of cockroaches, an accurate description because cockroaches were quite abundant. They were similar to today's cockroaches, except in size; some of them were over nine inches long. Overhead flew the ancestor of the modern dragonfly, with a wingspan of nearly three feet, that easily preyed on the cockroach. Another denizen of the lush forests was a centipede over six feet long. Primitive amphibians and reptiles also populated the earth, but no mammals had yet evolved.
In the coalfields of South Joggins, Nova Scotia, fossil tree trunks have been found buried in shale. Amazingly many of them are still standing upright on the very spot where they grew some 300 million years ago! Most of the preserved trunks range from one to three feet in diameter, and for some of them as much as nine feet of trunk has been preserved. Judging from the diameters, some of the trees must have reached gigantic heights.
The trunks were preserved by the accumulation of coal materials and clays that slowly buried them. As decay set into the exposed dead wood, only the buried portion of the trunk remained. The interior of each standing trunk was also completely decayed before final burial, leaving a great hollow interior. Actually, only the outer rim, including the bark of the tree, has been preserved.
The hollow interiors of the trees did not remain empty for long. As the standing trunk was slowly being covered by accumulating sediments, its summit must have for a time been level with the accumulating soil. The hollow thus served as a well, or pit, for snails and millipedes to crawl into. Many of them died and were buried by the clay material filling this hollow interior.
These pits in the soil were probably concealed by fallen leaves from nearby living trees. Many of the early land vertebrates, particularly amphibians, fell through the soft cover, unmindful of the trap that was concealed under the leaves. With a cavity too deep to climb out of, the hollow tree became a veritable death trap. From only 25 cataloged trees, over 53 individual specimens representing 12 different species have been recovered. The character of the sediments in the trunks changes often, indicating that filling the hollow trees was a very slow process that took a great period of time.
No doubt today's immense coal deposits were formed when these giant forests were buried where they grew. Coal has been formed in small quantities ever since vegetation began to grow on the land, although plants were not abundant enough before the Carboniferous Period to produce the deep coal beds mined today. The luxuriant forests of the Carboniferous are the source of almost all the coal that has fueled modern industry.
The conversion of plants into coal occurs when vegetation does not decay. This happens when the forest becomes a swamp of standing water; falling trees sink into mud that lacks enough oxygen for the agents of decay to rot them. Instead the plants slowly begin to turn into peat, the first stage of coal. Peat can lie in bogs for thousands of years without changing much. But in time sediments carried into the swamps by rivers bury peat deeper and deeper, compressing it into lignite, a low—grade coal. Under the weight of millions of years of additional deposits the lignite is further compacted into bituminous, or soft, coal. Finally additional pressure brought about by shifts in the earth's crust compresses the material in many places into anthracite, or hard coal.
The amount of compression needed in the conversion of coal is tremendous, for one foot of bituminous coal represents about 20 feet of original plant matter. The extent and luxuriance of these Carboniferous forests are demonstrated by coal seams upward of 400 feet thick in China. This represents about 8,000 feet of original vegetation. In Pennsylvania, Ohio, and West Virginia the coal seams are only 5 or 6 feet thick on average, but they blanket thousands of square miles. Apparently, widespread forests flourished on top of previous forests, and each in its turn became compacted into a layer of coal. One West Virginia coal bed has yielded 120 such seams, one above the other. The swampy environments were not continuous in time; they alternated with dryer conditions. But when they did occur they were always very widespread. Geology textbooks rightly refer to the Carboniferous Period as the age of coal.
The giant coal—yielding forests came to an end about 230 million years ago. Vast changes on the earth's surface resulted in mountains, glaciers, and deserts quickly wiping out the expanded swampy environments, and vegetation took on a more modern aspect. A new era began—the age of dinosaurs—but the overall green color persisted. About 100 million years ago, during the last period of the dinosaur era, the Cretaceous, the green blanket that covered the earth began to change. Flowering plants arrived at last, as did pollinators such as the bee and the hummingbird. Green was now one of many colors that blanketed the earth.