A New History of Life (26 page)

But even these plants from 700 million years ago may not have been first to get out of the sea, because an increasing number of geobiologists are concluding that there was land life far earlier: single-celled photosynthetic bacteria, making the water-to-land transition as much as 2.6 billion years ago. If so, these early colonists would have been long, long established when “higher” plants and animals finally climbed onto land as well.

What is known is that in less than 100 million years after the appearance of animals in the sea, some species of green algae, probably still living in freshwater, shed the shackles of a wholly aquatic lifestyle and migrated to the land, rapidly evolving from simple leafless twiglike plants not dissimilar to many moss species of today to true giants, thanks to one of evolution’s great innovations: the leaf.

From about 475 million years ago, when the aquatic green algae began the numerous evolutionary changes that would allow them to attain nutrients—and most critically reproduce—in the combination of air and soil rather than entirely in water, to about 425 million years ago, when the fossil record shows the beautiful unmistakable remains of the first true vascular plants (those with roots and stems), the necessary changes were slow, step-by-step, and largely invisible to the fossil record. The evolution of these first small spiky and leafless plants to the first plant with true leaves took another 40 million years. But once the first leaves appeared, a great revolution of rapid change was
unleashed. By around 370 to 360 million years ago, trees were up to twenty-five feet tall.

It took almost a hundred million years for the invading multicellular plants to change from small marine forms to the world-covering forests that were present by the end of the Devonian period. In one respect these plants had a far more significant effect on the land than the long-reigning microbes did, for the multicellular land-plant invasion utterly changed the nature of landforms and soil. It also changed the transparency of the atmosphere, for as more and more plants spread across the land, the restless sand dunes and dust bowls that had been the unceasing landforms of the Earth until that time were transformed. Roots began to hold the grit and dust of the land in place to a far greater extent than did the land-dwelling bacteria, which, as single cells or even thin sheets would have had little strength; as the primitive plants died and rotted in place, thicker and thicker soil began to form, and the ragged, rocky landscape that had always been Earth began to soften. From space the very air itself would have cleared; for the first time the edges of continents and seas, of large lakes and rivers would have become visible from short and great distances alike.

By the Late Devonian, forests had almost completely covered the land, changing the very way rivers moved across the landscape. And in so doing, plants ultimately caused atmospheric oxygen to climb far above the 21 percent found today, to levels as high as 30 to 35 percent—levels that allowed limbed lungless fish to crawl from the sea and survive the hundreds of thousands of years it would take to evolve an efficient, air-breathing lung. All of this conquest and change caused by land plants depended on a single great anatomical innovation—the evolution of the leaf.

Animal life emerged from the sea in a series of successive invasions, much like a succession of uncoordinated, ragtag, and poorly equipped and adapted armies might do—a few solders at a time, and most dying in the process. The standard explanation for this particular history is
that these invasions took place because animals had finally evolved to a point where conquest of land was possible, with the driver being the presence of unexploited resources, less competition, and less predation (for a while, anyway). In other words, the evolutionary advances in arthropods, mollusks, annelids, and eventually vertebrates—the major animal phyla involved in the conquest of land—had finally and coincidentally arrived at levels of organization
allowing
them to climb out of the water and conquer the land. But our view is that the first conquest of land by animals took place as soon as atmospheric oxygen rose to levels allowing it.

Let us first look at what was required of both plants and animals to allow terrestrialization, the adaptations allowing life on land. Let us begin with plants, for without a food source on land, no animals would have made the effort to gain a terrestrial foothold.

By 600 million years ago, plant evolution had resulted in the diversification of many lineages of multicellular plants, some familiar to us still: the green, brown, and red algae that are familiar members of any seashore in our world.
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But these were plants that had evolved in seawater. The needs of life—carbon dioxide and nutrients—were easily and readily available to them in the surrounding seawater. Reproduction was also mediated by the liquid environment. The move to land required substantial evolutionary change in the areas of carbon dioxide acquisition, nutrient acquisition, body support, and reproduction. Each required extensive modification to the existing body plans of the fully aquatic taxa. Much of this history is still disputed, especially with the understanding of how abundant and diverse various groups were in the Proterozoic era, even before the Proterozoic snowball Earth.
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While the press loves anything that includes “oldest,” “largest,” or some other absolute, there is a disconnect between the rapid rate of discovery of the antiquity of land plants, their biological affiliations, and the need to more accurately date them. For instance, in 2010 the discovery of the “oldest” land plants was trumpeted based on new fossil discoveries from Argentina.
⁷
These fossils appear to be related to the common liverwort, and were dated at 472 million years in age. But the error on any such dating from such ancient rock is substantial. And
besides, while these are indeed quite ancient “vascular” plants, kinds with complex internal transportation systems, in this case definitions of just what a plant is complicate the story. There were a lot of both body plans and species diversity of photosynthesizing organisms we can call plants well before 472 million years ago. Many paleobiologists suspect that a wide diversity of fungi as well as green photosynthesizing microbes to multicellular plants may have been on land earlier than is now considered, and that even a billion years ago there may have been a surprisingly vigorous and numerous assemblage of what collectively could be called plants, if we throw in lichens, fungi, and sheets of green microbes draping wetter landscapes and swamps.
⁸

It was the green algal group, the Charophyceae, that ultimately gave rise to photosynthetic multicellular land plants that all can agree are true “plants,” the kind of organism being described in most stories about oldest plants. Many obstacles had to be overcome; perhaps first among these was the problem of desiccation. A green alga washing ashore from its underwater habitat quickly degenerates and dies, as it rapidly desiccates in air, for there is no protective coating. But these green algae produce reproductive zygotes that have a resistant cuticle, and this same cuticle may have been used to coat the entire plant in the move onto land. But the evolution of this cuticle, which protected the liquid-filled plant cells inside, created a new problem: it cut off ready access to carbon dioxide. In the ocean, carbon in dissolved carbon dioxide was simply absorbed across the cell wall. So to accomplish this, in the newly evolved land plant, many small holes, called stomata, evolved as tiny portals for the entry of gaseous carbon dioxide.

The plant body must be anchored in place, and early land plants were probably anchored by fungal symbionts because there doesn’t appear to be any differentiation in the higher forms. Additionally, this symbiotic relationship would provide for a means through which water could be recovered from the soil.

Moving onto land also created the problem of support. Plants need large surface areas facing sunlight. One solution is to simply lay flat on the ground, and the very first land plants probably did this. This
kind of solution is still used by mosses, which grow as flat-lying carpets over soil. A visit to the Ordovician land probably would have been a visit to a moss world, where the world’s tallest “tree” was all of a quarter inch tall. But this is a very limiting solution. Growing upright enables acquisition of much more light, especially in an ecosystem where there is competition between numerous low-growing plants, and various harder materials were incorporated by early plants to allow first stems and finally tree trunks. Concomitant would have been the evolution of a transport system from the newly evolved roots up to the newly evolved leaves. Finally, reproductive bodies that could withstand periods of desiccation evolved, ensuring reproduction in the terrestrial environment.

With these innovations, the colonization of land by plants was ensured, and with the formation of vast new amounts of organic carbon on land for the first time, animals were quick to follow. New resources spur new evolution. If the first terrestrial plants evolved from a small group of predominantly freshwater green algae, as is the most accepted opinion, they certainly did so without a lot of paleontological fanfare or evidence in the fossil record. They left behind a very fragmentary fossil record. Unearthing this fossil record (in both the literal and philosophical sense) required detective-like sleuthing of the first order.

The recovery of the fossil record of the earliest complex land plants began with a seminal 1937 paper, and for much of the discussion here, as well as the scientific history, we are indebted to our acerbic but brilliant colleague and friend, David Beerling of the University of Sheffield, who in his revolutionary book
The Emerald Planet
rather unapologetically complains that his field of Earth history, paleobotany, “gets no respect” in an almost comically Rodney Dangerfield way. But he is totally correct, in the sense that while dinosaurs and dinosaur hunters garner the lions’ (raptors’?) share of scientific interest and glory, in fact, plants remain by far the most important group of organisms on Earth in terms of their effect on the history of life. A book about how our planet changed as a result of the “history” of life should have one chapter about animals and all the rest about
plants. In any event, much of our take on the role of plants overtly comes from David’s work, and especially his book.

The history of how land plants took over the terrestrial ecosystems, and in so doing changed the nature of life on Earth because of their effects on global temperature, ocean chemistry, and atmospheric inventory, can start with paleobotanist William Lander. Lander is the scientist who made these first discoveries and found the then-oldest-known land-plant fossil remains in 417-million-year-old rocks in Wales. (At the time these dates were completely unknown. In fact, the absolute age dates that we now use are a fairly new discovery.) While the 417-million-year-old fossils from Wales were thought to be the oldest record of land plants, soon other fossils began to appear in even older rocks, later dated as being 425 million years in age, also found in Wales.

This oldest plant was named as
Cooksonia
. From these early beginnings, land plants underwent a curiously long and much-delayed evolutionary radiation. Between 425 and 360 million years ago plants underwent their own version of the Cambrian explosion in animals; only this time it was an explosion of plants on land. But the newest view is that for at least 30 million years following the first appearance of land plants, not one of them had leaves. It now looks as if leafy plants were not firmly established until 360 million years ago.

There is indeed a mystery as to why leaves took so long. Even after the first appearance of leaves, it then took another 10 million years until they became widespread and distributed both in diversity and abundance throughout the planet. This extremely long period of time between the appearance of a land plant and that of a land plant with leaves can be compared with the much faster appearance of large and diverse mammals following the extinction of the dinosaurs, 65 million years ago. For the latter, it took no more than 10 million years for the major stocks of land mammals to appear, and appear not only in diversity but also in abundance and large size.

Once again we must look at the role of evo-devo and genes to understand this particular evolutionary history. Plants had to first evolve the genetic tool kit required to assemble leaves, but then they
had to be able to use it, and the use seems to have been delayed. The best evidence to date indicates that plants with leaves had the genes necessary to build leaves, but then had to await changes within the environment in which they lived. In this particular case it was not a wait for the rise in oxygen—as it was for animals—but something entirely different: a wait for a drop in atmospheric carbon dioxide, as least according to the latest paleobotanical interpretations of the twenty-first century.

Here again is an example where the modern day can inform past history—our history of life. Experiments on living plants show that they are extremely susceptible to the level of carbon dioxide in which they live. All plants need carbon dioxide to undergo photosynthesis, but to do this, the plant has to absorb carbon dioxide out of the atmosphere around it. If there is a leaf, carbon dioxide has to enter through the otherwise impenetrable outer wall of the leaf. This is done through tiny holes called stomata. But there is a two-way street here. While carbon dioxide can enter through the stomata, water within the plant can also exit through the same holes. A theme that recurs over and over in the evolution of land animals and land plants is that desiccation remains one of the major obstacles to life. In high carbon dioxide settings, there are very few stomata. But when carbon dioxide is reduced, the number of stomata increases.

One would think that high levels of carbon dioxide would be the most optimal condition for any land plant. In terms of this physiology, in fact this is true. However, we know that carbon dioxide is one of the principal greenhouse gases. Times of high carbon dioxide are times of high heat on the surface of the Earth.