Josh+Carson

Evolution, genetic change over time, has one major goal; create an organism that is invincible to its environment. Unfortunately, or fortunately, this goal is insurmountable seeing as the environment is ever changing. Take the wooly mammoth for instance genetically designed perfectly for low temperature; the creature was killed off as temperatures of the earth rose following the ice ages. Plants have had a similar up-breeding, shaken by changes to the earth plants have had to evolve around the changes. They had to make the change from sea to land, from being prone to water loss to having a controlled pore system, from non-vascular to vascular, etc. Some of the changes plants have made over time have been lost due to extinction; however the major and most beneficial changes continue to be used by plants today, which are reaping the benefits of millions of years of genetic alteration. At the source of this variation is an unlikely suspect; the atmosphere. Characterized by changing gas and particle concentrations, not only does the atmosphere protect plants and animals alike from deadly rays, it also has a direct effect on global temperatures and how sunlight hits the land. It is the major reason that climates change, and why as I mentioned above, evolution is and will for ever be a never ending game. It is because of this that I believe the atmosphere is the soul reason that major plant evolution has occurred. Indisputably the greatest evolutionary creation of plants is photosynthesis, however since the origins of the process are unknown, other then that it evolved some 2.5-2.6 million years ago¹, it is not a transcendent place to start our search for the evolution of plant life. Instead we will be beginning with the evolution of land plants which began about 450 million years ago with the establishment of green algae². Before getting into any of this however, a rudimentary understanding of the atmosphere and its history is necessary. Simply put the earth’s atmosphere consists of four ‘levels’ the Thermosphere, Mesosphere, Stratosphere (which contains the ozone layer), and the Troposphere. Along with nitrogen, oxygen, argon, carbon dioxide, as well as hundreds of other elements, the atmosphere also contains water vapor, and small sold molecules such as pollen³. Over the past 2.5 billion years the composition of the atmosphere has had abrupt and massive changes, the most important of these changes being oxygen and carbon dioxide levels. Prior to photosynthesis oxygen levels in the atmosphere were very low⁴, however, once organisms developed the ability to harness CO₂ in order to store energy and release, as a bi-product, oxygen, its presence in the atmosphere rose steadily for millions of years until it eventually leveled out to what it is today. Along with the rise in oxygen came the assembly of the ozone layer, which is made up of O₃ molecules, and protects the earth from deadly rays that exist in the universe. Although important for explaining how plants, and animals for that matter, can survive on earth, the creation of oxygen is not quite as vital for explaining how plants have evolved. Carbon dioxide, on the other hand, is. Being a ‘greenhouse gas,’ carbon dioxide concentration in the atmosphere is the major cause of earth’s mean temperature⁵. CO₂ acts as an insulator. During the day energy from sunlight is absorbed by the oceans and land on the surface of the earth, however at night the absorbed energy tries to re-radiate back into space which if allowed would lower the earth’s temperature so much that life could not possibly survive⁶. Luckily CO₂ and other ‘greenhouse’ gasses like methane absorb the energy trying to escape regulating the temperatures enough for life to continue living. The problem is that CO₂ levels fluctuate; when the weathering of silicate rocks paired with the recombination of CO₂ and other molecules into marine limestone’s does not offset the continual outpour of CO₂ from volcanoes and deep sea vents an influx of CO₂ comes into the atmosphere, or the opposite occurs and there is an outflow of CO₂ from the atmosphere⁷. In times when there is an increase in CO₂ levels too much of the energy trying to flee the earth is absorbed by the ‘greenhouse gasses,’ causing a warming affect, known today as global warming. Conversely in times when there is a plunge in CO₂ levels to little of the fleeing energy is absorbed, causing a cooling effect, leading to what we now call an ice age. It is these shifting of temperatures, thanks to atmospheric alterations, that have caused the major genetic changes in plants. In order to show a direct correspondence between climate change and plant evolution, an assessment of the earth’s average temperature over the past 500 million years had to be obtained, along with a timeline of major evolutionary changes in plants. Using data collected from geologist C.R. Scotese’s study on the earth’s average temperature from pre-Cambrian times to today, as well as combined data from a University of Cambridge interactive timeline about the evolution of plants, I was able to create the below chart to show how the two correspond. The first major evolutionary change for plants, besides their implementation of photosynthesis, was their move from water to land which happened about 450 million year ago. The fact that plants could even live on land was a new idea, seeing as oxygen levels up to this point were not high enough for the ozone layer to be fully formed. Because of this harmful UV radiation would have relentlessly attacked the land, making it impossible for any life to live there⁸. The ability to move from the sea to the land is very different from the necessity, however. Plants were ultimately forced from the sea because of changing temperatures of the earth thanks to a changing atmosphere. Although it is not visible in the above chart, the world had seen a very long warm period of at least 125 years from about 600 Ma to about 475 Ma⁹. In this long period sea levels were high thanks to the thermal expansion of water and the temperature of the water was high enough to sustain a diverse population of plant life. However, beginning around 475 Million years ago, the temperature of the earth hastily dropped ¹⁰, causing ocean temperatures to cool and the water, which had expanded began to contract. Additionally the cooling of the earth caused glaciation to occur lowering sea levels further¹¹. As the sea levels lowered plants became isolated on land, leaving them with an ultimatum, adapt or die. Unfortunately, many died in what is known today as the Ordovician Extinction. Although the extinction was catastrophic to marine life, killing up to 70% of it¹², it sparked new life on land. Green algae were the catalysts of this move. Adapting to the harsh variable conditions outside of the ocean was not easy, and green algae did it feebly. In order to survive the algae found the dampest locations possible, often in small puddles, allowing them to continue living life as usual with only a few adaptations. Since these small puddles were on land, however, they were prone to occasional drying, leaving the algae completely land bound¹³. The major adaptations that the green algae made were to the reproductive process. The zygotes of the green algae were used to surviving in water, so when they were introduced to the dry air, many dried out and could not carry out the reproduction process. Because of this green algae that may have been able to survive died off because they could not reproduce. Luckily, a mutation in DNA among at least one population of green algae, allowed for a layer of sporopollenin, a durable polymer, to encase the algae zygotes during dry spells, allowing them to stay fertile until water was available again¹⁴. This major reproductive tool paved the way for life to reproduce on earth, and is believed to be the pioneer for the adaptation of all plant spores to be encased by sporopollenin¹⁵. Thanks to the atmosphere changing the temperature of the earth and thus sea levels, plants had to genetically alter the way they reproduce in order to stay alive on land. Following the short ice age of the late Ordovician Extinction, temperatures began to rise once again and while green algae continued its liking of cool moist areas, its descendents were branching out. Cooksonia, what is believed to be the first all-land plant, first evolved about 420 million years ago¹⁶ when temperatures were back at pre-Ordovician times. Even though sea levels were back up to where they had been long before, plants had already conquered land, and with higher temperatures plants had to adapt to the problem of water loss. The first of these evolutions was to the cuticle of the Cooksonia. In order to make sure no dehydration occurred from the dry warm air, Cooksonia generated a thick waxy material to protect the cuticle¹⁷, and thus the plant from dry and arid times. However, this was only the beginning of its water conservation adaptations. In the ocean and in the early terrestrial plants pores were always opened allowing CO₂ to enter for photosynthesis to occur. The problem is that on land transpiration, the evaporation of water into the atmosphere from plants occurs. This is because of water potential. The water potential in the plant is generally much higher than the air outside. Water tends to move from high potential to low potential, and therefore water moves out of the plant and into the air. Unfortunately, water, being vital to photosynthesis just like CO₂, cannot leave the plant or the plant will end up dying. This leaves a rather large dilemma, how can plants obtain a high level of CO₂ intake while keeping the rate of transpiration as low as possible? Cooksonia answered this question with the creation of stomata. Found on almost every land dwelling plant on earth today, stomata are small pores that open and close, thanks to different concentrations of K+ molecules, depending on if the leaf needs to conserve water or obtain more CO₂¹⁸. It is no coincidence that as temperatures rose stomata formed. This is because the warming of the land killed off many of the plants who had no way of protecting themselves from losing water. Somewhere along the line a plant had a genetic mutation that created stomata like pores, and the plant survived passing along its genes, while the other plants died. Yet another advantageous and highly important creation that evolved via Cooksonia was vascular tissues. Of course, since these were the first vascular tissues to evolve, they were quite primitive when compared to vascular tissues in plants today. None the less, Cooksonia was what got the ball rolling, and it is why Cooksonia is best known as the ‘first vascular plant.’ As the earth warmed, sea levels rose again, however where there was still land the atmosphere was hot and arid¹⁹. This caused problems in that plants like algae, which sat on top of the ground, they were often isolated from water when the puddles dried up, and died because of this. Cooksonia on the other hand developed a new system, although not having roots per-se, it is hypothesized that they developed rhizomes²⁰, a horizontal underground plant stem ²¹. These rhizomes were most likely able to absorb water from the soil, and using its vascular tissues pass the water along to areas where photosynthesis was occurring. Thus, thanks to rising temperatures on earth due to changing atmospheric composition, Cooksonia was forced to develop what would soon evolve into roots, and vascular tissues to transport water from these roots. With the creation of protective layers over zygotes, waxy cubicles, stomata, roots, and vascular tissues plant diversification and evolution was now on a fast track. Over the next 200 million years billions of plant types were created, some becoming extinct, but all were given a chance to live thanks to these seemingly simple evolutionary developments. Of course, advancements continued to occur, evolution did everything it could to make sure that the plants became a perfect fit for their environment, but the environment kept on changing, and it was because of this that two more colossal innovations occurred. The first, occurring about 280 million years ago, was the creation of Ginkos trees. The time period this evolutionary development occurred was during the early Permian period, which was characterized by very cold weather, however it also had great fluctuating seasons²². Because much of the year was too cold for any photosynthesis to occur, Ginkos evolved very broad leaves, so that during the warmer seasons they could perform photosynthesis at alarming rates. At the first signs of cold weather, however they drop all of their leaves at once, counting on the stored up glucose from the summer months to get them through winter. The Ginkos have still not gone extinct, having lived through both ice ages and warm periods. Much of this is due to the evolution of there leaf structure. The second important leaf evolutionary development occurred about 218 million years ago the first conifer trees were developed. It was during the late Triassic period, which was characterized by hot and arid climates²³, that the firs made there ascent, and it was the hot and arid weather that shaped the trees existence. The major adaptation that firs made was the shape of their leaves. With temperatures high and with little water vapor in the atmosphere, stomata are simply not good enough for keeping water from leaving the leaf. Therefore, conifer trees changed the shape of their leaves. By rolling the leaves into a needle, the cuticle of the leaf is thicker and less prone to dehydration²⁴, with the added effects of wax to hold more water in, these leaves are perfect for hot arid climates, like that of the Triassic period. Another added, evolutionary change that these plants made was sunken stomata²⁵. This particular type of stomata it found deeper inside the epidermis of the plant leaf to protect it from winds, which could increase transpiration²⁶. These changes to the leaf, although originally helpful for warm weather, also tended to do well in colder weather as well, as seen by the fact that the tree has avoided extinction despite having to live through ice ages, and the fact that the conifer trees are well developed among colder climates. This is mainly because the leaves allow the tree to perform photosynthesis year round, and since the leaves are so thick the water within them do not freeze in cold temperatures. After the development of needle leaves in conifer trees, yet another major evolutionary change occurred in plants about 18 million years later. Precisely 200 million years ago, during the early Jurassic period, the first CAM pathways developed. As early Jurassic weather was characterized by hot temperatures and little amounts of water vapor in the air²⁷, plants were having large amounts of trouble with water loss via transpiration. In order to fix the problem a new form of carbon fixation occurred. During the night, when transpiration occurs at a slower rate because the atmosphere has a higher water potential then during the day, stomata are fully opened taking in as much CO₂ as possible, as the CO₂ enters not all of it goes straight into the process of photosynthesis, however. Instead some of the CO₂ goes into a different pathway and is fixed into malate²⁸, an acidic organic compound, where it can be stored overnight. During the day when transpiration is at its peak, the stomata of the CAM plant close completely, keeping most water from transpiring, and the malate is decarboxilated, releasing the CO₂ to allow for photosynthesis to continue during the day. This important evolutionary technique is still being used today in many cacti and dessert dwelling plants, and its creation is directly thanks to atmospheric changes effecting the temperature and humidity of the early Jurassic time period. After the CAM pathway the next important method to evolve was the C4 pathway, which is very similar to CAM pathway plants. The first C4 plants were believed to have been produced some 12 million years ago while carbon dioxide levels in the atmosphere dropped to near all time lows²⁹. The purpose of the C4 pathway in plants such as corn is to increase CO₂ intake while lowering water loss. The mesophyll cells of the plant take CO₂ from the atmosphere and combine with a phosphoenolpyruvate, a three carbon compound, that along with CO₂ makes the four carbon compound called oxaloacetate³⁰, which amusingly enough is the same 4 carbon compound that Acetyl CoA joins with in the Krebs cycle during cellular respiration. The enzyme that fixes the CO₂ with the phosphoenolpyruvate is so efficient at its job that stomata can be half opened and still receive the same amount of CO₂ as if the stomata were fully opened without the enzyme³¹. The oxaloacetate is then made into maltase thanks to chemical changes to the compound, and moves out of the mesophyll cell and into the chloroplasts of bundle sheath cells where the four carbon compound releases CO₂ into the Calvin cycle of photosynthesis³². The need to adapt to low CO₂ levels in the atmosphere was what ultimately created the C4 pathway. Today, approximately 10% of all plants utilize C4 and CAM pathways³³, showing clearly the importance their evolution has had. As I have clearly shown, the atmosphere, specifically CO₂ and water vapor levels within the atmosphere, have had a direct and major effect on the chief evolutionary changes within plants. However, there are some who tend to disagree, saying that the main causes of plant evolution are separate from the atmosphere. One specific disagreement is that roots developed as a way to anchor the plant first, and later developed absorption techniques. This would undercut my analysis that roots specifically evolved because of the lack of water on the surface of the earth, causing plants to go under the ground and into the soil to find the H₂O. It is an interesting debate seeing as fossils from time periods when the evolution of roots was actually occurring do not usually have roots in them. Therefore, it has left scientists with a lot of room to offer ideas and theories. The major point of scientists who see roots having evolved for support and not absorption say that the major reason for the evolution was not the atmosphere, but the plants need to compete for sunlight with other plants. With a completed root system, plants can grow taller, the taller they go the more sunlight they can obtain because no other plants are blocking their view of the sun. Another reason plants would want to grow tall is to keep their leaves out of the reach of herbivores, although this motive developed much later of course. Although there is no definite answer, it makes much more sense that roots would develop for absorption purposes before being used as an anchorage system. While plants were in the ocean they lacked a waterproof epidermis like what was created once plants moved to land. Because of this, and the fact that all of their tissue was lined with pores, sea plants could absorb as much water as was necessary from any portion of the body. Because of this there was no need for a vascular system to transport the water. When plants conquered land, however, the only place that was able to absorb water were the roots, seeing as the cubicle was lined with wax³⁴. Had roots been developed as an anchoring system first, not having an absorption system, there would have been no where on the plant for water absorption, and the plant would subsequently have died. It seems obvious, therefore, that the structural support function of roots was a later development. Similar to the belief that roots had an original purpose of structural support, is the belief that vascular tissues were developed in order to allow plants to grow taller. The theory ultimately states that the reason vascular tissues were developed was to aid in the plants urge to compete with other plants for sunlight. If this were true, my belief that vascular tissues developed as a way of bringing water from the roots to the places where photosynthesis occurred, however instead of aiding competition, it was actually developed as a way of survival. Although, there are some good points about why vascular tissues may have been developed for competitive purposes, my confirmation that roots were a product of absorption disproves it. Since roots were developed in order for survival and not for structural reasons, they had to have some sort of transportation system to bring the water from the roots to the leaves, or else there would be no point in the roots in the first place. Therefore it is quite obvious that even if the vascular tissues eventually aided in the upward growth of plants, its original function was in survival, like I had previously attested to. In conclusion, it is quite clear that the evolution of plants from the ocean to the land was a direct response to a change in atmospheric CO₂ levels. Therefore, not only would the evolution of plants that I have discussed in this paper never have been able to take place without the changing atmosphere, there would never have been any plant life on land, and consequently, possibly no animal life either. However, this is not the only correlation between the atmosphere and evolution of plants. Genetic mutations that benefit plant growth have occurred in direct association with the atmospheres carbon dioxide concentrations as well as its water vapor levels. These major evolutionary changes include the creation of thick waxy materials to cover zygotes so that they will not dry out, a waxy cubicle to prevent dehydration, as well as stomata for the same reason, roots to penetrate the land for water, vascular tissues to transport this water, broad leaves to increase photosynthesis, narrow leaves to protect precious water from transpiration, and CAM and C4 pathways for the same reasons. These evolutionary modifications have been past down through millions of plant types, helping them to better survive in their climates. However, the atmosphere is not done changing, and nor will it ever be. We are yet to see what sort of plants may develop from the next warming period on earth, or if a mass extinction will wipe out any of the afore mentioned developments. Plant evolution and life is ultimately at the disposal of the atmosphere, and with large atmospheric changes occurring now we can expect a new and improved plant to be developed that will be able to survive in whatever climate the atmosphere creates for us next.