[MUSIC] Hi, I'm Vivi Vajda. I'm professor in paleontology at Lund University in Sweden. In this lesson, I will take you on an exciting journey through time to witness the greening of the Earth's surface. We will start in the Ordovician, some 450 million years ago, and we will track the evolution of the land plants up until the rise of the flowering plants. We will examine what application fossil plants have and how plants have transformed the very air we breathe. Up to around 450 million years ago, the land was desolate. But some of those small, green algae in the rivers developed adaptations that would allow them to survive out of water. These plants began the long journey of conquering the land and transforming the Earth into the green world we know today. Plants' conquest of the Earth's surface paved the way for all other living creatures that we have on land today. Every creature living out of the oceans today relies either directly or indirectly on the existence of plants for their food. The activities of plants have even changed the concentration of the gases in our atmosphere, which may at various times have paved the way for dramatic climate changes. The study of fossil plants is called paleobotany. There is also an important subdiscipline of paleobotany that specifically deals with the study of fossil spores and pollen, and this field of study is called palynology. On our journey through the evolution of the plant kingdom, we will explore the diversity of plants through time, and the rise and fall of major plant groups, of some of the catastrophes that have affected life on the land. Plants represent an extraordinary diverse array of photosynthesizing organisms, from unicellular algae to the largest organisms on Earth, the Californian redwood trees. Land plants probably evolved from a group of green algae living in freshwater ponds on the continents. The closest modern relatives to all the land plants are the charophytes, also known as the stoneworts. Plants are multicellular and photosynthetic, which means that they produce their own nutrition with energy from the sunlight and with carbon dioxide from the atmosphere. Therefore, they are called primary producers, whereas humans and all other animals are consumers. We obtain our nourishment from the sunlight through the plants that we eat. In the early ecosystems that developed on land, plants were still very dependent on water for their reproduction. So only the wetter areas on land were initially covered with vegetation, while most of land remained a barren wasteland. These harsh environmental conditions must have served as a strong driver of evolution. And plants that could best adapt to their environment had a strong advantage to survival and were the winners of the evolutionary race. The challenges to grow on land were many, including drought, physical support, UV-light, and reproduction. So, can you guess what the adaptation to the new environment were? To combat drought, the successful plants evolved roots and advanced vascular systems. Vascular tissue is composed of cells that are specialized for transporting water and food nutrients around the plants. Some plants go to the extreme length in search of water and nutrients. In the modern flora, plants such as figs, mesquite and the curious welwitchia may have roots that extend over 50 meters below the surface to tap the supplies of underground water in the harsh desert environments. Other plants such as cacti solved the problem of desiccation by storing water inside their stems and reducing their leaves to spines in order to minimize evaporation. Land plants also developed a waxy outer layer called cuticle. This coating provides a protective shield to the plants to minimize water lost through evaporation. However, evolving a protective cuticle meant that plants also needed breathing holes so that they could take in carbon dioxide and emit oxygen. Such breathing pores are called stomata, and they are flanked by guard cells that expand or contract to regulate the flow of gases to the plant's tissue. This is what stomata look like through a microscope. You can actually fit in about ten stomata in a row to make up a millimeter, so they are really, really tiny. The inverse relationship between stomatal density, which is actually often recorded as the percentage of stomata relative to stomata plus epidermal cells on the leaf surface, and referred to as stomata index. So there's an inverse relationship between this and the relative atmospheric concentration of CO2 that has been repeatedly demonstrated for a wide range of plant taxa through time. Thus, measuring the density of stomata on plants through time allows us to calculate the past concentration of carbon dioxide, which is, as you probably know, a major greenhouse gas. So the global temperatures can be tracked through the geological time. In the rivers, lakes and oceans, algae could rely on the buoyancy of water to hold up their bodies. On land, air could not provide the same physical support as water. Therefore, plants needed to evolve a supportive tissue to keep their stems erect and successfully compete for the sunlight against their neighbors. Various supportive tissues were developed in the stems, with many cells strengthened by rigid material called lignin. Another really important step was to improve, their strategies for reproduction. So when plants had to survive on dry land, they needed to protect their male gametes, which are equivalent to human sperm cells, from desiccation and from the strong UV light. To partially overcome this problem, the spore was invented. Spore walls are made up by long carbon hydrogen and oxygen chains that are very resistant to degradation. And this resistant coating allowed plants to disperse their offspring without risk for desiccation. That is also the reason why we today can trace the evolution of the land plants. While most parts of those early plants were delicate and decayed away quickly, the spores were robust and easily fossilized. Thus, we can find spores from those early land plants preserved in the rock record from over 400 million years ago. The oldest spores are found in 450 million year old sediments. The first record of land plant spores are actually preserved in marine environments. These were spores that were produced on land but carried out by rivers into the oceans and embedded in the sediments of the sea floor. The plants producing these spores were probably very small and delicate, so that's the reason why we don't find these entire plants in the geological record. Moss-like plants, like these, are considered to be among the first land plants, though we have yet not found fossils of the whole plants, of these oldest spore-bearing deposits. We find the first evidence of vascular plants in around the end of the Silurian, about 420 million years ago. And this is when life truly began to diversify on land. Vascular plants are those that have specialized water and food-conducting cells. The oldest of these fossil plants were still very small as you can see when you compare them to a needle. This earliest vascular plant got the name Cooksonia in honor of Isabelle Cookson, an Australian paleobotonist active in the 1930s. These first vascular plants were characterized by simple forked branches with the sporangia containing the precious spores at the tips, and they had no leaves. Instead, the entire plant was photosynthetic, maximizing the chance for collecting energy from sunlight. Here we have a totally different plant, but actually, it, it looks very similar to the first land plants. These early vascular plants were still very dependent on water. So only the moist areas would have been vegetated, while dry areas would have remained barren during the Silurian. Slowly, the continents turned green and the plants interacted with the land. The roots interweaved between the sand grains and stabilized the soil. The plants also increase the mechanical and chemical erosion of rocks, and change the drainage pattern of the rivers. This is traceable in the geological record, and so are the traces of the first animals that progressed onto land once the vegetation provided sustenance and protection. We gain information about these early fauna by studying the tracks that these first animals left in the mud. As plants emerged in masses to green the continents, the oxygen level of the atmosphere rose. And with the accumulation of dry plant matter, fires began to appear, probably triggered by occasional lightning strikes. We find the traces of these fires in the geological record through the preservation of charcoal in the sediments. The next major plant group to appear were the lycophytes, also called the club mosses. Today, these are small herbaceous plants, but during the Devonian, some grew to huge sizes, up to 45 meters, and formed vast swamp forests. These are reconstructions of the Paleozoic lycophyte Sigillaria, and we find plenty of these in the fossil record. But why would plants invest so much energy in order to grow so large? Many scientists would say that this was a consequence of the sun rays, that the larger the tree is, the better is the chance to collect sunlight by outcompeting its neighbors. More sun gives more energy. The club mosses were not only larger during the Paleozoic, there were also much more diverse and abundant compared to present. As these plants died and fell into lakes and swamps, they did not always decay, but instead became buried in anoxic water where they slowly turned into coal as they were buried by more and more sediments. The result was that vast coal deposits formed on several continents during the Carboniferous. Locking up all this organic matter in the Earth also meant that carbon was taken out of the atmosphere, which led to record high oxygen levels and low carbon dioxide during this time period. [MUSIC]
