In this lecture, we'll look at the evolution of cellular life on the earth. We're talking about an era in the earth's history called the Proterozoic Eon, and it runs from 2.5 billion years ago to about half a billion years ago. This is the first time period in the earth where simple life begins to proliferate. The fossil record begins during this era. Also, we see the first rise of multicellular life rather than single widely dispersed cells in the environment. Also, this Eons sees the rise of eukaryotic cells which are cells with nuclei anaerobic respiration. The beginning of this era is marked by a period when average temperatures plunge and glaciers covered the surface of the earth, a very dramatic period in the Earth's history. It's called the Huronian glaciation or Snowball earth. It occurred somewhere between 2.4 billion and 2.1 billion years ago. Remember that for all talk about time this bygone in the earth's history, the evidence is indirect, because the Earth is a restless planet, where tectonic activity, and volcanism, and erosion, and weathering have altered the surfaces and the rocks. So, even rocks of this age are hard to find, but this evidence has been pieced together over a few decades of careful research. So, it's fairly clear that the earth underwent a dramatic period of extended climate change during this period. Basically, the oxygen produced by photosynthesis diluted the proportions of greenhouse gases in the earth's atmosphere. This of course reduced the warming effect of those greenhouse gases and the greenhouse effect was reduced. As a result of this cooling, more snow and ice formed in the polar regions. The result was a positive feedback whereas more ice and snow form, they reflect more of the sun's radiation further cooling the earth. So, the process accelerates until the earth is sent into the deep freeze. Evidence from geology indicates the glaciers nearly covered the planet for about 300 million years. In fact, the argument is whether the glaciers covered the entire climate a completely frozen snowball, or whether there was a small girdle of liquid ocean poking through near the equatorial regions. Current indications are that the earth was not completely frozen during this time. The core of the evidence is of course rocks indicating glaciation at equatorial regions where you wouldn't expect that in the fossil record, and this evidence is seeing all around the earth's surface. So, it was clearly a global event. The period leading up to the Snowball Earth Cataclysm was marked by the prevalence are prokaryotic bacterial life. Remember, prokaryotes were the first types of cells, cells without nuclei, simple life forms, all bacteria, all archaea, the other form of life also bacterial going back to the root of the three or like four billion years ago. Some of these bacteria were photosynthesizing using sunlight to directly drive their metabolic processes. But other bacteria use chemical energy from hydrothermal lens, and some from sulphur deposits. Prokaryotic life was limited to the oceans and the shallow seas, there was no life on land at this time. Prokaryotes that did not use photosynthesis evolved a different process to generate their energy. They evolved what's called anaerobic respiration and metabolism that does not use oxygen in the cellular processes that generate energy. Anaerobic respiration takes place in the cytoplasm and breaks down glucose by a fermentation to produce two energy carrying molecules of ATP, adenosine triphosphate. Anaerobes did not develop a mechanism for using free oxygen. Because essentially, there was no oxygen in the environment when these first prokaryotes were evolving. Remember, this is about a billion years before the grace glaciation in the Snowball Earth episode. As oxygen levels rose and the Earth froze, early life was faced with many challenges. Oxygen is highly reactive. It can react with and damage cellular components. So, even when oxygen is part of my metabolism, it's a double-edged sword for biology because of its highly oxidizing and corrosive effects. Remember, oxygen also likes to react with rocks, with metals to form rust, and dissolves into seawater to make it slightly acidic. So, free oxygen is hard to sustain, unless it reaches a certain level in the earth's atmosphere. Free oxygen reacts with nearby molecules to produce free radicals, which also damage cellular material. If a cell cannot generate enzymes to neutralize the oxygen or the associated free-radicals, then oxygen exposure essentially kills the cell. It is hypothesized that this environmental pressure is what led to the emergence of aerobic respiration in eukaryotic cells, essentially an entirely different mode of life's energy process. At some point, cells began to develop their aerobic oxygen based metabolism, the kind familiar to us on the present day earth. Anaerobic fermentation only breaks glucose in half. Because oxygen is such a powerful reactant, aerobic respiration can break apart nearly the entire glucose molecule. Aerobic respiration, once it's developed, releases 18 times more energy than anaerobic respiration. Therefore, it was a major improvement over anaerobic respiration. In the evolution, of life, conserving energy or utilizing energy more efficiently is a powerful selective advantage for life forms. So, we can see that the most efficient energy process will come to dominate the mode of life on Earth. Aerobic respiration was an invention of eukaryotic cells, cells with nuclei, that emerged around the time of the great Snowball Earth episode. Cells that could perform a wide range of metabolic tasks were more likely to be able to adapt and survive in this rapidly changing environment. Prokaryotic cells lack specific function organelles and anaerobic respiration takes place in the cytoplasm itself. So, as soon as you have eukaryotic cells and aerobic respiration, you can start to differentiate the function of cells, and a wide variety of cell types evolve with specialized functions in the organism. It is speculated that environmental pressure may have driven prokaryotic cells to establish symbiotic relationships with other prokaryotes. So, one of the ideas of how the first cells with nuclei evolved was that it was a commensial relationship between two types of cells without nuclei, where one cell subsumes the other cell to become its nucleus. This is a theory first put forward by Lynn Margulis about 40 or 50 years ago. In certain events, one prokaryotic cell absorbs another, and the resulting called a chimeric cell survives. There are various aspects of eukaryotic cells that support this theory. First, eukaryotic organelles physically resemble prokaryotic bacteria embedded in the cytoplasm because of their membrane separating the organelle from the cytoplasm. Also, mitochondria have their own RNA and DNA, and they have a membrane just like a prokaryotic cell. They also divide separately from the rest of the cell in a simple fashion like that of prokaryotes. So, this theory hangs together. Remember, however that the evidence for it is indirect because we have no fossil record going back this far in the Earth's history. Eukaryotic organelles are program for specific jobs that let the cell carry out a variety of highly complex function. For example, in a modern human being, there are about 50 different distinct cell types; liver cells, brain cells, and so on. Eukaryotic cells emerged sometime between 2.1 and 1.6 billion years ago. They fundamentally changed the evolution of biological life on the earth allowing much greater differentiation of function, much greater specificity of adaptation to the environment, and much greater complexity of the still small living organisms. All living organisms today are made up of eukaryotic cells except for bacteria and viruses, that is to say all plants and animals. Eukaryotic cells have continued to differentiate over time evolving different classes of cells program for specific tasks in the body. Unique organs that are entirely dedicated to the function of vision, such as eyes or detoxifying the blood like kidneys, are the result of this specialization of cell tasks. The chemical and physical complexity of life is amazing. In this video, we shrink by a factor of million to go inside a cell and see all the complex organisms. This is just a snapshot of some fraction and the interactions that go on in a cell. This video was called the inner life of a cell and it was created from the National Institutes of Health from research data. So, although it is an animation, it fairly represents the physical structures and the activities going on inside all of our cells. Rising oxygen in the earth's atmosphere led to a climate catastrophe called Snowball Earth, about 2.4 to 2.1 billion years ago. The earth went into a positive feedback cycle that essentially froze the planet almost to the equator. Emerging at the end of this process were cells with nuclei, eukaryotic cells. All plants and animals are eukaryotes. We don't know exactly how this happened, but it's possible that the first eukaryotes formed from symbiotic relationships of prokaryotic cells, cells without nuclei, where one absorbed the other. Regardless, after half billion years, the differentiation of these cells with nuclei led to specificity of function that has all the attributes of modern life.
