Transcript: The tree of life suggests that the first organisms on Earth were thermophiles, organisms that accustomed to and thrived in conditions of high temperature. The environments may have been hot springs or deep sea ocean vents. They came from two of the three branches of the tree of life, bacteria and archaea. They were prokaryotes or cells without nuclei. It’s possible that eukaryotic organisms also coexisted with the prokaryotes. They would not have left fossils, and we may not...

Transcript: In general, evolution proceeds from simple to more complex forms of life, but it’s not the unique and defining attribute of evolution. Simple organisms can be extremely well adapted. Anaerobic bacteria have survived on this planet for over three billion years. No large and complex creature has ever lived for more than a few tens of millions of years. Also, early life was not that much less complex than current large life forms. We share forty percent of our DNA with simple...

Transcript: The existence and abundance of extremophiles on Earth has implications for the search for extraterrestrial life. Remember that the entire idea of a habitable zone is predicated on a range of distance from a star where water can be in a liquid form. Perhaps this is too limiting an assumption. The first organisms on Earth were extremophiles, so maybe extremophiles formed first elsewhere in the universe and are relatively abundant. The wide range of physical conditions for...

Transcript: Certain life forms can exist under extreme chemical conditions. Examples include halophiles that prefer a high salt concentration, above five percent all the way up to thirty percent. For comparison, seawater has a salt concentration of three percent, and it’s toxic to humans. Environments where halophiles exist might be a great salt lake or other salty environments, either on land or in the ocean. Other examples of important organisms on Earth that have extreme chemical...

Transcript: Life forms on Earth can exist and thrive in extreme chemical conditions corresponding to acid or base. Acidophiles can thrive in conditions where the pH is less than five. That's similar to sulfuric acid. Examples include the sulfur pots in Yellowstone Park, volcanic soils, and of course the gastric fluid in the human stomach. At the opposite extreme are alkaliphiles which can exist and thrive at pHs greater than nine. Examples include soda lakes in Africa and soils that are...

Transcript: There is a set of organisms that can persist and thrive under conditions of extreme pressures. They are called barophiles. Barophiles can exist at pressures of hundreds of atmospheres. These environments are typically found in the deep sea. For example, there are entire colonies of eukaryotes on the continental shelf. Sea cucumbers and other creatures also exist under very high pressure, and there are microorganisms that thrive near deep sea black smokers. At the other...

Transcript: In the 1970s, scientists who used the first deep sea submersibles to go thousands of feet below the ocean’s surface got a huge surprise. They found entire ecosystems near deep sea ocean vents where hot superheated water existed due to volcanic activity on the ocean floor. This is a region of utter darkness. Light can of course only penetrate the first few feet under the ocean. It’s also a situation of extreme pressure, perhaps a hundred atmospheres, and yet in this extreme...

Transcript: Many microbial organisms on Earth can survive extremes of temperature. Thermophiles is the name for organisms that like heat. Thermophiles have been found that can exist in the temperature range forty-two to a hundred and thirteen degrees centigrade. That's a hundred and seven to two hundred and thirty-six degrees Fahrenheit, well above the boiling point of water. Examples of these organisms are found near deep sea geothermal vents and in Yellowstone National Park. At the...

Transcript: We tend to think of the evolution of life in terms of a sequence from simple organisms to large and complex organisms, but we should remember that even now, most life on Earth looks nothing like us. There are many more prokaryotic species, involving cells without nuclei, then eukaryotic species. There are many more species that can survive physical conditions that would kill us then there are large and complex organisms like us. Collectively, these species are called...

Transcript: If you looked at a biology book from a hundred years ago or even fifty years ago, you would have seen a diagram called the tree of life. In the traditional version of this diagram, it is represented as a tree. The simplest organisms are at the bottom of the tree, and more complex, sophisticated, or intelligent organisms are higher up the tree. In traditional representations of the tree of life, primates and humans are at the pinnacle of the tree or the apex representing the...

Transcript: The modern version of the tree of life based on the sequence of DNA can be used to speculate about the earliest organisms on the Earth. It’s important to realize that the tree of life is only tracing genetic differences in the DNA sequence, so it’s not possible to put an exact time sequence on these changes. Evolution occurred at different rates at different periods of the Earth’s history. For example, the mutation rate early on was much higher than it is now. It’s likely that...

Transcript: Similarity or dissimilarity in DNA can be used to trace human origins. The oldest human civilizations date back only ten thousand years, the oldest human artifacts about thirty or forty thousand years. Yet changes in the structure of DNA can be used to trace human origins back hundreds of thousands of years, even millions of years. The first human ancestor dates back to two or three million years ago. By watching the change in DNA as artifacts are found in different parts of...

Transcript: Fossils have existed only since creatures had skeletons or hard bodies or shells, about a half a billion years, but life existed on Earth at least three billion years before that. Almost all of these life forms were microscopic. Scientists can learn about the history of microscopic life over the whole span of Earth’s history from DNA itself, which acts as a kind of living fossil telling the story of life. Imagine the DNA of an early living organism that was the ancestor of all...

Transcript: Natural selection operates at the level of species interacting with their environment. At the microscopic level, DNA copies itself, a mechanism that is generally extremely efficient and effective, but its not perfect. The human cell copies the entire information in the human genome, three billion bases, in only a few hours. The error rate is tiny, one in a billion. However errors do occur, errors in transcription, alterations caused by ultraviolet radiation, and even the...

Transcript: The earliest solid evidence for life on Earth comes from three and a half billion years ago in the form of stromatolytes. Stromatolytes are prokaryotic organisms, colonies of microbes, bacterial mats. They take the physical form of layers of sediment interspersed with microbes. Today they are found in their fossilized form, but the layered structure is very clear evidence of a colony of organisms. The energy source for the prokaryotes was photosynthesis near the top of the...

Transcript: The first biological systems capable of independent life on Earth were prokaryotes. Bacteria are familiar example, a single long strand of DNA with several thousand genes. Most prokaryotes are harmless to humans, and in fact they are essential for our form of life and for the survival of more complex organisms. Prokaryotes may have less genetic material than eukaryotes, but they are highly complex chemical factories, many of which are still not understood. In fact, the...

Transcript: There are two fundamentally different types of cells in life on Earth: prokaryotes and eukaryotes. The prokaryotes are cells without nuclei. They are ten times smaller then the eukaryotes, and they are far less complex in a chemical sense. Eukaryotes, which are larger, have their DNA contained in a nucleus which provides a higher level of functioning and complexity. Examples of prokaryotes are E coli and salmonella. Examples of eukaryotes are amoeba and of course the...

Transcript: The history of life on Earth is a story of experimentation; initially, the experimentation of chemical reactions, then the experimentation of biological processes, and finally that of genetic variation itself. These genetic variations and earlier variations are molded to the environment, which ensures the survival of the fittest organisms; this is the basis of natural selection. At a chemical and biochemical level, life has formed specific and efficient solutions to its...

Transcript: Cells are tiny chemical factories, and they are essential for every complex life form on Earth. They have several major ingredients that are important for their function. Carbohydrates are the basis of all the structures in cells. In the every day world, we are familiar with carbohydrates because they can store energy. Other carbohydrates involve cotton, wood, cellulose for example. Lipids are important because they store energy, and they were used to form the first barrier...

Transcript: A major step in the development of life is the transition from free floating organic molecules in water to cells. Biology cannot develop without some form of compartment or membrane to concentrate chemical reactions and protect the entity from the environment. Scientists do not yet know how this transition occurred on the early Earth, but it’s been shown that the heating and cooling of solutions of amino acids can lead to the formation of a cell-like structure called a...

Transcript: The construction of larger organic molecules from smaller pieces, even if there’s a mechanism for it to operate, is essentially a random process and not directed in any way. So how did this process continue to build larger and larger chain molecules? RNA strands can catalyze their own creation, and this leads to a microscopic version of natural selection. The RNA strands that replicated faster and more efficiently and with fewer copying errors were able to dominate the...

Transcript: If RNA was the first replicating molecule on Earth, how was it built? Fred Hoyle, a theorist in astronomy, speculated that it was incredibly unlikely to build long complex chain molecules from small pieces by chance. He said that it was as unlikely as if a whirlwind passed through a junk yard and fully assembled a jumbo jet. Turns out he was wrong because both in theory in computer simulations and in the lab simple autocatalytic networks of chemicals can rapidly build...

Transcript: DNA was probably too complex to have been the first replicating molecule in the history of life. RNA is much simpler. It has only one strand, and it would be easier to construct from smaller pieces. However, neither molecule can reproduce itself without enzymes. Enzymes are proteins that take their instructions from the DNA itself which leads to a classic chicken and egg problem. This problem was solved in the 1980s by Thomas Cech who showed that RNA can catalyze biochemical...

Transcript: In addition to being built up from smaller pieces by natural chemical reactions, organic molecules were brought to Earth by impacts early in the Earth’s history. Meteors have been shown to contain amino acids, a significant number of them, and other complex molecules. Complex molecules and organic material therefore can form and survive in deep space and can probably survive in the interior of an impacting object when it lands on the Earth. Thus, especially early in the Earth’s...

Transcript: How did life begin on Earth? In the 1950s, a classic set of experiments were conducted by Miller and Urey. The Miller-Urey experiments tried to make life in a bottle. A flask containing water and the basic chemical ingredients of the oceans and the Earth’s atmosphere is given extra energy and left for weeks. After watching this closed experiment for weeks, it turns out that amino acids and complex molecules and other organic material is created. The Miller-Urey experiments...