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Formation of the Solar System (5 BYA)
Solar System Around five billions years ago, the solar system was still forming at this time. It was only a huge mass of gas and dust. Later, gravity pulled debris together to make the sun and the Earth. The solar system was formed by the explosion of a supernova. Waves in space squeezed the cloud of gas and dust together. -
Formation of the Earth (4.6 BYA)
Earth After the solar system formed, the sun and the plants began to form. Earth began to form at this time due to collisions with space debris. These collisions released a large amount of thermal energy, which would have melted part of Earth's surface. Earth continued to increase in size as gravity pulled in more debris. -
Organic Molecules (4 BYA)
Organic Molecules Organic molecules began to accumulate around four BYA. Using a dating method based on the decay of uranium and thorium isoptoes in rocks, scientists have determined Earth's approximate age. The oldest, unmelted rocks tell scientists when Earth started to cool. The age of the oldest known of these rocks is about four billion years, so scientists think that organic molecules appeared at this time. -
Archaea Fossils (4 BYA)
Archaea Scientsits have fossils of archaea, which lived in harsh conditions, from this time. Specifically, they have a fossil of the archaea Methanosarcina bakeri. It was thought to be similar to the first forms of bacteria that appeared at this time. Many archaea also obtain energy through chemosynthesis, rather than photosynthesis. -
Stromatolite Fossils (3.5 BYA)
Stromatolites The oldest known fossils are similar to modern-day cyanobacteria. An example of cyanobacteria is lynbgya. Fossils of stromatolites, layered structures of lynbgya, are from this time. -
Oxygen and Organisms
Oxygen and Unicellular Organisms Oxygen was damaging to unicellular organisms at this time. In these organisms, oxygen destroyed the coenzymes needed for cell function. However, in other organisms, oxygen bonded to other compounds, so it was not harmful to the organism. This was an early form of aerobic respiration. Also, scientists have found that some forms of life from this time became photosynthetic. -
Earth's Modern Appearance (2.2 BYA)
Pangaea At this time, Earth probably became similar in appearance to how it is today. Pangea, the unicontinental mass, broke off into the 7 modern continents and the oceans filled in around the continents. Even so, as the continents continue to shift, Earth's appearance will continue to change as well, very slowly. -
Oxygen Levels (2 BYA)
Oxygen in the Atmosphere Oxygen levels in the atmosphere reached today's levels. Later, oxygen gas reached the upper part of the atmosphere, where it was split into O atoms. This was helpful in ozone formation (see the event labeled Ozone Formation). -
Endosymbiosis (1.5 BYA)
A small aerobic prokaryote was engulfed in and lived in a larger anaerobic prokaryote. These prokaryotes provide a way to synthesize energy. This is called endosymbiosis, where "endo," or "endon," means within, "syn" means together, and "biosis" means ways of life. The prokaryote later evolved into modern mitochondria. (For more information, see the event Lynn Margulis' Experiment). -
Ozone Formation (1 BYA)
Ozone Formation The ozone formed at this time. As oxygen gas traveled to the upper part of Earth's atmosphere, the sun split it into single O atoms. These atoms reacted with O2 to form O3, or the ozone. The ozone is especially important because it protects organisms from the sun's harmful UV rays and allows life to flourish on planet Earth. Without the ozone, the DNA of organisms would be damaged, hindering life on Earth. -
First Microscope(s) (1665)
Leeuwenhoek Robert Hooke studied nature with an early light microscope. These microscopes magnify objects by bending light rays. Later, Anton van Leeuwenhoek observed living cells with a more precise microscope (1673). Leeuwenhoek viewed cells, and thought they looked like the cell where a monk lived. The name "cell" stuck for the smallest life form. -
Pasteur's Experiment (mid-1800s)
Pasteur's Experiment
Louis Pasteur placed boiled chicken broth in a curve-necked flask. After a year, the broth was still clear, though there was residue in the neck of the flask. This residue, which was once organisms, was not able to travel into the flask, therefore no life could "generate" in the flask. However, Pasteur later removed the neck, and after one day, the broth was cloudy. -
Oparin's Hypothesis (1920s)
Oparin's Hypothesis Alexander Oparin worked wtih John Haldane to learn more about the compounds in Earth's atmosphere and how they formed. Oparin formed a hypothesis that the gases might have formed simple organic compounds. He proposed that when Earth cooled, the organic compounds collected in water and went through a series of chemical reactions to eventually produce macromolecules needed for life. -
Radiometric Dating (1950s)
Radiometric Dating Radiometric dating is the process by which sciencetists determine the age of an artifact. They use the known half-lives of certain radioactive elements (Carbon-14 is very popular) to determine the age of the speciman. By measuring how much of the the element has disappered, the scienctists can calculate the age. This process is useful for archeological artifacts as well as fossils. -
Urey and Miller's Experiment (1953)
Stanley Miller and Harold Urey tested Oparin's hypothesis because he was not able to do so. The experiment consisted of a chamber containing the gases thought to be in Earth's atmosphere. Electric sparks supplied energy in lieu of chemical reactions. The experiment produced many organic compounds, such as amino acids, which supported Oparin's hypothesis. The experiment was performed to study conditions for the formation of organic molecules on Earth. -
Fox's Experiment (1960s)
Fox's Experiment Sidney Fox, along with many others, sought to find out which physical structures eventually created the first cells. These structures form in simpe organic compound solutions. Microspheres and coacervates are examples of these structures. They can spontaneously form under certain conditions, of which were present in Fox's lab. These conditions were similar to those on early Earth. -
Thomas Cech's Experiment (early 1980s)
RNA Thomas Cech found that RNA in unicellular eukaryotes can act as a chemical catalyst, which is similar to an enzyme. Based on this, Cech used the term "ribozyme" to describe RNA that serves as a catalyst. There have also been other studies and experiments conducted to see RNA's other roles in an organism. -
Spallanzani's Experiment (1700s)
Spallanzani's Experiment Lazzaro Spallanzani also wanted to test the theory of spontaneous generation. To do so, he placed boiled (to kill any living organisms) chicken broth in flasks. One flask was sealed and the other was not. After multiple days, the covered flask remained clear, while the uncovered flask had become cloudy. This supported Redi's experiment. (The covered flask remains clear today.) -
Redi's Experiment (1668)
Redi's ExperimentFrancesco Redi sought to discover if organisms spontaneously generated, or if they came from other living organisms. He tested this by using meat in a jar. One jar was covered with cheese cloth and the other was left uncovered. After a while, there were no maggots on the covered jar, but there were maggots on the uncovered jar. Based on these results, Redi concluded that living organisms only come from other living organisms, disproving the spontaneous generation hypothesis. -
Lynn Margulis' Experiment (1960s)
Lynn Margulis proposed that endosymbiosis led to the evolution of eukaryotic cells. She believed that mitochondria evolved from a symbiosis of bacteria and plant + animal cells. Overall, her hypothesis was that organelles (in cells) came from endoymbiosis (see event labeled Endosymbiosis). Margulis tested her hypothesis using electron microscopy. The results showed that organelles are similar to bacteria, therefore endosymbiosis is accepted as the reason for the evolution of eukaryotic cells.
