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Planet Formation (5 BYA)
Space material such as gas and dust were pulled together by gravity and formed the sun. Planets then formed through repeated collisions of space debris. -
Growing Earth (4.6 BYA)
Earth grew larger because gravity kept pulling in more space debris. Collisions between Earth and space debris created large amounts of thermal energy. -
Old Rocks and Crystals (4 BYA)
The oldest rocks and crystals were formed at about this time. This date was determined using a dating method based on the decay of uranium and thorium isotopes. -
Archaea (4 BYA)
Types of life very much like archaea first populated the Earth at this time. These unicellular organisms thirive in harsh conditions. -
Stromatolites (3.5 BYA)
Stromatolites are a form of cyanobacteria that have grown in layers. -
Photosynthesis (3 BYA)
At this point, some forms of life had become photosynthetic. This was discovered because of geological traces of photosynthetic material. -
Earth Appearance (2.2 BYA)
Earth appeared as it does now from space. The major continental plates have reached their current location. -
Oxygen Levels (2 BYA)
Oxygen levels on the planet Earth reached where they are today. -
Prokaryotes (between 2 BYA - 1.5 BYA)
During this time period, evidence suggests that a small aerobic prokaryote was engulfed by a larger, anaerobic prokaryote, where it began to live and reproduce. This theory is called endosymbiosis. -
Ozone Layer (1 BYA)
The oxygen in the atmosphere rose and the heat caused the bonds to break in some molecules leaving single oxygen atoms with O2 atoms which bound together forming the poisonous gas, ozone. -
Redi's Experiment (1668)
Francesco Redi hypothesized that only meat exposed to flies would contain flies. The control group, the meat in the open jars, contained flies. However, the meat in the sealed jar did not contain any flies. This experiment was a large step to disproving spontaneous generation. -
Pasteur's Experiment (mid-1800's)
French scientist Louis Pasteur conducted an experiment to end the controversy over spontaneous generation. To do this, he boiled broth inside a flask with a curved neck that prevented solid particles from entering. The broth remained clear for a year. However, when he removed the curve neck, the broth became cloudy after only one day. Because of his experiment, biogenesis became a widely accepted fact. -
Radiometric Dating (1905)
Radiometric dating is used by knowing the half-life of a radioactive element and the amount of the element in the specimen. Then using that information, the scientist can calculate the specimen's age. -
Oparin's Hypothesis (1924)
Alexander I. Oparin was a Soviet scientist who hypothesized that the compounds in the early atmosphere (at high temperatures) could form amino acids and create more complex chemical structures in water with lightning as a source of energy. He believed that the early atmosphere consisted of ammonia, hydrogen gas, water vapor, and hydrogen and carbon compounds. He did not test his hypotheses in an experiment though. -
Urey and Miller (1953)
An American student (Miller) and his professor (Urey) tested Oparin's hypothesis by assembling a system of tubing that included the gases thought to be in the early atmosphere. Electric sparks were used to simulate lightning. Many compounds were formed from the experiment, including amino acids. -
Fox (1960)
Scientist Sidney Fox researched extensively on the structures that may have given way to the earliest cells. He discovered to structures: Microspheres- spherical shape made up of protein molecules organized as a membrane & Coacervates- droplets of certain molecules such as lipids, amino acids, and sugars. -
Lynn Margulis (1970)
Researcher who proposed that early prokaryote cells may have developed a mutulally beneficial relationship (endosymbiosis) where the aerobic prokaryote was engulfed by a larger anaerobic prokaryote. The aerobic prokaryote had an acceptable living environment while the aerobic prokaryote provided an energy source. -
Thomas Cech (1981)
Researcher who discovered that a certain type of RNA found in unicellular prokaryotes can act as an enzyme. Cech called this catalystic molecule that can promote a certain reaction a ribozyme and led to many future discoveries about RNA. -
First Microscopes (1665)
The first microscopes were invented around this time. An early light microscope was used in 1665 by English scientist Robert Hooke. This technology has been improved over time and more advanced models help scientists today. -
Spallanzani's Experiment (mid-1700's)
Italian scientist Lazzaro Spallanzani hypothesized that spontaneous generation does not exist and that all living things come from other living things. To test this, he boiled broth in flasks to kill all the microorganisms. After they were boiled, he sealed one group of flasks and left one group open. The sealed group's broth remained clear while the other became cloudy. He concluded that the broth became cloudy only because microorganisms entered.
