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(5 BYA) The Solar System Forms
A cloud of rotating gas and dust slowly gained speed as it condensed. The shear from different speeds created eddies in the disk around which planets formed. The center of the disk condensed further until it ignited; nuclear fusion blew the nearby residual gas away from the center, which was now the sun. The particles remaining in the disk coagulated to form four small inner planets and the asteroid belt, while the gravity of the large gas giant planets cleared the outer solar system. -
(4.6 BYA) The Earth Forms
As the fragments of planets grew larger, their mutual gravitational attraction increased, causing the collisions between them to intensify. Earth formed from violent collisions of planetesimals, liberating enough heat to melt the rock and metal into an enourmous molten sphere. Heavier parts like iron gathered at the core while lighter parts like gases and feldspars floated to the surface. The trapped gas eventually would create the atmosphere. -
(4 BYA) Life Forms
Gases in the atmosphere combined under the influence of temperature and lightning to form amino acids, ATP, and nucleic acids. The organic molecules formed microspheres and coacervates, membrane-bound, organized spheres which are not alive but developed into the first cells with the introduction of RNA, the first hereditary molecule. The cells initially consumed organic molecules for food, but then some developed chemosynthesis, the metabolism of sulfur compounds. -
(4 BYA) The Earth Cools
After the Earth formed, it cooled over hundreds of millions of years. Volcanoes spewed gases outward (the exact mixture is debatable but probably included nitrogen, water, and methane or carbon dioxide) that were trapped under the surface. They formed the first atmosphere of Earth, created a shield from cosmic rays, and provided the materials from which the first organic compounds formed. -
(3.5 BYA) Cell Colonies Cevelop
Cells grew together in colonies of cells called stromatolites which were dated to 3.5 billion years old, the oldest discovered fossils. -
(3 BYA) Photosynthetic Cells Develop
Organic molecules became less abundent for cells' food as more cells reproduced. Cyanobacteria, which featured a mutation that produced chlorophyll and enabled photosynthesis, were able to reproduce more easily than heterotrophic cells. Oxygen gas is a product of photosynthesis; it harmed some early bacteria, but others evolved an immunity to oxygen by incorporation of oxygen atoms into certain molecules, a step towards aerobic respiration. Photosynthesis developed autotrophs and heterotrophs. -
(2.2 BYA) Earth Resembles Present Image
The Earth physically resembled its present form, with a cool surface and oceans. -
(2 BYA) Oxygen Levels Reach Present Level
As a result of the great oxygen event, the atmosphere contained 20 percent oxygen. Aerobic cells could develop to use the atmospheric oxygen advantageously and gain extra energy. -
(1.5 BYA) Eukaryotes Develop
Prokaryotic cells engulfed smaller prokaryotic cells, and the two cells formed an endosymbiotic relationship. The first small prokaryotes to be engulfed were aerobic bacteria, and these provided the larger cell with ATP in exchange for food and protection. These cells descended into modern mitochondria. Then, cells engulfed cyanobacteria, which, in the same manner, descended into photosynthetic chloroplasts. -
(1 BYA) Ozone Formed
In the stratosphere, ultraviolet rays broke diatomic molecules of oxygen into two atoms. Each atom attached to another molecule of oxygen to form triatomic oxygen, ozone. Ozone accumulated in thin layer of the stratosphere, where it absorbed ultraviolet light and decomposed into diatomic oxygen and atomic oxygen. Ozone protected cells from ultraviolet light that would have mutated nucleic acids and harmed their metabolisms, so cells could from then on expose themselves more to direct sunlight. -
(1768) Spallanzani Disproves Spontaneous Generation
Spallanzani performed an experiment to test spontaneous generation. He placed some broth into sealed containers, while he placed the rest into open containers, an then he boiled the broth in every container. The broth sealed from air did not become cloudy, while the broth exposed to air did. Spallanzani concluded that foreign agents in the air must pollute the broth; they do not spontaneously form. His ideas were rejected by most people during his life. -
(1862) Pasteur disproves Spontaneous Generation
Pasteur poured broth into a flask in the same manner as Spallanzani, but he drew out the neck into a long curve to inhibit microbes from entering but permitting air to. The broth did not cloud until he broke the neck off, proving spontaneous generation is false. -
(1668) Redi Disproves Spontaneous Generation
During the seventeenth century, people thought maggots spawned spontaneously. Redi took pieces of meat and sealed them from outside air, including flies. He observed that maggots appeared only on meat that was exposed to flies, so he concluded that flies must create the maggots and maggots do not appear spontaneously. -
(1929) Early Atmosphere Hypothesis
Oparin and Haldane formed a hypothesis that hydrogen, water, ammonia, and carbon compounds formed the atmosphere of Earth in its early life. They reasoned that these gases, likely to have been abundant on the early Earth could provide the substances to create organic compounds, but did not test it. -
(1953) Miller-Urey Experiment
Miller and Urey created an apparatus to test the validity of Oparin and Haldane's hypothesis. They linked flasks together and filled them with the gases thought to be present by the hypothesis. With electric discharges to simulate lightning, Miller and Urey synthesized amino acids, ATP, and nucleic acids from the gases, which supported the hypothesis. -
(1957) Microsphere Theory
Sydney Fox discovered that the amino acids created in the Miller-Urey experiment could polymerize into polypeptides with addition of heat. Not only could they form proteins, but the proteins would form microspheres several micometers in diameter. He hypothesized that they could be a step between organic molecules to living cells. -
(1966) Endosymbiosis Theory
Lynn Margulis formulated the theory that the first eukaryotic cells formed when a large prokayote engulfed a small prokaryote and they formed an endosymbiotic relationship; the larger cell provided protection and food, and the smaller cell provided extra energy. Her theory gained popularity in 1980 when scientists discovered that mitochondrial DNA is different than nuclear DNA. -
(1980) Ribozymes
Cech disovered that ribonucleic acid segments can act as catalysts when they twist into enzymes. Ribozymes can perform many functions within cells, including in DNA replication, and scientists found that some ribozymes can self-replicate in controlled environments. Ribozymes may have played a part in the creation of the first cells. -
(1949) Radiocarbon Dating
Libby developed the technique of dating objects using carbon-14, which can date organic objects between 26,000 years and 60 years old. Other methods of dating include uranium-238 and potassium-argon-40, which scientists used to determine that the age of Earth is 4.6 billion, not millions, of years old, which was a critical step to understand the timescale of the development of Earth and life on Earth.
