Structural Biochemistry/History of the Earth

Beginnings
The Earth was formed about 4.5 billion years ago as theorized by the Big Bang. Life did not arise on the primordial earth due to reducing conditions and super-hot climates. After billions of years; however, signs of molecules began to arise. It was not until 3.9 billion years ago did these molecules begin to form into protobionts. The early reducing atmosphere and conditions on earth also promoted organic synthesis. Energy was everywhere on earth due to climate, lightning, ultra-violet rays and volcanoes. The Oparin-Haldane hypothesis was proposed to explain earth's early conditions and help shed light on how the first molecules were formed. Other theories have hypothesized that the Earth could have been bombarded by asteroids that contained both D and L amino acids, but as billions of years passed the L-isomer amino acid predominated and is in all organisms today.

Evidence of Life's History: By studying fossils and through relative dating and absolute dating, geologists were able to establish a geologic record of the Earth’s history. Fossils are the remains of ancient life or activity. Types of fossils include imprint fossils, trace fossils, petrification, and amber. Relative dating provides a geologic time scale. The principle of superposition states that in sedimentary rocks, the highest stratum is the youngest and vice versa. Absolute dating is a technique that use radioactive isotopes and half life of elements to determine the actual ages of strata in geological column.

The current geologic record divides Earth’s history into three eons: the Archaean, Proterozoic, and Phanerozoic eons. The Archaean and Proterozoic eons lasted approximately 4 billion years together leaving the Phanerozoic eon to encompass the most recent 500 million years of Earth’s history [2].

The Phanerozoic eon represents most of the time for the existence of animals on Earth. This eon is split into three eras on the geologic record: the Paleozoic, Mesozoic, and Cenozoic eras.



Early Earth - Precambrian Era (4.5 Billion Years ago - 545 Million Years Ago)
Conditions on the earth before the Cambrian explosion and prior to the three domains of life we know today were very extreme and harsh. However, protobionts (organic molecules that formed abiotically) began to form through time and were theorized to be the first molecules, and the precursor to the prokaryotic cells. After a few billion years prokaryotic cells formed and along with these prokaryotes, cyanobacteria began to grow and photosynthesize, thus releasing oxygen into the atmosphere and changing the oxygen level of the earth. Prior to this earth had little to no oxygen. With newly diversified cells, bacteria began to conjugate. Conjugation of bacteria over time therefore led to single celled eukaryotes, the protist. As protist began to replicate and grow, multicellular eukaryotes began to arise and led to multicellular organisms. These conditions over time paved way for the massive Cambrian explosion where diversity of life on a massive scale took place.

Precambrium - Archean: 3.5 bya (billions years ago) - oldest fossils (of bacteria) found. There were only prokaryotes for the first 1.5 billions years of life. Fossil of cyanobacteria were found, they were oxygenic and could carry out photosynthesis. 2.7 bya - atmosphere accumulated Oxygen and iron precipitated out of the oceans.

Precambrium - Proterozoic: 1.9 bya - oldest Eukaryotes found. 1.2bya - multicellular eukaryotes -alga- found. 700mya, there were diverse algae and soft-bodied animals. 650mya - there was a massive glacier (snowball earth) cooling of earth. 540mya - Cambrian explosion - diverse animals and algae come about.

The Oxygen Revolution
During Earth’s history, there were many key events that resulted in major evolutions. One such event was the “oxygen revolution”. Most of the oxygen gas in the atmosphere was of a biological origin, particularly from the water-splitting step of photosynthesis in ancient cyanobacteria. The oxygen gas was first dissolved by the surrounding water. The oxygen gas then started to react with dissolved iron once the concentration of oxygen was high enough. This reaction produced iron oxide which precipitated to form sediments. As the oxygen gas concentration kept rising, the seas and lakes soon became saturated with oxygen gas and the oxygen started to “gas out” of the water and into the atmosphere [2]. The amount of atmospheric oxygen gas was increasing at a gradual rate between 2.3 and 2.7 billion years ago; however, the rate of oxygen gas being added to the atmosphere spiked up rapidly causing an increase of 1% to 10% of its present level [2]. This is known as the “oxygen revolution” and it had a huge impact of the life inhabiting Earth at the time. Many prokaryotic groups became extinct due to oxygen being toxic to these groups by attacking chemical bonds, inhibiting enzymes, and damaging the cells. The species that survived did so by inhabiting an anaerobic environment. Diverse adaptations also resulted from this revolution, one of which was cellular respiration. The oxygen revolution also brought about the first eukaryotes. The origin of the eukaryotes can be further explained by the endosymbiont theory.

Paleozic Era (545 Million Years Ago - 251 Million Years Ago)
The Paleozice Era is marked by the cambrian explosion in which organismic diversification exploded, forming different life forms on earth. Due to lack of a fossil record prior to the Paleozic era, little can be said about when the cambrian explosion actually occured but new novelty structures such as jaws, appendages, bones, and other features on today's organisms began to form. There were also marine diversification. The first vertebrates appeared 525 mya. THere were invasion of land by many. The first land plants and fungi formed 460mya. THe first land animal Arthropods appeared. Following this, the first insects was found 396mya. Furthermore, the first land vertebrate (amphibians) appeared 360mya. The appearance of amniotes (terresterial animal) in the late Paleozoic.

Also during this time the earth due to continental drift began to move together and form a supercontinent, pangea. There were a reduction of shallow coastal habitats and along came cold harsh interior climates. However, the Paleozoic Era ended with a mass extinction termed the Permian Crisis. During the Permian Period about 251 million years ago, 96% of species went extinct. This occurred because massive volcanic eruptions released SO2 and CO2, causing global warming feedback. This massive release of methane by the underwater volcanoes led to a change in ocean circulation and an anoxic ocean, or an ocean depleted of oxygen.

Mesozoic Era (251 - 65 Million Years ago)
The Mesozoic era, also known as the age of the dinosaurs shows more growth and diversification in organisms. Gymnosperms appeared in the early Mesozoic. During the mid-mesozoic, dinosaurs were introduced. The late Jurassic introduced birds. Origin of mammals are from early Mesozoic. 127 mya angiosperms (flowering plants) appeared. Also during this time the earth due to continental drift Pangea began to break. The end of the Mesozoic era is also marked with another mass extinction (especially of dinosaurs),the Cretaceous Crisis, theorized to have been caused by a gigantic asteroid, hitting the area of Yucatan. Layers of iridium are found around this area, thus proving that there could have been a catastrophic asteroid hitting this area affecting the plants and organisms that lived around that area during that time. The global effects included (1) crustal material ejected into stratosphere, (2) widespread fires nearer to impact, (3) reduced photosynthesis for several years, and (4) reduced global temperatures.

Cenozoic Era(65 Million Years Ago - Present)
The Cenozoic also known as the age of mammals began after the extinction of all the dinosaurs (except birds). The first grass appeared 40mya. At the beginning of Pleistocene, 2mya, there were glaciations. Human evolution occured during this time. Homo-sapiens as we know today began to populate the earth 100,000 years ago. There are many misconceptions: our ancestors are not chimps, not half-stooped cave dwellers, and are not contemporaries of dinosaurs.