absorbs UV light in the upper atmosphere. This decreases the mutation rate on earth. Oxygen was probably toxic to many lifeforms that existed on earth at the time. Plate tectonics have reconstructed the continents. The tectonic plates have an edge, which is marked by an orogenic belt. There has been more than one supercontinent throughout earth s history. Pangea was the last of the supercontinents. There were at least two snowball earths in the Proterozoic era. These were related to the CO2 levels on the earth, which keep the earth warm.
EARLY FORMS OF LIFE The oldest rocks still found on earth are about 4 billion years old. Evidence of life shows up shortly after that. Fossils of a microbial mat were found in Australia from 3.5 billion years ago. Cyanobacteria in Greenland were discovered dating back 3.7 billion years. These were what stromatolites were made from. Scientists do not know if there were earlier forms of life that did not show up in fossil records. There are no fossil records from the Hadean period. There were many asteroids and meteorites bombarding the planet but life may have existed during this time. Evidence for this is the finding of graphite carbon in crystals of zircon dating from 4.1 billion years ago. Graphite cannot exist without the presence of organic life. Some researchers are skeptical and believe that asteroids could have brought the carbon to earth. As mentioned, the earliest forms of life were prokaryotes. They probably used nutrients like phosphorus from the environment and likely thrived in extreme conditions. There are about 335 proteins that are shared by all modern Archaea and Bacterial organisms, indicating a common ancestor. It indicated also that these were probably anaerobic, living without oxygen, and lived near hydrothermal vents. There are those who believe life originated on earth more than once or that life came to earth on asteroids. Most believe that the earliest organisms were based on RNA instead of DNA. This feature changed over time. No one knows, though, how RNA first came into being. It seems astronomical that RNA could have formed spontaneously. Others believe that it s not so astronomical and that life lives elsewhere in the universe.
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