Era Report WAC - 9/22/17

       According to the Geologic Time Scale, there are four major eras in history that shape Earth as we know it today. The Precambrian era is the first era that occurred on Earth. This era covers almost 90% of Earth's entire history. The beginning of the Precambrian era started with the formation of Earth 4.6 billion years ago and ended at the first sign of complex life about 540 million years ago. This era is divided into three eons. This includes the Hadean eon, the Archean eon, and the Proterozoic eon. The Hadean eon started when the planet first began to form as the Earth's core was building. The solar system was constructing within a cloud of dust and gas, known as the solar nebula. The Archean eon was a time of continental building and showed the first stages of early life on Earth. At this time, the Earth's crust cooled enough that rocks and continental plates began to configure. The final eon apart of the Precambrian era is the Proterozoic eon. During the Proterozoic eon, life began to evolve into more complex organisms.

       Since the Earth was barely formed during the Precambrian era, there were little to no plants present during this time. According to the Penn State University department of science, Lichens are believed to have been the first fungi existing on Earth. A Lichen is a composite organism that arises from algae cyanobacteria living among filaments of multiple fungi in a symbiotic relationship. During the Proterozoic eon of the Precambrian era, Lichens teamed up with photosynthesizing organisms. The organisms that Lichens collaborated with, include cyanobacteria and green algae. Together these plants conceivably boosted oxygen levels in the atmosphere high enough for animals to develop skeletons, grow larger, and diversify.

        Toward the end of the Precambrian era, the first life formed. The first one-celled organisms formed during the Archean eon of the Precambrian era. Early evidence of life that was once living in the oceans of Earth were microscopic bacteria that were capable of photosynthesis. These one-celled organisms helped make the air and water around Earth become full of oxygen. Once there was plenty of oxygen in the atmosphere and in the ocean, new life could form. New varieties of bacteria began to harness the power of the sun through the biochemical process known as photosynthesis. This energy source ultimately lead to the creation of simple plants and opened the planet up to animal life. The Precambrian era ended with the emergence of complex, multi celled life forms. By the end of the Archean eon, the ocean floor was covered in a living mat of bacterial life.

         Scientists named this eon after the Greek god Hades, who ruled underworld. The surface of the Earth during most of the Hadean eon must have been like our image of the underworld. During the Hadean eon the Earth released tremendous amounts of heat as the Earth's core was forming, due to gravitational sinking. In fact the surface temperature of Earth at this time, was at 1200 degrees Celsius. Throughout the Archean eon, the outer covering of the Earth cooled and hardened into a crust. The hot molten leaked out at weak places in the crust, creating volcanoes. Water from comets and hydrated minerals condensed in the atmosphere and fell as torrential rain. This rainfall formed and filled oceans. The presence of micro organisms on Earth during this time, indicates that 4 billion years ago the Earth's surface temperature must have cooled to less than 100 degrees Celsius. Furthermore, the oceans must have been above 0 degrees Celsius. Therefore, the Earth's surface temperature was between 0 degrees Celsius and 100 degrees Celsius. These temperatures would have had to occur on Earth during this eon in order for these organisms to have lived.

           The basis of Earth's growth happened during the Precambrian era. There are three eons that make up the Precambrian era, including the Hadean, the Archean, and the Proterozoic. The Earth had just formed during the Hadean. At scorching temperatures, there were an abundance of radioactive elements and collisions with other solar system bodies. Approximately 70% of continental landmass was formed during the Archean eon. Huge pieces of crust began to move in different directions, called tectonic plates. This tectonic activity formed the first continent, Rhodinia, about 1.1 billion years ago. The modern-day continents South America, Australia, and Antarctica surrounded North America which was located in the middle. The Earth's crust started to cool down to between 0 degrees to 100 degrees Celsius for micro-organisms to live. However, the Proterozoic was the time when life actually evolved into more complex organisms. These organisms include bacteria life and multi celled organisms such as the Lichen fungi. All of these events that have occurred billions of years ago, are components that form the Earth we live on today.