History of Life on Earth Chapter 6

Evidence of the Past How do fossils form? The heat and pressure from being buried in sediment can sometimes cause the tissues of organisms to release hydrogen and oxygen, leaving behind a residue of carbon. This process — which is called carbonization, or distillation — yields a detailed carbon impression of the dead organism in sedimentary rock.carbon The most common method of fossilization is called permineralization, or petrification. After an organism's soft tissues decay in sediment, the hard parts — particularly the bones — are left behind. Water seeps into the remains, and minerals dissolved in the water seep into the spaces within the remains, where they form crystals. These crystallized minerals cause the remains to harden along with the encasing sedimentary rock.sedimentary rock In another fossilization process, called replacement, the minerals in groundwater replace the minerals that make up the bodily remains after the water completely dissolves the original hard parts of the organism. Fossils also form from molds and casts. If an organism completely dissolves in sedimentary rock, it can leave an impression of its exterior in the rock, called an external mold. If that mold gets filled with other minerals, it becomes a cast. An internal mold forms when sediments or minerals fill the internal cavity of an organism, such as a shell or skull, and the remains dissolve.

Evidence of the past How can we estimate the age of a fossil? dating-rocks-and-fossils-using-geologic-methods Radiometric Dating (Absolute Dating) Geologists use radiometric dating to estimate how long ago rocks formed, and to infer the ages of fossils contained within those rocks. Radioactive elements decay. The universe is full of naturally occurring radioactive elements. Radioactive atoms are inherently unstable; over time, radioactive “parent atoms” decay into stable “daughter atoms.” When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. Afterwards, they decay at a predictable rate. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed. Bracketing the fossils Fossils are generally found in sedimentary rock—not igneous rock. Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years. So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil. Scientists date igneous rock using elements that are slow to decay, such as uranium and potassium. By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be; this is known as “bracketing” the age of the sedimentary layer in which the fossils occur.

geological time scale

Geologic Time Scale The standard method used to divide the Earth’s long natural history into parts. In our text, time is divided into eras and eras are divided into subgroups called periods. Mass extinctions, periods when many species suddenly become extinct, have occurred several times throughout Earth’s history. Scientists do not always know exactly why or how these happened. The disappearance of dinosaurs is an example of this. There was a supercontinent on Earth called Pangea that moved due to plate tectonics. Plate tectonics is the theory that explains how the Earth’s outermost layer, called tectonic plates, move and change shape. This is a very gradual process.

precambrian era MYA- Precambrian time covers the vast bulk of the Earth's history, starting with the planet's creation about 4.5 billion years ago and ending with the emergence of complex, multicelled life- forms almost four billion years later. Early Earth’s atmosphere did not contain much oxygen. Therefore, the earliest lifeforms did not need oxygen... these were single-celled prokaryotes. There is evidence that cyanobacteria, a type of photosynthetic bacteria, formed about 3 billion years ago. These helped form a new layer of gas in the atmosphere called the ozone and put oxygen into the atmosphere. The ozone allowed life to move out of the Earth’s ocean and onto land. About a billion years after bacteria formed, multicellular organisms formed.

paleozoic era MYA- Paleozoic mean “ancient life” in greek. Period of time where life emerged on the land. Plants, fungi, and air-breathing animals colonized land (only plants that did not appear during this era were flowering plants). Included ages of invertebrates, fishes, and amphibians. Three mass extinctions present during this era. There were six periods during this era.

mesozoic era MYA- Pangea cracks (splits into Laurasia & Gondwanna), Dinosaurs appear, first mammals appear, first birds appear. End of this period marked with mass extinction (extinction of dinosaurs). Flowering plants appear and Earth starts to look closer to present-day Earth. Made up of three periods

cenozoic era 65.5 MYA to present- First large mammals appear and become dominant, known as “age of the mammals.” Means “recent life” in Greek. One mass extinction during this era. Climate has changed many times during this period, two ice ages.

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