EARTH VIEWER ANCIENT EARTH HADEAN EON – INFORMAL ERA Earth forms (4567 Mya) Earth formed from interstellar dust and debris in a process called accretion. During accretion, the gravitational compression of the emerging mass elevated the temperature, and with additional heat supplied by the decay of radioactive elements, the proto-Earth became a hot molten mass that stratified according to density. As a result, Earth has a core rich in iron and nickel and a crust made up of silicon, magnesium, and iron. Moon forms (~4500 Mya) Scientists hypothesize that about 4.5 billion years ago, a planetesimal the size of Mars collided with Earth and ejected the material that became the moon. The moon's chemical composition and lack of an iron core are consistent with this hypothesis. Oldest minerals show evidence of liquid water on Earth (~4400 Mya) Volcanic eruptions on early Earth contained lots of water vapor. Only when the planet cooled to moderate temperatures could liquid water exist on the surface. The oldest known minerals are 4.4-billion-year-old zircon crystals from Australia. Able to withstand erosion, extreme temperatures, and pressure changes, zircon crystals provide a durable record of geological history. These crystals have the chemical signature of water–an elevated ratio of oxygen-18 to oxygen-16–leading to the conclusion that water was present on Earth at a much earlier age than previously believed. ARCHEAN EON – EOARCHEAN ERA Oldest rock (4000 Mya) Dated to 4 billion years ago, the Acasta Gneiss is the oldest known exposed rock on Earth. It is located in the Slave craton formation in the Northwest Territories of Canada. A rock is an intact aggregate of one or more minerals. Rocks can be destroyed through chemical and physical processes and broken down into their constituent minerals. Therefore, some mineral grains have been found that predate the Acasta Gneiss. Oldest evidence of life: Isotopically "light" organic carbon (3850 Mya) The earliest evidence of life consists of rock samples in which organic carbon compounds show an isotope signature low in carbon-13. Both carbon-12 and carbon-13 are stable isotopes of carbon, but biological processes preferentially incorporate carbon-12, and carbon that has been processed by biology becomes depleted of carbon13. Slightly younger (3400–3500 Mya) rocks contain several lines of evidence indicating the presence of biological carbon and sulfur cycles by this time. ARCHEAN EON – PALEOARCHEAN ERA Strelley Pool Formation (3400 Mya) Stromatolites in this geological deposit in Western Australia provide the oldest visible evidence for life on Earth. When these structures formed, the atmosphere was devoid of oxygen. The early organisms that built these stromatolites probably obtained energy from reduced sulfur, iron compounds, and light. ARCHEAN EON – MESOARCHEAN ERA Earliest evidence for plate tectonic movements (3000 Mya) Plate tectonics is the unifying theory of earth science and explains Earth's major geologic features: continents, oceans, mountains and valleys, volcanoes and earthquakes. However, the crust on early Earth may have been too hot, too buoyant, or too pliable to support plate tectonics. Mineralogists studying diamonds have found the earliest evidence for subduction of ocean crust dating back 3 billion years, thus indicating a minimum age for the onset of plate tectonics PROTEROZOIC EON – PALEO-PROTEROZOIC ERA Great oxygenation event (2400 Mya) Prior to 2.4 billion years ago, Earth's atmosphere was devoid of oxygen. Produced by ancestors of modern cyanobacteria, oxygen was consumed as quickly as it was produced. About 2.4 billion years ago, oxygen began to persist in the atmosphere, reaching 1% to 4% of current levels, and has been present ever since. Snowball Earth (2200–2000 Mya) Several times throughout Earth’s history, the globe has been covered in ice or nearly so, much like an enormous snowball. Snowball events could begin by runaway ice-albedo positive feedbacks and end by the greenhouse effect from accumulation of carbon dioxide from volcanoes. However, the exact mechanisms continue to be debated by scientists. Vredefort Age of Impact Structure: 2.03 billion years Diameter of Crater: 160 km Modern Coordinates: S 27.00°, E 27.50° Grypania spiralis (1880 Mya) Several sites around modern-day Lake Superior have yielded important fossils from the Precambrian Era. One of them, a single-celled organism called Grypania spiralis, is the oldest known fossil of a eukaryote. Sudbury Age of Impact Structure: 1.85 billion years Diameter of Crater: 130 km Modern Coordinates: N 46.60°, W 81.18° Oldest eukaryotic fossil (1800 Mya) The oldest eukaryote yet found in the fossil record is Grypania spiralis, a coiled fossil two millimeters wide and more than 10 centimeters long that dates from 1.8 billion years ago. The cell walls of single-celled eukaryotes are also preserved in rocks of this age. Early supercontinent Nuna (~1850–1500 Mya) With a landmass of roughly 50 million square kilometers, Nuna (sometimes known as Columbia) is counted as the earliest supercontinent, forming about 1.8 billion years ago by the amalgamation of smaller continents. PROTEROZOIC EON – MESO-PROTEROZOIC ERA Origin of chloroplast by symbiosis (1600 Mya) Chloroplasts evolved from cyanobacteria that were engulfed by an aerobic eukaryotic cell. Bangiomorpha pubescens (1200 Mya) Bangiomorpha an ancient genus of red algae. It is the earliest known complex, multicellular, sexually reproducing organism in the fossil record. While some older fossils consist of multiple cells, Bangiomorpha is composed of different types of clearly specialized cells. Such specialization was an important step in the evolution and diversification of complex organisms. PROTEROZOIC EON – NEO-PROTEROZOIC ERA Supercontinent Rodinia (1200-800 Mya) The supercontinent Rodinia formed about 1 billion years ago and began to break apart 800 million to 750 million years ago. This breakup may have facilitated major environmental changes, which in turn provided new opportunities for life. Spitsbergen (800 Mya) Fossils found in 800-million-year-old rocks from Spitsbergen Island, above the Arctic Circle, exhibit the same morphology and life cycle as those of cyanobacteria that today live on tidal flats in the Bahamas. Being able to compare fossils to modern organisms allows scientists to reconstruct what the ancient environment might have been like. Snowball Earth glaciations (716–635 Mya) After a billion and a half years during which there is no evidence for extensive ice sheets, two glaciation events again spread ice around the globe. Doushantuo Formation (600 Mya) The Doushantuo Formation is a phosphorite deposit in China that contains diverse fossils, many of them preserving exquisite cellular detail. They provide a rare glimpse of Earth’s biodiversity just before the evolution of larger macroscopic animals. Oxygen increases (635–560 Mya) During the Ediacaran Period, just before the beginning of the Phanerozoic Eon, oxygen levels increased again, paving the way for large animals in the oceans. Beaverhead Age of Impact Structure: 600 million years Diameter of Crater: 60 km Modern Coordinates: N 44.25°, W 114.00° Acraman Age of Impact Structure: 590 million years Diameter of Crater: 90 km Modern Coordinates: S 32.02°, E 135.45° Ediacaran diversification of large animals (580 Mya) The Ediacaran Period saw the first diversification of large, complex multicellular organisms in Earth history. Many of the organisms are not similar to existing groups of animals, and most became extinct by the end of the period. Cambrian explosion (540 Mya) During the Cambrian Period, there was an unprecedented diversification of life across the planet. The Cambrian explosion followed the rise of oxygen to near-present levels and eventually gave rise to the diverse animal lineages on land and in the sea.
