Dawn of Animal Life
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|Formation of Earth|
This exhibit is recognized by the Exploratorium under their "10 Cool Sites" pick in November 1988
The Orion Nebula
Orion Nebula, 1500 light-years from Earth. Research by Dr. C. Robert O'Dell, Rice University
The Earth is incredibly old. The image to the left (from the Hubble Space Telescope, Space Telescope Scientific Institute) shows us how our planet probably formed. Clouds of gas and dust called nebulae are scattered throughout the universe. This image shows the nebula in the constellation Orion. Within this nebula, new suns are forming as the dust cloud slowly condenses. The inset image shows a close-up of one young star surrounded by a pancake-shaped cloud of dust. These proplyds as they are called will theoretically condense even further to form planetary systems surrounding the star.
Studies of meteorites tell geologists and astrophysicists about the age of our solar system. Some scientists feel that meteorites once formed a small planet in our solar system that fragmented in space, while others view them simply as "leftover" material from the original nebula. Whatever the origin, they are materials from the beginnings of our solar system that have changed relatively little since their original formation as rocks.
Geologists use radioactive age dating methods to determine the ages of rocks. Certain atoms in the rock spontaneously change into different atoms over time through radioactive decay. By taking a rock (meteorite) apart atom by atom and finding out how many atoms have decayed, and knowing the rate of change (or half-life of the atom) scientists can work out how many years have passed since the rock formed. This is a tremendous simplification, but basically it is somewhat like counting rings in a tree to see how old it is. Many meteorites age-date at about 4.6 billion years. This is the currently accepted age of the Earth.
To give you an idea of just how big this number is, let's pretend that you wanted to count to 4.6 billion. If you started right now, and counted at 3 numbers per second without ever stopping to sleep and without ever slowing down, you could count to 1 million in just under 4 days. A billion however is 1000 million, so it would take you over 10 and a half years to count to one billion! 4.6 billion would require 50 years of non-stop counting. Now, imagine that every number that you counted is a whole year long!!! Perhaps this gives you a better idea of just how long the Earth has been around, and how much time and change the Earth has been through...
Approximately 3.5 billion years ago Archean Beginnings ... evidence of the first life.
The early Earth was quite different from today with constant bombardment by meteorites, intense volcanism, and a poisonous atmosphere. However, fossils known as stromatolites in rocks 3 to 3.5 billion years old show geologists that life evolved very early in the Earth's history. During the time known as the Archean (~4 billion to 2.5 billion years ago) stromatolite fossils are very rare. Canada has three of the world's known examples at Red Lake, Ontario, Steep Rock Lake, Ontario, and near Yellowknife, Northwest Territories.
Stromatolites are layered mounds, columns, and sheets found in the rock. They were originally formed by the growth of layer upon layer of cyanobacteria, a single-celled photosynthesizing microbe growing on a sea floor. Cyanobacteria are prokaryotic cells (the simplest form of modern carbon-based life) which lack a DNA-packaging nucleus. This simple organism would be the only life on Earth for the next 2 billion years.
Modern Stromatolites, Shark Bay, Australia. Photo courtesy Marjory Martin, Deakin Univ, Australia
Very rarely, stromatolites are still forming today in places such as Shark Bay, Australia. The salinity of the water is very high in this bay, and basically the only life that can survive is cyanobacteria. Under normal ocean conditions, the cyanobacteria would be eaten by marine creatures such as snails. In this special case however, columns and mounds are forming as can be seen in the picture to the left.
Although simple, cyanobacteria was ultimately responsible for one of the most important "global changes" that the Earth has undergone. Being photosynthetic, cyanobacteria produce oxygen as a by-product. Photosynthesis is the only major source of free oxygen gas in the atmosphere. As stromatolites became more common 2.5 billion years ago, they gradually changed the Earth's atmosphere from a carbon dioxide-rich mixture to the present-day oxygen-rich atmosphere. This major change paved the way for the next evolutionary step, the appearance of life based on the eukaryotic cell.