TIME PERIODS
What is the Pre-Cambrian era?
The Pre-Cambrian era represents the time of Earths beginning to just before the big explosion of life in the oceans from about 4.54 billion to about 543 million years ago. During this time, Earth was cooling, developing its oceans, and building the continental crust; in addition, scientists believe that life began during the early part of the Pre-Cambrian. The following lists one interpretation of three Pre-Cambrian divisions, the approximate dates, and major evolutionary events during these times:
Hadean 4.5 to 3.8 billion years ago, the time when Earth was forming in the early solar system.
Archaean 3.8 billion years to 2.5 billion years ago, the oldest bacteria evolved.
Proterozoic Era 2.5 billion to 543 million years ago, a time in which multi- celled eukaryotic (a cell with a definitive nucleus) evolved in other words, animals.
Why do scientists believe that several ice ages occurred during the late Pre-Cambrian era?
Chemical and isotopic analysis of rocks found in Africa show that Earth may have gone through at least four ice ages between 750 and 570 million years ago. These were very deep ice ages, essentially turning Earth into a snowball planet. From the evidence to date, some scientists think the oceans were covered with ice almost 300 feet (91 meters) deep, and the land was completely dry and barren of life.
Some scientists believe the Pre-Cambrian ice ages may have been caused by Earths tilt toward the Sun. The planet may have been tilted at a much larger angle upwards of 55 degrees than todays angle of 23.5 degrees. This large degree of tilt meant that the polar areas received most of the Suns warmth, keeping them ice-free. But the areas around the equator would have been colder, allowing glaciers to form. If this was true, the buildup and melting of the glaciers around the equator during the Pre-Cambrian era may have created enough force to move the planets axis to its modern position. Some scientists have equated this process to repeatedly pushing on a swing at just the right moment in its movement, adding energy to make it go higher. The influence of the alternating advance and retreat of the glaciers could have caused the axis to straighten to its present angle.
Some scientists believe the heroes that thawed the snowball planet and paved the way for an explosion of life were none other than volcanoes. As these surface blisters erupted toward the end of the Pre-Cambrian era, they sent massive amounts of carbon dioxide into the atmosphere, an increase of approximately 350 times its present concentration. This increase trapped re-radiating solar energy, warming the planet as it created a super greenhouse effect. The temperatures rose enough to melt the ice-covered oceans and end the ice age.
Why was the Cambrian Explosion, also called the evolutionary big bang?
Just after the end of the Pre-Cambrian era, about 543 million years ago (during the Cambrian period), a great burst of evolutionary activity began in the worlds oceans.
Based on the fossil record of the Cambrian period, scientists estimate that the number of orders of animals doubled roughly every 12 million years. At this time, too, most of the modern phyla of animals began to appear in the fossil record.
For some reason, new animals appeared at breakneck speed, geologically speaking, filling the oceans with life. No one really knows why the animals started to appear, and scientists have suggested theories ranging from a change in climate to an overall natural threshold reached. For example, some scientists believe temperature or oxygen levels reached a point that allowed the proliferation of organisms.
Trilobites were one of the most successful creatures to ever inhabit our planet, with some 15,000 species that survived from the Early Cambrian through the Permian periods (iStock).
By looking at the genes preserved in and common to modern animals, researchers are trying to determine a possible cause. One study found that an ancient common ancestor a worm-like animal from which most of the worlds animals were derived had special genetic machinery that was so successful that it survives to this day. These genes, used to grow appendages (arms, legs, claws, fins, and antennas), were operational at least 600 million years ago. With appendages, animals swam faster, grabbed tighter, and fought with greater efficiency, and, thus, they could eventually dominate the globe.
