(Determining the date of a rock layer is not as precise as knowing your own age.) In addition, there is often disagreement as to the extent of certain time periods, since rocks and fossils found on different continents vary. Even radiometric dating does not reveal the true age of a rock or mineral because there is always a certain amount of estimation involved.
Who first developed an absolute geologic time scale using radiometric dating?
In 1911, British geologist Arthur Holmes (1890–1965) began to formulate a geologic time scale based on absolute time, using the uranium-lead dating method to determine the age of rocks. In 1913, he published The Age of Earth, in which he outlined how radioactive decay methods, in conjunction with geological data, could be used to construct an absolute geologic time scale. In 1927, Holmes estimated that the age of Earth’s crust, based on his radiometric techniques, is approximately 3.6 billion years old.
TIME PERIODS
What is the Pre-Cambrian era?
The Pre-Cambrian era represents the time of Earth’s 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 multicelled 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 Earth’s tilt toward the Sun. The planet may have been tilted at a much larger angle—upwards of 55 degrees—than today’s angle of 23.5 degrees. This large degree of tilt meant that the polar areas received most of the Sun’s 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 planet’s 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 world’s 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 world’s 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.
What were trilobites and when did they exist?
Trilobites were some of the most successful creatures to ever live on Earth—and finding one of their fossils is one of the joys of modern fossil collecting. These hard-shelled, segmented animals lived for hundreds of millions of years in the oceans; they are called “three-lobed” because of their head, thorax, and tail. They were one of the first arthropods (animals with jointed legs), comprised over 15,000 different species, and ranged in size from about a half inch (3 millimeters) to 2.3 feet (70 centimeters) long.
Trilobites first appeared in the Early Cambrian (during what is called the “Cambrian Explosion”); they increased in diversity in the Devonian period; and their numbers decreased in the Silurian (probably due to the appearance of sharks and other predators). They became extinct after the Permian mass extinction. Although they did not exist during the time of the dinosaurs, scientists respect this species for its tenacity and abundance.
What theories attempt to explain why the Cambrian Explosion occurred?
In the past, paleontologists have offered many theories as to why the great blossoming of life, known as the Cambrian Explosion, occurred. Some point to the mass extinction of Ediacara organisms (the earliest known complex multi-cellular organisms); another theory states that the development of eyes in animals changed the predator-prey dynamics; and yet another theory suggests that an increase in size of many organisms accelerated diversity.
There is also one study that places the blame, or credit, for this evolutionary explosion on our planet itself—and although not everyone agrees, it is an interesting theory. More than 500 million years ago, shifting masses in the interior mantle of Earth essentially unbalanced the planet, or “tipped” it, causing the entire surface to reorient itself in an effort to become balanced again. In a process called “true polar wander,” the ancestral North America moved from near the South Pole up to the region of the equator; the large continent of Gondwanaland (made up of South America, Antarctica, Australia, India, and Africa) traveled all the way across the Southern Hemisphere. This movement all
