Precambrian time represents the longest geologic time unit of Earth’s history. This time lasted from 4.6 billion to about 544 million years ago. Although the Precambrian was the longest unit of geologic time, relatively little is known about Earth and the organisms that lived during this time. Why is the fossil record from Precambrian time so sparse?
Precambrian rocks have been deeply buried and changed by heat and pressure. They have also been eroded more than younger rocks. These changes affect not only the rocks, but the fossil record as well. Most fossils can’t withstand the metamorphic and erosional processes that most Precambrian rocks have experienced.
Scientists look for evidence of what Earth looked like during Precambrian time and what life-forms were present. Lightning or the sun may have provided the energy necessary to build amino acids out of the simple compounds in Earth’s Precambrian atmosphere. Amino acids are the “building blocks of life.” These amino acids reacted with each other and combined to form the compounds from which life may have evolved.
It wasn’t until fossilized cyanobacteria forming layered mats, called stromatolites, were found that scientists could begin to unravel Earth’s complex history. Cyanobacteria are bacteria thought to be one of the earliest forms of life on Earth. Cyanobacteria first appeared on Earth about 3.5 billion years ago. As these organisms evolved, they contributed to changes in Earth’s atmosphere. During the few billion years following the appearance of cyanobacteria, oxygen became a major gas in Earth’s atmosphere. The ozone layer in the stratosphere also began to develop, shielding Earth from ultraviolet rays. These major changes in the air allowed species of single-celled organisms to evolve into more complex organisms.
Animals without backbones, called invertebrates, developed near the end of Precambrian time. Imprints of jelly fish and marine worms have been found in late Precambrian rocks. However, because these early invertebrates were soft-bodied, they weren’t easily preserved as fossils. This is another reason the Precambrian fossil record is so sparse.
1. How long did the Precambrian Era last in millions of years?
2. Why is the fossil record from Precambrian time so sparse?
3. What are amino acids?
4. What might have provided the energy necessary to build amino acids out of the simple compounds in Earth’s Precambrian atmosphere?
5. What is Cyanobacteria? How long ago did this bacteria first appear on Earth?
6. After the appearance of cyanobacteria, what became a major gas in Earth’s atmosphere?
7. What developed in the Earth’s stratosphere and what did it help to do?
8. At the end of this first era, animals without backbones, or invertebrates, developed. Why don’t we have many records of them?