Approximately 543 million years ago, a tremendous change took place on the Earth. This change came in the form of a rapid diversification of life. This change marks the beginning of a period in the history of the Earth known as the Cambrian era, and has come to be called the Cambrian explosion. It was not the beginning of life on the Earth; life had already existed for some time prior in microbial and soft-bodied forms. However, it was the beginning of many of the recognizable life forms seen today. Shells, skeletons, claws, and teeth first appeared during the Cambrian explosion.
It should be noted that the Cambrian explosion, like many geological and biological events, did not occur overnight. Rather, the entire event took 20 million years to unfold. This is an extremely brief period of time when compared to the 4.6 billion-year history of the existence of this planet, but it is plenty of time for evolution to take place. For this reason, the Cambrian explosion does not, as some have suggested, pose a challenge to the theory of evolution. Fossil records from this period clearly demonstrate the evolutionary process underway.
Several theories exist to explain the triggering of this massive event. Some have suggested an astronomical explanation: an earth-grazing comet or an ancient star that went supernova with its contents falling toward the planet. Others voice the more mundane explanation that declining ocean levels due to glaciation forced creatures into smaller and more close-knit habitats where things like increased predation and competition would have forced evolution of more efficient means of protection and food collecting. Recently, however, research of a structure known as the Great Unconformity has suggested another possibility.
The Great Unconformity is a worldwide layer of rock that contains sediments from Cambrian seas. It was first discovered by explorer and geologist John Wesley Powell in 1869 on a trip through the Grand Canyon. The Great Unconformity had long been viewed as an oddity and a gap in the evolutionary record. Recent research into the structure, however, has revealed that the Great Unconformity may provide the answers to the very question that scientists have been asking for over 100 years. Shanan Peters, a geoscience professor with the University of Wiconsin-Madison, who led the research, has proposed the hypothesis that biomineralization evolved as a response to the increased weathering of rocks during the formation of the Great Unconformity. Essentially, this means that the same processes which created the layer also led to biomineralization and triggered the Cambrian explosion.
During the early part of the Cambrian age, shallow seas which covered much of the earth began to advance and retreat across the continent, eroding the surface rock and uncovering the fresh basement material beneath. The newly exposed rock reacted with the ancient environment in a chemical weathering process that released ions such as calcium, iron, potassium and silica into the oceans, altering the seawater chemistry. This change is evidenced by high rates of carbonate mineral formation in the rock and by the extensive presence of glauconite.
The flow of ions in the oceans would likely have posed a challenge to organisms. According to Peters, the body has to maintain a balance of ions to function properly. “If you have too much of one, you have to get rid of it, and one way…is to make a mineral.” Fossil records from the Cambrian age show that the three major biominerals found in all organisms today; calcium carbonate in shells, calcium phosphate in bones and teeth, and silicon dioxide, which is found in the skeletons of radiolarians, appeared about this time and in a diverse array of organisms, making it very likely that biomineralization probably evolved as a response to the change in seawater chemistry that occurred during the early Cambrian.
It appears that there is much that can be learned by studying the gaps in evolution, as can be learned by studying evolution itself. The Great Unconformity has given science new insight as to the actual cause of the explosion that took place during the early Cambrian age. Biomineralization as a response to chemical change is only a hypothesis, but the evidence supporting it is strong.
The question of what caused the chemical change still remains. Was it simply due to the exposure and weathering of new terrestrial material, or did it come from the skies, as suggested by the astronomical theory? One thing is certain; one-half billion years have passed since the Cambrian explosion, and no other event of the same magnitude has occurred.