What Caused the Cambrian Explosion?

Austin Holbrook

Dr. Caddick and Dr. Nesbit

Geology Research Paper

12 November 2017

What Caused the Cambrian Explosion?

The story of Earth’s formation is an epic account of planetesimal collisions and fiery devastation. Where oceans of molten magma and an atmosphere of toxic fumes existed, it was genuinely hell on Earth. How could living organism ever survive these unbearable conditions? Over time this sea of lava cooled forming crust on Earth. It is thought that the first liquid water was brought to Earth through collisions of large meteorites from the asteroid Vesta. These meteorites are thought to have contained enough water to cover most of Earth’s surface.1 Fast-forward about 4 billion years to the Cambrian period and an explosion of life starts to appear in the fossil record. Scientists have long debated on how this expansion of biological diversity started to come about. The hypothesis of how the Cambrian explosion occurred are put into three categories: developmental/genetic, ecologic/ behavioral, and abiotic/environmental, with geochemical hypotheses forming an abundant and distinctive subset of the last.2  Most of these hypotheses claim to be the cause of the explosion, but most likely the actual cause is a combination of all.

        The first life on Earth began to appear in the Archean period, these were bacteria that could survive the highly toxic atmosphere that existed. Earth’s crust began to cool forming clouds, thus producing large amounts of rainwater enlarging the ocean.3 About 3.5 Ga photosynthesis began to occur releasing oxygen into the atmosphere. Earth’s conditions were finally becoming habitable enough to spark a diverse biological community.  

The first hypothesis states that a diversity in genetic change is what caused the Cambrian explosion. Early fossil records show the first appearance of animals in the marine ecosystem with hard shells and exoskeletons. An important component in the evolution of organisms is the bilaterian developmental systems. Bilaterians are animals with a longitudinal plane of symmetry and specialized internal organ system, and include most living animals.3 The bilaterian gut and the ability to feed on large prey is considered the change that enabled the evolution of large body sizes and skeletons in response to marine predation. These adaptations would help organisms survive harsher conditions and predation. The theory of natural selection in which certain organisms who acquire favorable adaptions will reproduce, helps support this hypothesis.3 

        One of major feature of the Cambrian explosion, is that it erases any notions of increasing evolutionary complexity. Creationists in the past have focused on such things as the complex fully formed trilobite eyes in the Cambrian.  Erwin and Valentine essentially concur with these premises—if anything stating them even more strongly—as they write,

“The most remarkable pattern to emerge from any analysis of early Cambrian metazoan diversification is the extraordinary breadth of morphologic innovation. It is evident at many different scales, from the obvious generation of morphologically distinctive groups to diversity in anatomical details. For instance, one might expect that complexity and sophistication of eyes improved through the Phanerozoic, but the recent discovery of exquisitely preserved eyes from arthropods in the early Cambrian Emu Bay Shale in Australia illustrates that highly advanced, compound eyes with more than 3,000 ommatidial lenses had evolved very early in the history of the clade. Surprisingly, many of the recovered eyes preserve a ‘bright zone’ within the ommatidia that has higher light sensitivity and, perhaps, acuity.”4

The second hypothesis suggests large ecological and behavioral changes are what brought about the Cambrian explosion. Timothy Lyons, a geobiologist at the University of California at Riverside, believes that the increase in oxygen crossed an ecological threshold, resulting in an ecological arms race.5 The increase in oxygen levels allowed animals to grow in size and increased mobility. Animals now depend on oxygen to survive instead of using oxidizing agents to break down food like carbon dioxide and sulfur. The process of metabolizing food in the presence of oxygen releases much more energy than most anaerobic pathways. Animals rely on this potent, controlled combustion to drive such energy-hungry innovations as muscles, nervous systems. With increased oxygen levels, predators emerged setting off a chain reaction for other organisms to evolve or be killed off. Adaptations also emerged for defense and predation such as mineralized shells, exoskeletons and teeth.5 

With the introduction of macrophagy, the feeding on large particles or prey, intricate food webs emerged. Changes in the food web caused animals to start adapting rapidly to their environment. These adaptations introduced free swimming animals, burrowing, and biomineralization. The adaptations were new evolutionary products that rapidly became adopted in diversification. Trace fossils from the early Cambrian show that animals started to burrow in the sediments to gain easy access to previously untapped nutrients. This would also act as a refuge from predators.6 

The third and final hypothesis suggests that the Cambrian explosion was caused by abiotic and environmental changes. Photosynthesizers reduced the amount of carbon dioxide, which is a greenhouse gas, that caused the planet to cool. The harshest ice age in Earth’s history occurred between 800 to 600 Ma. After the glacial period, temperatures rose with oxygen levels to form what could have been the ignition for the Cambrian explosion. The rise in oxygen levels caused by photosynthesis allowed larger animals to roam the ocean. As described in the second hypothesis, metabolizing food into energy is much more effective and led to predation. This chemical process gave organisms enough energy and a reason to start swimming, either for catching prey or escaping from predators. The largest predator was Anomalocaris, a free-swimming animal that moved through the water by flexing its body.7 

During the late Precambrian, a supercontinent was breaking up creating favorable habitats such as continental shelf edges and shallow seas.8 With the return of a warmer global climate, the first large and complex multicellular organisms appeared suggesting environmental conditions had become just right for them to evolve.9 With sea level rising caused by glacial melting, Neoproterozoic erosion had resulted in weathering of crystalline basement rock giving the ocean an abundant amount of calcium, phosphate, and other ions.9 This made biomineralization more widespread to both predators and prey in the Cambrian.9 Also, a large amount of nutrients that eroded from the continents and dead organic matter was trapped under sediment causing organisms to start burrowing. The environment in the Cambrian was finally becoming able to sustain large amounts of various life. Increase in oxygen, abundant nutrients, and a verity of habitats helped this period become an explosion in life like the world has never seen before.