An Early Cambrian Success Story
Published in The EarthSphere Blog. Feature Image: Jellyfish — 540 Million Years of Success: by WM House (ArcheanArt)
Prologue
The Forgotten Origins series has taken us into the Cambrian Period, where an explosion of life occurred in a frenzy of hyper evolution during the Early Cambrian. But why then?
Why the Early Cambrian?
The fossil record is clear about a period of hyper evolution in the early Cambrian. What is less clear is the cause of this anomalous event. In a universe of cause and effect, it’s reasonable to suspect the Cambrian explosion relates to significant changes in environmental conditions. It is also likely that the early Cambrian evolutionary frenzy was stoked in part by an increase in the availability of critical resources. So, the two prime suspects for change are oxygen and food/nutrients.
The Cambrian commenced approximately 2 billion years after the Great Oxidation Event. We also know the Ediacaran biota required oxygen, so an oxygenated atmosphere was present long before the Cambrian began. But the degree of oxygenation at any particular time is difficult to determine.
The simplest models propose two major steps associated with rising atmospheric oxygen levels. The first was the Great Oxidation Event, where oxygen levels rose from negligible to measurable. The second step was a Neoproterozoic phase, when oxygen levels rose enough to support Ediacaran animal life. But this two-step model doesn’t explain why the explosion of life didn’t occur earlier in the Ediacaran.
A second possible factor in this story is nutrient supplies. The creatures roaming early Cambrian oceans needed food, nutrients, and trace chemical compounds to maintain their bodily functions. Successful species also needed sufficient quantities of these items to fuel population growth.
Oxygen levels factor heavily in the survival of individual organisms, and food and nutrients are often critical for maintaining working ecosystems and large species populations.
Cambrian Oxygen
We can’t look at the geological record for fossilized oxygen, so studies of ancient environments use the presence or absence of oxygen-dependent compounds to decipher the mystery of oxygen levels. This practice requires rock samples spanning a relatively continuous period of time, so the data provide a chronological sequence of measurements.
One such set of fossil-rich limestone strata in Siberia provides a continuous record of the early Cambrian. Researchersanalyzed an extensive set of samples from these rocks, examining carbon and sulfur isotopes. Changes in isotope levels indicated oxygen variations in the ancient Cambrian atmosphere.
They didn’t detect a steady increase or decrease in oxygen levels. Instead, they uncovered a history of wild variations, creating a record of boom and bust oxygen fluctuations. The estimated average oxygen level during the early Cambrian atmosphere was about 40 percent of the modern level (21%). But the data from Siberia indicate oxygen levels fluctuated by about 50 percent, meaning atmospheric oxygen varied between 4 and 12 percent during the early Cambrian.
The work identified five oxygen spikes, with each peak corresponding to an increase in biodiversity. The oxygen lows correlated to high species extinction rates. These data indicate a coupling between higher oxygen levels and spurts of hyper evolution (adaptive radiation).
Looking deeper into the next level of cause and effect, we don’t know exactly why more oxygen caused biodiversity to spike. One line of thought is that extra oxygen expanded the geographical range of ecosystems, giving life more opportunities to evolve. Another explanation correlates higher oxygen with increasingly complex predator development, creating an evolutionary arms race where the evolve or perish axiom necessitated rapid evolutionary change.
Nutrients
Another approach to understanding the Cambrian explosion focuses on food webs. The breakup of the Rodinia supercontinent and the formation of Gondwana and Laurentia probably created more coastline and shallow seas, while the tectonic collisions forming these supercontinents caused uplift and mountain building. This geomorphic arrangement, along with a warmer climate and more weathering, caused massive continental erosion, and the eroded sediments into the flooded into the shallow Cambrian seas.
This influx of nutrients stimulated plankton growth at the base of the food chain, creating a biological chain reaction as massive amounts of organic carbon entered the food web. The extra food provided energy for the scavengers and predators to flourish.
Combine the oxygen story with a greatly expanded food web, and we see how mother nature rolled the dice in the early Cambrian and came up with a winner. Expanding ecosystems rich with food provided a lucrative environment for life to thrive and evolve.
An added bonus was that each dip in oxygen created an extinction event, and we know mass extinctions are often followed by the rise of new species and lifeforms. The early Cambrian had five oxygen-related extinctions, or conversely, five opportunities for evolution to reset and come up with something new.
The Cambrian explosion was a unique turning point in life’s 4.5 billion year journey. We still have more questions than answers about what happened during this brief 15 million year period at the beginning of the Cambrian. We also don’t fully understand why the original nine major phyla have remained, and evolution didn’t replace them with something even better. Maybe sticking with what works is a fundamental principle of life.
Sources:
Global atmospheric oxygen variations recorded by Th/U systematics of igneous rocks (By He Liu, Robert E. Zartman, Trevor R. Ireland, and Wei-dong Sun; PNAS)
The Cambrian explosion (by Derek Briggs; Science Direct)
Rapid Oxygen Changes Fueled an Explosion in Ancient Animal Diversity (by Jonathan Lambert; Quanta Magazine)
