Urozean: Oxygen "Ping-Pong" as Evolution Drive?

Multiple anoxic phases disrupted the oxygenation in the Cambrian seas

Life in the Cambrian Sea © Ghedoghedo / CC-by-sa 3.0
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The transition from the oxygen-poor primal seas to oxygen-rich oceans was far from uniform, but more like a ping-pong game: in many cases, the gradual increase some 500 million years ago was interrupted by anoxic phases that were deadly to many lifeforms. This is proven by a study published in "Nature". It was probably these fluctuations in oxygen that drove the explosive speciation in the Cambrian.

Oceans and atmosphere of our planet were the first scarcely four billion years of the earth's history relatively oxygen poor. This changed according to popular doctrine 600 million years ago, in the late Ediacarium and allowed only the rise and development of multicellular organisms. But now a team of scientists from various American universities has found that this transition in the ocean was anything but smooth. Instead, conditions fluctuated several times between the two extremes, with periods of millions of years of anoxic periods recurring.

For their study, the researchers studied the isotopic ratios of carbon, sulfur and molybdenum in limestone and shale that had been deposited during the Cambrian. The combination of this isotope data allows conclusions about how much oxygen has been present in the seawater of different time periods.

Ping-pong game of oxygen levels

The data revealed that after a first phase of oxygenation, the oceans of Earth experienced at least one, but probably more, relapses into oxygen-depleted conditions. "Our research shows that the ocean swayed back and forth between the different oxygen states 499 million years ago, " explains Timothy Lyons, professor of biogeochemistry at the University of California at Riverside. Very soon after the development of the first multicellular animals in the sea, conditions dropped and the sea remained anoxic for two to four million years. Such phases of oxygen depletion could have been repeated several times in the following years, the researchers suspect.

Anoxic phases as a driving force of evolution?

According to the scientists, these fluctuations could even have been the driving force behind the explosive speciation in the Cambrian, which already provided the basis for the further development of the large groups of organisms. Such fluctuations have played an important, perhaps even dominant, role in the early evolution of life on this planet. Because they triggered extinction events, which paved the way for new organisms

Researcher Benjamin Gill on Cambrian rock strata Steve Bates

"Life and the environment in which it lives are closely connected, " explains Benjamin Gill, lead author of the Harvard University study. When oxygen availability changes rapidly in the ocean, as was the case then, some organisms can not adapt quickly enough. At the same time, the altered oxygen balance also endangers other material cycles of biologically important elements such as iron, phosphorus and nitrogen. "The disruption of these circuits is another way to trigger biological crises, " says Gill. Therefore, switching to oxygen-depleted conditions in the ocean can cause major extinction events.

Scientists had previously hypothesized that the Cambrian species explosion could have been promoted by rapidly changing environmental conditions. The extinction of species releases ecological niches and creates new possibilities for the development of the surviving species.

Cause of fluctuations still unknown

However, why the oxygen levels passed through such a ping-pong game some 499 million years ago is still unclear. What we have found so far is proof that it happened. We have the effect, not the cause, "explains Gill. The anoxic status was maintained for two to four million years, probably until the increased fall of organic matter in a kind of negative feedback to a gradual oxygenation in the atmosphere and sea f hardness.

Findings also for today's "death zones"

According to the researchers, the new findings also help to better understand today's developments in the sea. Einige Today, some areas of the oceans have also become depleted of oxygen Chesapeake Bay and the so-called death zone in the Gulf of Mexico are just two examples, according to Gill. We know that Earth has undergone similar scenarios in the past. Understanding the causes and consequences of events at the time can therefore give us crucial clues as to what the future of our oceans might look like

(University of California - Riverside, 06.01.2011 - NPO)