Plants kick-start evolutionary drama of Earth's oxygenation

Oct 08, 2010 by Jenny Green
A panser shark (predatory fish greater than 30 feet long) is a consequence of the Earth's oxygenation event of 400 million years ago. Credit: Staffan Waerndt/ Swedish Museum of Natural History

An international team of scientists, exploiting pioneering techniques at Arizona State University, has taken a significant step toward unlocking the secrets of oxygenation of the Earth's oceans and atmosphere.

Evolution of the Earth's multitude of organisms is intimately linked to the rise of oxygen in the oceans and atmosphere. The new research indicates that the appearance of large predatory fish as well as vascular plants approximately 400 million years ago coincided with an increase in oxygen, to levels comparable to those we experience today. If so, then animals from before that time appeared and evolved under markedly lower oxygen conditions than previously thought.

The researchers, including collaborators from Harvard, Denmark, Sweden and the United Kingdom, made use of a method developed at ASU by Ariel Anbar, a professor in the department of chemistry and biochemistry and the School of Earth and Space Exploration in the College of Liberal Arts and Sciences, and his research group. The method can be used to estimate global in ancient oceans from the of ancient seafloor sediments.

Their important findings are presented in a paper published in this week's online Early Edition of the (PNAS), titled "Devonian rise in atmospheric oxygen correlated to radiations of terrestrial plants and large predatory fish."

"There has been a lot of speculation over the years about whether or not oxygen in the atmosphere was steady or variable over the last 500 million years," explained Anbar, who leads ASU's Astrobiology Program. "This is the era during which animals and land plants emerged and flourished. So it's a profound question in understanding the history of life. These new findings not only suggest that oxygen levels varied, but also that the variation had direct consequences for the evolution of complex life."

The Earth is 4,500 million years old. has probably thrived in the oceans for most of that time. However, until about 2,300 million years ago, the atmosphere contained only traces of oxygen. During that time, some microbes in the oceans likely produced oxygen as a byproduct of photosynthesis. But the quantities they produced were insufficient to accumulate much in the atmosphere and oceans. The situation changed with the "Great Oxidation Event", 2,300 million years ago. Oxygen levels rose again around 550 million years ago. The first animals appear in the at this time, marking the beginning of an era that geologists call the "Phanerozoic" – a Greek word meaning "evident animals". This new work explores how oxygen levels changed during the Phanerozoic.

The new study was led by Tais W. Dahl while he was a postdoctoral scholar at Harvard. Dahl spent several months in Anbar's lab at ASU during his graduate research learning how to make the necessary measurements from Gwyneth Gordon, Ph.D., who is also an author of this paper. Other authors include geochemist Don Canfield, Dahl's Ph.D. mentor at the University of Southern Denmark, and paleontologist Andrew Knoll, Dahl's postdoctoral mentor at Harvard.

Dahl returned to ASU to perform the measurements for this study, which involved measuring the relative amounts of different isotopes of the element molybdenum in rocks called "black shales". These rocks are formed from ancient ocean sediments.

Isotopes are atoms of an element, in this case molybdenum, that differ only in their mass and therefore can be easily distinguished from one another. Molybdenum has seven stable isotopes. Chemical reactions fractionate heavy from light isotopes. For example, carbon 12 is enriched by three percent in plants relative to the carbon in carbon dioxide in the atmosphere. Similarly, molybdenum isotopes are fractionated during their removal from seawater into ocean sediments. The magnitude of this fractionation is sensitive to the presence of oxygen.

The data Dahl obtained at ASU reveal that there were at least two stages of oxygenation during the Phanerozoic, separated by the oxygenation event 400 million years ago. This inference from molybdenum isotopes is corroborated by the appearance of large (up to 30 feet long) predatory fish in the fossil record 400 million years ago, coincident with the rise in oxygen. Animals of that size consume energy rapidly, requiring high levels of oxygen for their metabolism. "Tais's data indicate that early animals evolved in an environment with less oxygen than today," said Anbar. The newly discovered oxygenation event therefore explains the puzzling appearance of these fish in the fossil record. "It's always satisfying when we can demonstrate how an environmental change drove biological evolution," Anbar explained.

"But the real kicker is that these data also show us the reverse - that biological innovation can drive environmental change" continued Anbar. He points to the fact that vascular plants also appear in the fossil record around 400 million years ago. The bodies of such plants decompose with difficulty, making it easier for organic carbon to be buried in sediments. When that happens, the organic carbon – produced by photosynthesis – is not available for reaction with oxygen. The consequence is a rise in the amount of oxygen in the environment.

"It's a push-me-pull-you situation," explained Anbar. The biological innovation of vascular plants led to more carbon burial, and therefore to more oxygen. Then, the rise in made it possible for larger animals to evolve. "This is a great example of what we call the "co evolution" of life and the environment", enthused Anbar "Geoscientists talk about this idea a lot, but we rarely find such nice examples."

Explore further: Images released of shipwreck in San Francisco Bay

Related Stories

How plants drove animals to the land

Sep 30, 2010

(PhysOrg.com) -- A new study of ancient oxygen levels presents the first concrete evidence that after aquatic plants evolved and boosted the levels of oxygen aquatic life exploded, leading to fierce competition ...

Climate change and the rise of atmospheric oxygen

Mar 23, 2006

Today's climate change pales in comparison with what happened as Earth gave birth to its oxygen-containing atmosphere billions of years ago. By analyzing clues contained in rocks, scientists at the Carnegie Institution's ...

Toxic to aliens -- but key to health of planet

Jun 18, 2008

Scientists at the University of Leicester are using an ingredient found in common shampoos to investigate how the oxygen content of the oceans has changed over geologically recent time.

The rise of oxygen caused Earth's earliest ice age

May 07, 2009

(PhysOrg.com) -- Geologists may have uncovered the answer to an age-old question - an ice-age-old question, that is. It appears that Earth's earliest ice ages may have been due to the rise of oxygen in Earth's ...

Recommended for you

NASA sees last vestiges of Tropical Depression Jack

15 hours ago

Tropical Cyclone Jack had weakened to a tropical depression when NASA and JAXA's Tropical Rainfall Measuring Mission (TRMM) satellite passed above on April 22, 2014 at 1120 UTC/7:20 a.m. EDT.

New discovery helps solve mystery source of African lava

18 hours ago

Floods of molten lava may sound like the stuff of apocalyptic theorists, but history is littered with evidence of such past events where vast lava outpourings originating deep in the Earth accompany the breakup ...

Climate change likely to make Everest even riskier

19 hours ago

Climbing to the roof of the world is becoming less predictable and possibly more dangerous, scientists say, as climate change brings warmer temperatures that may eat through the ice and snow on Mount Everest.

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

DamienS
not rated yet Oct 08, 2010
Excellent article.

More news stories

On global warming, settled science and George Brandis

The Australian Attorney General, Senator George Brandis is no stranger to controversy. His statement in parliament that "people do have a right to be bigots" rapidly gained him notoriety, and it isn't hard to understand why ...