Extinctions of Early Life Probably Happened Slowly Over Time, Not With a Bang

The ocean is the primary cause of the world's mass extinctions over the past 500 million years.

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Since the advent of life on Earth 3.5 billion years ago, scientists think there may have been as many as 23 mass extinction events. Most wiped out simple forms of life such as single-celled microorganisms. The five well-studied mass extinctions that happened in more recent geological time—say, the past 540 million years—put the kibosh on 75-95 percent of mostly marine plant and animal species.

Except for the demise of the dinosaurs 65 million years ago from a massive asteroid impact, most of the catastrophes happened for no obvious reason. Paleontologists have been chipping away at the causes of mass extinctions for almost 60 years. Now, a new study suggests the epic ebbing and flowing of sea and sediment for eons upon eons account for world's periodic mass extinctions over the past 500 million years.

"Impacts, for the most part, aren't associated with most extinctions," said Shanan Peters, a University of Wisconsin-Madison assistant professor of geology and geophysics and leader of the study. "There have also been studies of volcanism, and some eruptions correspond to extinction, but many do not."

According to the study, changes in ocean environments related to sea level drive extinctions and generally determine the composition of life in the oceans. "The expansions and contractions of those environments have pretty profound effects on life on Earth," said Peters.

Arnold I. Miller, a paleobiologist and professor of geology at the University of Cincinnati, says the new study is striking because it establishes a clear relationship between the tempo of mass extinction events and changes in sea level and sediment. "Over the years, researchers have become fairly dismissive of the idea that marine mass extinctions like the great extinction of the Late Permian might be linked to sea-level declines, even though these declines are known to have occurred many times throughout the history of life. The clear relationship this study documents will motivate many to rethink their previous views."

Peters measured two main types of marine shelf environments preserved in the rock record, one where sediments are derived from land erosion and the other composed primarily of calcium carbonate, which is produced in-place by shelled organisms and by chemical processes. The differences between the two areas in sediment stability, temperature and the availability of nutrients and sunlight have important biological consequences, according to Peters.

Over the course of hundreds of millions of years, the world's oceans have expanded and contracted in response to the Earth's shifting tectonic plates and climate changes. During some periods, vast areas of the continents were flooded by shallow seas such as the shark- and mosasaur-infested seaway that neatly split North America during the age of the dinosaurs.

As those flooded areas drained back into the sea, animals that swam in them went extinct; conditions on the marine shelves that once supported a vast diversity of life also changed.

That's not to say volcanic eruptions, killer asteroids, or disease and competition among species could not have played a role, Peters said. But the new study provides a common link to mass extinction events over a significant stretch of Earth's history and suggests that few of them were controlled by just one environmental influence.

The study, which was supported by the National Science Foundation, was published online June 15, 2008, in the journal Nature.

—By Diane Banegas/NSF

This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report.