15 fevereiro 2012

Life on Earth Began on Land, Not in Sea?

First cells likely arose in steamy mud pots, study suggests.

Steam rises from geothermal plains in Kamchatka, Russia.
Photograph by Michael Melford, National Geographic

Dave Mosher
Published February 13, 2012

Earth's first cellular life probably arose in vats of warm, slimy mud fed by volcanically heated steam—and not in primordial oceans, scientists say.

(Also see "All Species Evolved From Single Cell, Study Finds.")

The concept, based on the latest cellular and geologic research, resembles a suggestion by famed naturalist Charles Darwin that life could have sprung from a "warm little pond" rich in nutrients.

(Find out about Darwin's scientific inspirations in National Geographic magazine.)

Despite this early musing by Darwin, marine-origin theories for life have been popular in recent years, because oceanographers continue to find oases of life thriving on the seafloor.
In these deepwater ecosystems, simple yet hardy microbes munch on noxious minerals spewing from hot volcanic vents—a setting many experts think could resemble the birthplace of the first cells.

(Related pictures: "'Lost World' of Odd Species Found Off Antarctica.")

But in the new study, researchers argue that the fluid all cells struggle to keep within their thin cellular membranes couldn't be more dissimilar to ancient ocean water.
Instead, the team discovered, this cellular fluid is very similar to condensed vapors found in volcanic mud pots on land.
Such terrestrial environments boast the high ratios of potassium to sodium found in all living cells. Marine environments, meanwhile, are far too rich in sodium.
"For cells to synthesize proteins—their molecular machines—they need a lot of potassium. Sodium blocks these activities," said study co-author Armen Mulkidjanian, a biophysicist at the University of Osnabrück in Germany.
"Life cannot live without synthesizing proteins, so it must keep potassium high."

Keep It Simple, Cells

Cells today rely on complex proteins to pump excess sodium out through their membranes, so the cells can function properly.
The first cells, however, had no such machinery at their disposal—just rudimentary cellular membranes and whatever nutrients the cells were lucky enough to trap inside.
As a result, the first cells were highly permeable and completely at the mercy of their environments. The ratio of potassium to sodium therefore had to be greater than one to one, in favor of potassium.
But in ancient seawater—as well as in modern seawater—sodium outnumbers potassium 40 to 1.
With this hurdle in mind, Mulkidjanian and his colleagues enlisted the help of geologists to understand where else life might have originated between 4.3 and 3.8 billion years ago.

(Related: "Life Ingredients Found in Superhot Meteorites—A First.")

The team realized that geothermal fields on land could do the job, particularly the mud pots found in places such as Yellowstone National Park.
"Mud pots are where steam is coming out of the earth and condensing, carrying with it many minerals, including potassium," Mulkidjanian said. "They look like slime coming out of the earth and would make a nice kind of hatchery for the first cells."
Scientists had long ignored mud pots as possible analogs to primordial ooze, because the modern-day versions are swimming in sulfuric acid, a deadly chemical that forms when hydrogen sulfide encounters oxygen in the atmosphere.

(Related: "Space Poison Helped Start Life on Earth?")

"People were scared away by the acidic condition, but Earth used to have very little oxygen in its atmosphere," Mulkidjanian said.
"These anoxic environments were stable over millions of years and were probably conducive to supporting the first life on Earth."

The new study arguing that life started on land was published online today by the Proceedings of the National Academy of Sciences.

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