Science News - May 19, 2012

and create an acidic environment in the surrounding ocean, the Lost City vents are formed by a reaction between mantle rocks and seawater, leading to an alkaline environment.

The vents spew methane and hydrogen into the water, which react to form limestone chimneys, acetate (a potential energy source for early life) and hydrocarbons (important building blocks for life). What’s more, pores in the limestone chimneys suggest a way for chemicals to undergo reactions without floating away.

“These microcompartments serve the function of providing a way for chemicals to be concentrated in a physical way without cellular membranes,” explains Martin, who has collaborated with geologists to propose theories on the origins of life at the Lost City vents. Early cells could have lived off the chemical cocktails within the compartments. “Lost City is the most exciting thing that’s happened in the past decade in this field,” he says.

While today’s Lost City vents wouldn’t have been around 4 billion years ago, similar ones could have spewed life-sustaining chemicals into the early oceans. It’s up to geologists to determine whether such vents existed and up to biologists to figure out whether life could have thrived there.

Armen Mulkidjanian of the Univer-sität Osnabrück in Germany has a different idea about where life began. He studies what features modern single-celled organisms share with each other, a favorite topic among many evolutionary biologists. Two major domains of such organisms exist —bacteria and archaea — and both are thought to have evolved from a common ancestor that existed at least 3. 5 billion years ago.

While Earth’s first cells were probably more primitive than this ancestor, studying its characteristics may help scientists piece together a picture of even older cells. So far, researchers have found that bacteria and archaea have about 60 genes in common. Thus, scientists deduce, the ancestor, dubbed LUCA (short for Last Universal Common Ancestor), had these same genes.

“What we decided to do was to analyze what organic ions are required by each of the proteins coded by these ubiquitous genes,” Mulkidjanian says. His team’s analysis, reported in Proceedings of the National Academy of Sciences earlier this year, revealed that these shared proteins require potassium more than any other element. And sodium, the researchers showed, blocked the function of some of the cellular elements, most of which are involved in the translation of genetic material to proteins.

“We know that original membranes were very leaky. Cells could keep proteins or nucleotides inside, but not potassium,” Mulkidjanian says. So this means that LUCA must have been living somewhere with more potassium than sodium; otherwise potassium would have flooded right out of the membrane.

Here’s when the biologists consulted with geologists. Researchers familiar with ancient geology agreed that all the evidence from Hadean rocks suggests the oceans back then were rich in sodium. But one place on the ancient Earth, the geologists said, would be replete with potassium: ponds created by vapor from volcanic systems, which the Hadean planet had plenty of.

When magma from volcanoes heats rocks, some water evaporates, pulling certain elements from the rocks and leaving behind others. The resulting vapor can condense back into water and form freshwater ponds, rich not only in potassium but also in zinc and phosphate — also substances that could have driven early cellular processes. The

Vaporous ponds, like those found today at Hot Creek in California, could have provided a potassium-rich setting for early life.

cellular requirements of LUCA proposed by Mulkidjanian’s team matched the geological descriptions of these geothermal fields. “That geochemical knowledge is really what fed our biology story,” says Mulkidjanian.

Ideas about where life began, whether it was in an ocean or a pond or somewhere else entirely, are still just proposals, hypotheses with bits of evidence. The same is true for existing views about when life emerged and what it looked like. But as geologists and biologists continue to learn from each other, they’re turning up new evidence that can strengthen existing scenarios and lead to new ones.

For geologists, the challenge going forward is to find and analyze more ancient rocks to flesh out the picture of the Hadean planet. For biologists, the next task is to combine their theories on early cells with geologists’ descriptions of the early Earth.

“We’re starting to narrow the gap between microbiology and geochemistry,” Martin says.

As new zircons are uncovered and chemically favorable environments are explored, the tale of how life began may gain an agreed upon time, a setting and, eventually, a plot. s