New Theory Drastically Rethinks Evolution of Early Life
for National Geographic News
|January 30, 2003|
Long ago, before microbial organisms first took shape as organic cells
and began to colonize the biosphere, naked living processes may have
commenced within the confines of hollow bubbles of deep-sea rock.
This take on the origins and early stages of biochemistry, laid out in a bold new scientific treatise, could dramatically rewrite the opening chapters of the story of life on Earth. It also implies that extraterrestrial life might exist in much greater abundance that has been traditionally presumed.
"Under our model, you would not need an atmosphere [to foster life]. Rocks and water would be enough," said William Martin of the University of Dusseldorf in Germany.
"Any wet, rocky, sunlit planet will have life," said colleague Michael J. Russell of the Scottish Environmental Research Centre in Glasgow. "It's a matter of course. Life is absolutely inevitable."
If Martin, a biochemist, and Russell, a geologist, are right, then traditional hypotheses about how the Earth's earliest forms of life evolved are due for a rewrite.
The pair has published their far-reaching theory in this month's issue of Philosophical Transactions: Biological Sciences, a journal of the Royal Society of London. The document, at 24 pages, is a hefty and exhaustive tome by the standards of scientific publishing, which can usually pack any idea into six pages or less. But that's because describing early lifelike developing itrequires a bit of time and effort.
A Rocky Start
All living organisms are made up of cellsmembrane-enclosed organic sacksthat contain proteins and strands of genetic material. Without the biological machinery inside, cellular membranes couldn't grow, multiply, or even repair themselves. The most fundamental of life's processes, reproduction, would be inconceivable.
Without the membrane that encloses its fragile components, the molecular machinery of life would be unshielded from the harsh forces of its surrounding environment and would be torn apart before it could do its work. "Life starts with a cell wall or a membrane," Russell said. "Otherwise, it bleeds to death."
What scientists have so far lacked is a convincing explanation for how an organic cell wall could have developed before there was the biological apparatus to build it. And thus arises a vexing microcosmic variation on the chicken-and-egg riddle: Which came first, the apparatus inside, or the membrane that holds each bundle of life together?
Martin and Russell believe they've solved the conundrum by thinking outside the biological box. The first containers of life, they suggest, were themselves neither products nor producers of biochemistry. They were tiny, hollow chambersenveloped by rock.
Cavity-riddled masses of iron sulfide formed naturally where hydrothermal vents spewed warm, compound-rich fluids into deep-sea waters, explains geologist Russell. These "culture chambers" provided just the sort of incubator that the chemical ingredients of life needed to initiate biochemistry.
"These inorganic compartments were the precursors of cell walls and membranes found in free-living [cells]," Martin and Russell wrote.
Trapped within their inorganic incubators, "the first cell couldn't feed itself," said Russell. "Like a child in the womb, it had to be fed, to be nurtured" by the stream of nutrients and energy that continued to bubble up from the hydrothermal vents beneath them, he said.
It doesn't sound like an easy way to live, but there must have been multiple ways to pull it off, Martin and Russell argue, because there are three fundamentally different forms of life. Two of themthe eubacteria and their simple cousins, the archaebacteriadiverged evolutionarily even at this early stage in the history of life, they suspect.
From these humble beginnings, however, life would soon burst forth into a new and exciting worldthe juvenile Earth.
First, the researchers hypothesized, biological processes produced and deposited fatty molecules along the inner surfaces of the iron sulfide cavities. Eventually, these fats formed into cohesive membranes that fully enclosed the biological activity inside. Archaebacteria and eubacteria arose independently from parallel occurrences of this series of events, Martin and Russell proposed.
That landmark development rendered obsolete the original, inorganic compartments, and the first true cellular life was ready to emerge. When the first of these organisms broke free from the confines of their iron sulfide cocoons, said Martin, "they were all alone in an uninhabited planet."
"This lonely little spot" of life might have evolved quickly, Martin hypothesized, because virtually every adaptation that arose opened up whole new classes of untapped chemical resources in the organisms' submarine world. "It's as if you went into a bank [for the first time], and they gave you all the money."
The Whole Trinity
Eventually, organisms became numerous enough that competition among them ensued. That, Martin and Russell suggested, may have been when some eubacteria and archaebacteria hit on the strategy of joining forces, thereby forming the third class of life, the eukaryotes.
These organisms, composed of complex cells with complete, organ-like internal structures, include all multi-cellular life forms such as plants and animals. The ancient strategic merger that created them explains why most eukaryotes resemble eubacteria in certain biochemical respects but depend on archaebacteria-like internal organelles such as mitochondria or chloroplasts, Martin and Russell asserted.
That argument, like many the pair of researchers has made in their new model of life's origins, broadsides conventional wisdom.
Martin and Russell fully expect their theory to be held to the fire that burns in the crucible of rigorous scientific discourse.
|© 1996-2008 National Geographic Society. All rights reserved.|