Signs of Life in Acid-Washed Rocks
|"… some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity etc…", Charles Darwin, on the origins of life in tidal pools
Extraordinary clues to the history of biological evolution on Earth often come from something as mundane as rocks. To better understand the close connection between life and geology–and how one affects the other–new laboratory methods are being developed to tease out the information that ancient rocks contain.
Pioneering one such method is Dr. Frances Westall, paleobiologist for the Lunar and Planetary Institute and member of the NASA Astrobiology Institute. She and her colleagues are using acid vapor to isolate the remains of tiny microbial life forms. These fossils, entombed within ancient sedimentary fossil structures known as stromatolites, were once beyond the reach of scientific probing. But when acid washed, the fossils turn out to be tougher than the surrounding rock.
As recently described in the journal Precambrian Research, the technique used by Westall and her colleagues involves delicately etching the rock with fumes of the corrosive acid hydrogen fluoride. The rock dissolves away to reveal microscopic crystalline structures that look like sausage-shaped rods, spheres with wrinkled cell walls and clusters of bacteria–the telltale signs of ancient biology.
Modern-day stromatolites are created from alternating layers of sediment and cyanobacteria–a type of bacteria that photosynthesizes, giving off oxygen in the process. To form a stromatolite, the layers of cyanobacteria photosynthesize, grow and reproduce, creating a slimy mat that traps bits of clay, rock, sand, mud and organic debris. As this sediment accumulates and blocks sunlight, the microbes migrate up to start a new living layer.
|The biomolecule, DNA, that twists throughout the cell nucleus|
The old layer will gradually turn to stone–if it is sediment-rich and the moisture trapped within it evaporates quickly. As long as the bacteria continue living, reproducing and growing on top of previous populations, the layers of rock keep increasing.
Ancient stromatolites, however, formed in a slightly different way than modern-day structures.
"The very earliest stromatolites were formed by photosynthesizing, filamentous bacteria, which did not produce oxygen," says Westall.
"The very ancient stromatolites, in fact, were a mixture of layers of microbial mats coated with layers of precipitated minerals–which were again covered by more layers of microbial mats, and so on," says Westall. "The process of trapping debris was not a major method of growth in the ancient stromatolites–the precipitation of minerals on the microbial mats was more important to growth and fossilization."
Over time, minerals gradually replaced the structures of the organisms. The minerals eventually formed quartz, but the crystal structure of the organic-laced quartz is different from crystals that formed in the surrounding rock without the presence of organics.
"When the acid etches away the pure quartz which surrounds the fossils," says Westall, "the fossils stand out because the quartz which replaces them is ‘dirty’ and has a slightly different crystal structure to the quartz which surrounds them."
There have been some doubts as to whether ancient stromatolites really were constructed by bacteria, because previously no fossils were ever observed within them.
The acid vapor process, however, used in conjunction with high-resolution imaging and microscopic analysis of thin sections and rock chips, has revealed what the skeptics said were missing: microscopic life forms–smaller than cyanobacteria.
Cleaning Up after 3.5 Billion Years of Neglect
Studying ancient fossils, Westall says, is a multi-step process. The rock specimen is examined for evidence of life, such as very fine, wavy or hummocky layering typical of ancient microbial mats. If such evidence is present, they then slice the sample into thin sections and use a microscope to check for any larger biological structures, such as filamentous bacteria.
The final step uses acid vapor to etch the thin sections and other rock chips. This allows the scientists to search for smaller structures that are not readily visible in the thin sections.
The acid vapor method thus makes it possible to identify many more types of organisms than could be identified previously. Westall believes that her research can contribute to building a database for understanding the diversity of early microbial evolution on Earth.
In a broader sense, Westall is trying to unravel the connection between geology and life. She said her goals are to understand the geological context in which early life evolved and to understand the distribution of early life among a variety of environments.
"Basically," says Westall, "early life coped extremely well in the extreme conditions existing on the early Earth, such as the lack of oxygen in the atmosphere, high ultraviolet radiation, possibly higher ambient temperatures, severe volcanic activity, and so on. But those ‘extreme’ conditions are only extreme by modern standards. They were normal by the standards of the early Earth, Mars, Europa, or even Venus.
"When the Earth starts dying down and conditions become ‘extreme’ again–meaning a more hot, desert-like situation because of the expansion of the sun before it explodes–bacteria will be there to the bitter end."
Moreover, Westall argues, "extreme conditions on other planets would not be any impediment to life." Which is why, she says, "I’m interested in trying to understand Martian geological evolution, with a view to determining likely Martian environments for life."
Westall remains concerned about the problem of misidentifying microfossils.
"I am working with some colleagues in the hopes of establishing some biochemical technique to determine if there is any signal still from the degradation products of microbes in the very old rocks from South Africa and Australia," she states.
"But the search for ancient life and its distribution is a painstaking slow process requiring much methodical examination of the ancient rocks," she adds. "It will take years before we have a reasonable overview of what early life was like on Earth."
Dr. Westall’s research involves collaborations with many colleagues at the Lunar and Planetary Institute on Martian issues, as well as early Earth formation. Other collaborators include: Maud Walsh, Louisiana State University, on Early archaean microfossils; Bruce Jakosky, University of Colorado Boulder, on early Mars; David Deamer, University of California Santa Cruz, on prebiotic molecules and films; Andre Brack, Orleans, France, on prebiotic molecules; Andrew Steel and Jan Toporski, University of Porstmouth, UK, on ToF-SIMS analysis of organics, Martian meteorite contamination and fossil bacteria; Wouter Nijman and Sjoukje de Vries, University of Utrecht, Holland, on early Earth geology; Maartin de Wit, University of Cape Town, South Africa, on early Earth geology; and Martin van Kranendonk, Geological Survey of Western Australia, Perth, on early Earth geology.