It's a bright, clear day in the Proterozoic. A microbial mat is basking in the sand, happily soaking up the sun's rays. The bacteria that make up the mat are using the sunlight to produce some much-needed food. The only thing missing is something cool to drink - there often is some water in this sandy depression, but the last bit evaporated a few days ago. However, there are storm clouds on the horizon, and soon some rain will fall.
Wait a minute - life on land during the Proterozoic? That can't be right - everyone knows that life only existed in the oceans during this period of Earth's history, 2.5 billion to 550 million years ago. Life isn't supposed to hit land until the Silurian, 440 million years ago, or possibly during the Ordovician, 490 million years ago. Until then, the continents were barren, lifeless wastelands.
But emerging evidence is beginning to suggest that life may have come ashore much earlier than previously thought. Scientists have found geochemical evidence that indicates microbial life could have been on land as early as 2.7 billion years ago. A molecular clock study conducted by Blair Hedges of Penn State University found that mosses appeared on land about 700 million years ago, and lichens around 1.3 billion years ago. Many scientists are skeptical about molecular clock data, however, and geochemical evidence indicating life also can be controversial. Fossils, like it or not, are still considered the ultimate hard evidence for any paleontological theory.
Yet fossils dating back to the Proterozoic are hard to come by. Life forms during this era were soft-bodied, which don't preserve as easily as the later hard-shelled life forms.
|Sandstone features developed on upper bedding-plane surfaces in
Diabaig Formation. Prave believes the sandstone features are the remnants of microbial mats.
Credit: A.R. Prave
However, Tony Prave of the University of St. Andrews in Scotland recently found some rocks that may prove life not only arrived on land early, but it also may have been widespread. In a paper published in the journal Geology, Prave describes features he discovered in 1 to 1.2 billion-year old sandstone deposits in Scotland's Torridon hillsides.
Prave believes the sandstone features are the remnants of microbial mats. The organic material of the mats has long since decayed, but sand grains that were bound up in the mat's sticky mucus layers still remain. These sand grains show evidence of mats that tore, possibly due to rainstorms. They indicate how small pieces of the mat were carried away, becoming curled up into sausage-like shapes.
Where chunks of the mat tore away, the sandy soil beneath was suddenly exposed to the elements. These newly exposed areas developed ripples, probably from the same water action that tore the mat in the first place. Pieces of the torn maps settled over these newly formed ripples and became embedded in the muck.
"It's like you have a perfect green lawn, and then a storm blows a chuck of sod away," says Dave Bottjer, professor of geological sciences at the University of Southern California. "The loose soil underneath then becomes exposed to the storm, and ripples form when water washes over it. Perhaps pieces from your neighbor's lawn also fall onto the soil."
Prave says that the microbial mats developed well away from wet areas like shorelines or fluvial channels. They may have existed in temporary pools of water, but since the water tended to evaporate these microbial mats can be considered the earliest known life to exist on land. Prave says he has no way of knowing what the original elevation of the land was, but anything further up than a river's edge or ocean shoreline would have been, for life at this time, dramatically high and dry.
"Even if the rocks were deposited near sea-level, if my interpretations are correct than microbial biota had expanded to at least those 'dizzying' heights," says Prave.
Making the Leap to Land
Throughout most of the Proterozoic, life was single-celled. Some of these organisms developed photosynthesis and began to produce oxygen as a waste product. Still, the atmosphere during the Proterozoic was mostly nitrogen, with a little water vapor and carbon dioxide. Oxygen didn't become a major component of the atmosphere because it tended to react chemically with iron and other elements.
|Cyanobacteria create microbial mats that can stack up to form large rock-like structures known as stromatolites.
Credit: A.R. Prave
The introduction of oxygen into the atmosphere was important for the development of an ozone layer. For ocean-dwelling organisms, the ozone layer isn't that important because the water acts as a protective shield against UV radiation. But for any organisms trying to make the transition to land, an ozone layer would've been necessary to prevent them from being fried alive.
The first life forms to poke their head out of the water and brave the Sun's rays may have been cyanobacteria living in inter-tidal environments. These bacteria create microbial mats that can stack up to form large rock-like structures known as stromatolites. As sediment collects in the mat's sticky layers, sunlight is prevented from penetrating and photosynthesis can't occur. The bacteria then migrate up to create a new layer on top of the old. This process occurs again and again, creating multiple sediment layers over time.
Such biologically produced stromatolites have been found dating back to at least 2.2 billion years ago (there are stromatolites dating back to 3.5 billion years ago, but whether they were produced by life or by other means is still under debate). Stromatolites can still be found in highly saline shoreline waters today, in such places as Australia's Shark Bay.
Because there is no organic material remaining in the Scottish sandstone features, it's not possible to tell what sort of organisms made up the presumed microbial mat. But Prave thinks the most reasonable assumption is that they were similar to the cyanobacteria that were forming stromatolites.
"Microbes were inhabiting near-shore marine environments, including shoreline and tidal settings, for a couple billion years prior to the time of deposition of the Torridonian rocks," says Prave. "In all that time, isn't it conceivable that the microbial biosphere could have adapted to and migrated up river systems and into lacustrine settings? Or, to turn that around - especially given what we know about extremeophile lifestyles - isn't it more unlikely to think that microbes stopped at the shoreline for all that time?"
Dave Bottjer says the presumed microbial mats reported by Prave may be very similar to microbial life found in desert topsoil. These organisms - mostly cyanobacteria, lichens and mosses - create what is known as cryptobiotic soils. They have sticky filaments that adhere to soil particles, producing an intricate mat of fibers that make the soil resistant to wind and water erosion.
"If you go out to the Desert Parks of the West, such as Arches National Park in Utah, there are now signs posted asking you not to step off the path onto the desert soil," says Bottjer. "When the soils get stepped on, they can take centuries to reform. People had always looked out on desert surfaces and said there's nothing there - no life. We've only just realized that instead, there is life. Microbes are what holds the desert topsoil together."
Likewise, the traditional view of the Proterozoic was that there was no life on the land. Bottjer says that instead, he can envision the Proterozoic continents covered with microbially-bound soils similar to those in the desert.
"You and I wouldn't have seen them - the land would have looked barren just as it does in the desert," says Bottjer. "But if we were worms, then we would've seen a lot of life."
Prave plans to follow up his Geology paper by examining other localities where similar rocks are exposed. He hopes to find more evidence that the rippled features resulted from microbial activity.
"More importantly, though," says Prave, "I would hope that the paper generates enough excitement - or irritation - that it would cause other geologists to go out and re-examine their favorite non-marine Precambrian rocks. My personal feeling is that as more and more effort goes into examining the Precambrian sedimentary rock record, more such discoveries are likely."