“It’s only in the last five years that this 20-year-old discovery has begun to strongly influence the field,” said Stanley Awramik, a geobiologist at the University of California, Santa Barbara. Researchers like Strother have begun to find more evidence of Precambrian land fossils.
Advances in geochemical and genetic technologies are partially responsible for the surge of interest in searching for microbial life on Precambrian lands. “People now realize that there was a world of microscopic ground huggers that did not leave behind the wealth of fossils that make it easy to find marine life,” Knauth said. And the search for traces of life in Martian deserts has heightened researchers’ interest in spotting primal Earth life forms in harsh, non-marine environments, he observed.
As far as Knauth is concerned, the simplest explanation for the fact that low levels of carbon 13, those geochemical ghosts of ancient photosynthesizers, coexist with cell-like microfossils is that there were massive amounts of life on land during the Precambrian. And his formerly outlandish theory is now widely accepted by geobiologists. “Knauth’s microfossils are tangible evidence that terrestrial life existed during the Precambrian,” said Shuhai Xiao, a geobiologist at Virginia Polytechnic Institute and State University.
Knauth’s evolutionary model goes far beyond debunking continental sterility. According to him, the biomasses inhabiting Death Valley and Arizona were comparable to the ones now growing in the Bahamas and the West Indies. Studies of the islands’ geology, conducted by other scientists, support this idea. These tropical islands, which are currently experiencing the same sculpting process the Beck Spring Dolomite underwent nearly a billion years ago, have a similar carbon signature.
“Death Valley was as lush with photosynthesizers as the Bahamian Islands are today,” Knauth said, as we dribbled hot water onto our tongues in the noonday sun.
He is also convinced that multicellular oxygen breathers — the ancestors of modern animals — may well have lived in and fed upon photosynthesizing microbes that were spewing millions of tons of oxygen into the atmosphere. In fact, rather than these Precambrian animals (called metazoans) colonizing land tens of millions of years after the Cambrian explosion of life in the seas, Knauth thinks that the reverse may have occurred; land-based animals crawled into the sea, spawning marine metazoans with shells.
In 2005 Knauth published a geochemical paper showing that Precambrian seas were hot and super-salty, suitable only for microbes that could withstand salt and hydrogen sulfide. Because salt and oxygen do not mix well in solution, it was an inhospitable climate for oxygen breathers. As continental plates collided, some seas were trapped in shallow, uplifting basins. Huge amounts of seawater evaporated, leaving behind enormous quantities of salt that were sequestered in the land. As a consequence, the salinity levels of the oceans plummeted, allowing the oceans to absorb more oxygen. The existing marine microbiota were poisoned or driven to deeper water, leaving behind an empty niche.
Meanwhile, Knauth theorizes, oxygen-utilizing metazoans could have evolved on moist mats of oxygen producers kept hydrated by rains, lakes, streams and rivers during the late Precambrian. In other words, the protein-packed mats of photosynthesizing bacteria and algae would have been a great food source for larger creatures, such as tardigrades, microscopic animals that eat bacteria. Knauth hopes to find fossil evidence of such creatures in his latest round of excursions to Death Valley and western Australia.
In 2009, Knauth and Martin Kennedy of the University of California, Riverside, shocked their more conservative colleagues with a meta-analysis of thousands of geochemical records from around the planet. They reported additional evidence that a land-based explosion of photosynthesizing algae, mosses, fungi and other organisms was likely to have greened the continents and facilitated the global expansion of multicellular life (including animals) as long ago as 850 million years ago, giving even more geochemical teeth to Knauth’s reverse Cambrian-explosion model.
This idea is still quite controversial. Some paleoscientists who agree with Knauth that Precambrian lands were green with microbiota do not believe that early animals, or their precursors, were necessarily lurking in those bacterial jungles. “It is unlikely that animals evolved on land prior to the Cambrian explosion,” Awramik said, noting that Knauth’s theory is not supported by hard fossil evidence. Yet he keeps an open mind about the issue, given that new fossil finds have a historical record of shattering paradigms that once seemed patently obvious.
But just as with his earlier theories, evidence for early land-based animals is beginning to build. In a 2011 Nature paper, Strother and his co-investigators reported direct evidence that multicellular structures may have dominated freshwater terrestrial settings by 1 billion years ago, half a billion years before the Cambrian explosion.
A study published last year by Carrine Blank of the University of Montana shows that the genomes of today’s cyanobacteria look as though they evolved from early life that inhabited terrestrial lakes, rivers and streams more than two billion years ago. Furthermore, the Precambrian fossil record shows that early microbiotic organisms adapted to land-based stresses, developing a “lotion” to screen out solar radiation, for example. This counters the assumption that sterile terrains and ultraviolet radiation would have prohibited the evolution of life on the Precambrian continents.
Last year, Gregory Retallack, a paleobotanist at the University of Oregon, and collaborators pushed back the arrival of life on land even further, reporting that land-based microfossils unearthed in South Africa could be as old as 2.2 billion years. This would break Knauth’s previous record of 1.2 billion years.
Retallack attributes the notion that life sprang only from the oceans to the Greek philosopher Anaximander, who, stunned by the diversity of life in the Aegean Sea, claimed that humans had evolved in the mouths of fish. “Anaximander would not have been so sure if he had had a microscope handy,” quipped Retallack.
With evidence of terrestrial life during the Precambrian pouring in from all corners of the paleosciences, the iconoclastic Knauth is inspired to keep working on this burning issue, which he was so instrumental in igniting 20 years ago. He is getting ready to explore a steaming-hot, fly-infested, billion-year-old site in an Australian desert to look for vanishingly small fossils.
Knauth gazed philosophically at the hole in the canyon wall before we began the treacherous hike down. “We will never know for sure what happened in the distant past, since we were not there,” he said. “But by eliminating the improbable, as Sherlock Holmes says, science closes in on the probable. And the most likely scenario of events is usually the simplest interpretation of the facts on hand.”
Reprinted with permission from Quanta Magazine, an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.