The drop stones found in tropical rocks were one of the first signs that something unusual had occurred on Earth.
Drop stones are pebbles that land on the seafloor with enough power to distort the sediment.
However, no drop stones should have been present in these rocks.
The most common source is glaciers; ice sheet bellies accumulate rocks like ticks and then shed them when they go to sea.
The bands of limestone interspersed with the drop stone–bearing rocks, however, indicate that they were created under periodically hot tropical waters. Glaciers couldn’t possibly exist in the tropics, could they? Right?
Joe Kirschvink, a geologist at the California Institute of Technology, looked at this and other information that had been circulating for a few decades and came up with a novel hypothesis: snowball Earth.
Around 650–700 million years ago, our globe was completely buried in ice up to a kilometer thick, according to the theory.
Surface temperatures were considerably below zero everywhere, and life, in whatever shape it took at the time, had to adapt.
And the data implies that this disaster occurred not once, but twice around that time.
The initial encapsulation appears to have lasted 58 million years, which, I feel compelled to point out, is more than 24 times as long as T. rexes have been extant (a mere 2.4 million years). After 10 million years, the second snowball lasted another 5–15 million years.
Although partial glaciations would gradually creep into the temperate zone hundreds of millions of years later, close to our own time, ice would never again consume the Earth, as far as we know.
Cave fungi may have helped drag the globe out of the second snowball, according to a study of new Chinese fossils released in January.
If genuine, it would be significant because the earliest known terrestrial fungus fossils are nearly 200 million years old.
If the world was frozen solid, it stands to reason that once all of the heavy ice melted, the unburdened land rebounded and was bathed in the fresh air. Rainwater crept into gaps in the newly exposed rock, weathering the surface but also producing caves.
Scientists announced in January in Nature Communications that they had discovered both cave formations and pyrite-fossilized filaments that look a lot like mushrooms inside what they claim are the remains of these spaces in the Ediacaran Doushantuo rock strata of China.
There are branching and fusing curved filaments—including A- and H-shaped structures—as well as small branch buds that appear to be seeking each other out; hollow spheres (spores?) both integrated into filaments and at their termini; and two different gauges of fiber, implying at least two species.
The fibers also lack septa, which are internal barriers that separate tubes into cells.
The authors claim that the only fungus has this particular set of characteristics, while no other group of species does.
Abiotic look-alikes appear to be ruled out by the twisted and bending filaments.
Physical fungal fossil imposters are uniformly wide, but real fungi are narrower and vary in a variety of sizes.
Outside evidence suggests that a fungus is the most likely explanation. Fungi could be 0.9–1.5 billion years old, according to molecular clocks, which utilize predicted rates of DNA mutation to estimate when distinct groups of organisms emerged.
The scientists speculate that structures such as stalagmites, stalactites, and grapelike botryoids coated the cavities’ walls shortly after they developed, colonized, and catalyzed by fungi and other microorganisms; huge spheres in the fossils penetrated by the filaments could represent symbionts—or food.
Modern cave formations, for example, include comparable microorganisms, including fungi that look like fossils.
Some modern fungi can mine rock and extract nutrients. If moldy rock pockets in fresh-out-of-the-snowball Earth were widely distributed, the weathering of continental rocks already occurring at the surface and the transfer of phosphorus, a fertilizer, into the ocean would have been expedited.
Algal blooms would have pumped oxygen into the atmosphere as a result.
Before the snowball Earth, atmospheric oxygen levels appear to have been significantly lower than they are now.
It wouldn’t be long before life forms capable of utilizing all of that new free redox power took advantage of the situation, resulting in the Cambrian explosion.
The most complex life—and the total of over 3.5 billion years of evolution—seems to have been a sponge before snowball Earth.
Titanosaurs, morning redwoods, and huge funguses come after snowball Earth.
Was there a link, and did all that extra oxygen play a role? This is a hotly debated topic.
More broadly, the fact that humans only recently learned our planet likely underwent a 58-million-year transformation Mr. Freeze is disconcerting because of what it says about what we don’t know as well as what we do know.
I once heard a lecture by a classicist who said that, given the terrible state of the existing evidence, what we know about the ancient human world is comparable to what could be learned by peeping through a keyhole about the Palace of Versailles.
Paleontology, particularly the paleontology of life before bones and shells, and the geology of Earth under remote and alien environments are examples of this.
After Joe Kirschvink first developed the snowball hypothesis and shared it with him, geologist Paul Hoffman told Astronomy.com that it took them a while to do anything about it because such a scenario was so different from known Earth history that they didn’t know if any piece of evidence in the rock record was for or against it.
And if determining important events in Earth history where we have a rock record has proven challenging, what else do we not know about our planet’s history and owner’s manual because the relevant rocks didn’t survive or aren’t currently at the surface?
For example, much of the Earth appears to be missing a distressingly big one-billion-year chunk of the geologic record, an obvious loss known ominously as the Great Uncomformity by geologists.
They argue about why it’s gone, but I’m more concerned about the fact that it’s gone.
It’s as if Earth went on a drinking binge for a quarter of its life and now “completely forgets” what happened or where its keys are.
As a result, we must be grateful for the memories that our planet does possess.
If the following interpretations are right, the drop stones and rock pockets depict a dramatic picture: Just before the greatest burst of life the earth has ever seen, a Europa-like planet metamorphoses into a freshly scraped set of weathering continents packed with moldy caves, softly fertilizing the oceans and oxygenating the atmosphere.
The author is: Jennifer Frazer, an AAAS Science Journalism Award–winning science writer, authored The Artful Amoeba blog for Scientific American. She has degrees in biology, plant pathology and science writing.
