Issue Date: December 7, 2015
If These Researchers Are Right, Life On Earth Started Earlier Than Once Thought
Even though it takes up no more space than Abraham Lincoln’s eye occupies on a U.S. penny, an ancient zircon grain plucked from the Australian Outback more than 30 years ago has recently stirred fervent debate among geochemists.
Researchers at the University of California, Los Angeles, analyzed the 4.1 billion-year-old gem and found inside what they believe could be the oldest known chemical evidence of life on Earth: a speck of graphite containing a high concentration of carbon-12, an isotopic hallmark of a living system (Proc. Natl. Acad. Sci. USA 2015, DOI: 10.1073/pnas.1517557112). If the researchers’ analysis and interpretation of this tiny sample are correct, the new data would push back the widely accepted date for the origin of life on Earth by hundreds of millions of years. Other geochemists, however, aren’t ready to rewrite history just yet and think the UCLA team’s findings could be explained by other phenomena.
The zircon grain in question came from an outcropping of quartzite in Jack Hills, Australia, discovered during geological surveys by the Australian National University in the late 1970s. Survey teams search for Earth’s oldest rocks in locations such as the Outback because they’re undisturbed by tectonic movements, severe weather, and civilization. Using uranium-lead dating—a method that measures the age of a sample on the basis of how much uranium has decayed to lead—the researchers determined that many of the zircons in the quartzite were more than 4 billion years old. These hardy deposits of zirconium silicate make up some of the only known material that survived from the first half-billion years of Earth’s history, the Hadean eon, and offer a rare glimpse into what Earth may have been like at that time.
Because the UCLA researchers had access to a collection of about 10,000 zircon grains from the Jack Hills deposit, they wondered whether they might find any material captured within the specimens that could reveal more about the Hadean carbon cycle. “You get a lot of foreign crystals included in zircons,” says Elizabeth A. Bell, one of the UCLA team members. As is the case with flaws in other gemstones, these small bits of unfamiliar substances float in the molten material from which the zircon forms and can eventually become trapped within the crystallized product.
Bell and her colleagues visually examined all 10,000 of their zircon grains to painstakingly single out those containing dark specks. The researchers then analyzed the 656 remaining specked samples with Raman spectroscopy to determine whether carbon or another material, most commonly iron oxide, was responsible for the discoloration. Just two of these contained graphite crystals. One of the gems displayed cracks and therefore was potentially contaminated. But the other was deemed usable: The grain appeared pristine, and the graphite species had probably been there since the zircon formed. The UCLA team was surprised to find that this graphite had a relatively high concentration of carbon-12. That’s because carbon-12 usually only appears at such high concentrations in materials produced by a living system, and previous estimates of the start of life on Earth did not go as far back as the Hadean eon.
Scientists believe that living systems prefer carbon-12 over carbon-13—another common carbon isotope—because it passes more easily through cell membranes and is more likely to be processed by enzymes associated with metabolism. Carbon-13 is like a linebacker, and carbon-12 is like a jockey, in terms of their weight classes, explains Stephen J. Mojzsis of the University of Colorado, Boulder. Mojzsis was not involved in the current study but has analyzed isotope compositions of graphite inclusions in ancient material in the past. He and his colleagues published similar findings of isotopically light carbon found in 3.8 billion-year-old apatite crystals from Greenland about 20 years ago (Nature 1996, DOI: 10.1038/384055a0).
These carbon samples and other mineral inclusions from possibly once-living sources are known as chemofossils. Like traditional morphological fossils—such as impressions of leaves or parts of dinosaur bones—chemofossils offer evidence of past life. And some researchers believe they may provide a better estimate of the beginning of life on Earth—a time period for which morphological fossils may not exist.
Imprints of cyanobacteria called microfossils make up the oldest, essentially irrefutable, evidence of life on Earth and date back to around 3.5 billion to 3.6 billion years ago. But the rock record from that period is sparse, so life may have started even earlier—researchers may just not have found all the microfossils yet.
Until the past decade or two, however, most scientists were drawing the line for life’s first appearance at about 3.8 billion years ago, during the Late Heavy Bombardment. This cosmic event—in which an unusually high number of celestial objects pummeled Earth and the moon (leaving behind craters in the latter that can still be seen today)—was thought to have left Earth too inhospitable to nurture life.
But as techniques to investigate ancient minerals have improved, geochemists have challenged that assumption with isotopic evidence for liquid water going back further, to the Hadean eon. Liquid water supports living organisms and helps run photosynthesis. “In looking at the zircons, it looks like early Earth was quite different than that original conception and in many respects much more like modern Earth,” Bell says.
But despite the picture that scientists are building of a hospitable Hadean Earth, many in the research community are hesitant to believe that chemofossils provide adequate evidence of life. These skeptics think nonliving systems might give rise to isotopically light graphite or that some of the zircon samples in question have been contaminated.
In 2008, for instance, Swedish researchers believed they had found diamond that contained isotopically light carbon in another set of zircon grains from Jack Hills (Nature 2008, DOI: 10.1038/nature07102). Later investigations revealed that the researchers accidentally introduced diamond to the specimens when they used a polishing agent to prepare the gems for study. Since that mix-up, explains T. Mark Harrison, another member of the UCLA team, “everyone’s very gun-shy. We went to great lengths to say this bit of graphite was incorporated in that zircon 4.1 billion years ago.”
Scientists do not have a method to directly date very old carbonaceous material. Carbon-14 dating—a standard among archaeologists and geochemists—works only for relatively young material such as wood because the isotope involved decays completely after about 60,000 years. So the only way researchers can determine the age of graphite in ancient zircon specimens like the ones from the Outback is to analyze the minerals with uranium-lead dating and ensure that the carbon sample within was incorporated into the rock during its birth and has remained unchanged over time. The team must establish that the grain didn’t crack, says Steven B. Shirey, a geochemist at the Carnegie Institution for Science who was not involved in the new study.
Although Harrison, Bell, and colleagues imaged their zircon sample with transmission electron microscopy and saw no typical evidence of a healed or unhealed fracture, Shirey says that 4.1 billion years is a lot of geological time and that a fracture could have healed without a trace. However, he adds, the graphite could very well have crystallized within the zircon more than 4 billion years ago as the UCLA team suggests. “It’s basically a leap of faith,” he says.
“If you think light carbon means life, then you would go with their interpretation,” Shirey continues. But living systems are, as previously mentioned, not the lone possible source of light carbon. The UCLA researchers acknowledge this and provide a few alternative explanations for the unusual lightness of the graphite they measured. Most probably, Bell says, the carbon may have originated from a carbonaceous meteor that just happened to contain light carbon. Less probably, Mojzsis says, hydrogen and carbon monoxide may have become trapped in the zircon during formation and later converted into isotopically light carbon in a reaction similar to the Fischer-Tropsch process used to create synthetic motor oil. In either case, such sources of light carbon are largely theoretical and have not been established as naturally occurring processes. Those who think the new zircon study represents evidence of early life say that particular interpretation of the data represents the simplest explanation. “We don’t have time machines to go back and see what happened,” Mojzsis says, “but we can look at the chemistry.”
Geochemists in both camps agree that more zircons with isotopically light carbon inclusions must be found before researchers can draw more confident conclusions. Bell, Harrison, and their colleagues at UCLA are currently analyzing their zircon collection to find more usable grains and hope that future geological surveys reveal more sources of ancient material for analysis.
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