Is evolution predictable, or was it heavily shaped by random events? Biologists have argued over this question for decades. Some have suggested that if we replayed the history of life on our planet, the resulting species would be different. Opponents counter that life is largely deterministic.
Recently, researchers have begun to ask the same questions about rocks. About 5,000 minerals — crystalline substances such as quartz, zircon and diamond — have been found on Earth. But minerals didn’t just appear all at once when the Earth formed. They materialized over time, each crystal arising in response to the conditions of the particular epoch in which it formed. Minerals evolved — in some cases, in response to life. And so geologists are left to ask: Are today’s minerals a predictable consequence of the planet’s chemical makeup? Or are they the result of chance events? What if we were to look out at the cosmos and spot another Earth-like planet — would we expect its gemstones to match ours, or would they shine with a luster never seen before?
Robert Hazen, a mineral physicist at the Carnegie Institution of Washington’s Geophysical Laboratory, and his colleagues are publishing a series of four papers this year that reveal broad insights into whether geology is a matter of fate. Minerals on Earth may indeed have been guided by some deterministic rules that could apply to other worlds as well, they found. But our planet is rife with extremely rare minerals, which suggests that chance occurrences also play a significant part.
In addition, if we found an Earth-like twin elsewhere in the universe, many common minerals would likely be the same — but that planet would probably also hold many minerals unlike any that exist here.
The findings aren’t just a matter of curiosity. Some minerals may have helped early organisms emerge. And understanding which minerals could have formed on Earth-like planets may help scientists better predict which worlds are likeliest to harbor life. Conversely, some minerals arise only in the presence of organisms. So finding patterns in Earth’s mineral distribution could help scientists identify a mineralogical signature for life, which they could then search for on other planets.
Time and Chance
Traditionally, mineralogy has been dominated by analyzing the structures and formation of individual minerals. But in a 2008 study in American Mineralogist, Hazen and his colleagues took a more historical view. The researchers assessed Earth’s known minerals and tried to figure out when the conditions were right for their formation. The team concluded that about two-thirds of Earth’s minerals would not have emerged until life was present.
For example, early microorganisms seeded the atmosphere with oxygen, which interacted with existing minerals to yield new ones. The so-called Great Oxygenation Event “was a huge game changer,” said Hazen. “You open the door to literally thousands of new minerals.”
Hazen and collaborators then set out to investigate the role that chance played in mineral formation. First, the researchers studied the relationship between mineral diversity and the abundance of individual elements in Earth’s crust. They found that the more abundant the element, the more minerals it formed, a relationship that was published last month in The Canadian Mineralogist. They then performed the same exercise with minerals from the moon. A similar relationship held, even though the number of known minerals there is much smaller. This common trend suggested an element of determinism: Given starting chemical conditions, one could predict, to a certain extent, which minerals would form.
The team did find outliers, however. For instance, the element rubidium forms fewer minerals than expected, given its abundance. However, Hazen’s team believes there are chemical reasons for the discrepancies. Rubidium frequently substitutes for potassium in minerals and thus gets “used up” in existing potassium-dominated minerals. Meanwhile, some elements such as copper form more minerals than expected because they have several chemical states that allow them to combine with other atoms in multiple ways. These results still support the idea of determinism, said co-author Ed Grew, a petrologist at the University of Maine, because “we can explain why they’re not obeying the rules.”
Peter Heaney, a mineralogist at Penn State, University Park, notes that the correlation for Earth’s minerals is fairly weak. But he said that the reasons given for the outliers make sense. “What I think is really important is that [Hazen’s] asking these questions and making us think about mineral diversification in a way that no one has really done before,” said Heaney, who was not involved in the study.
Hazen’s team also found evidence for the role of chance. The researchers used a crowd-sourced database to retrieve more than 650,000 mineral observations at specific locations around the world. Twenty-two percent of all minerals were reported in only one place, and 12 percent were found in only two places. The presence of so many “onesies and twosies” suggests that randomness does play a role, said Chris McKay, an astrobiologist at NASA Ames Research Center in Moffett Field, California, who was not involved in the study. “That’s the hallmark of chance events.” These rare minerals might appear only under fortuitous circumstances, such as an unusual assembly of rocks that concentrates elements together. “It’d be like if you threw together a whole mess of ingredients and cooked it up, and it came out to be a prize-winning culinary dish,” said Grew.
And so what would happen if you replayed Earth’s history? There are about 15,300 plausible ways to combine naturally occurring elements into unique minerals, the researchers estimate. In a rerun of Earth, they say, at least one-quarter of the planet’s roughly 5,000 minerals would come out differently.
In addition, the chances that another planet has exactly the same set of minerals as Earth is less than 1 in 10300, the researchers report in a paper that will be published next month in Earth & Planetary Science Letters. In other words, our planet’s precise mineral composition is unlikely to be found anywhere else in the universe.
Life’s Rocky Start
Life adds another wild card. Earlier work by Hazen and other scientists showed that minerals and life likely coevolved. Minerals might have prodded life along by catalyzing reactions that produced biomolecules, for example. And life certainly changed the biosphere in ways that affected how minerals formed. “The origin of life depends on minerals, but the origin of minerals depends on life,” said Hazen.
Because of this relationship, the presence or absence of certain minerals on distant planets could affect the chance that the planet harbors detectable life. For example, astronomers know that some stars have different ratios of elements than the Sun does. The star’s chemical makeup affects the abundance of elements on any orbiting planets, and thus which minerals might form. Those minerals in turn could influence geological processes, the chances of life emerging and whether signs of life would be visible. If scientists can factor the likelihood of various minerals into their models, they might be able to more accurately pick the most promising planets to study. “It’s a game of statistics,” said Patrick Young, a theoretical astrophysicist at Arizona State University, Tempe.
But which minerals are needed for life, if any, is still murky. Stephen Freeland, an evolutionary biologist at the University of Maryland, Baltimore County, points out the example of the element phosphorus. It’s not that abundant, but it’s critical for life on Earth. Did a mineral gather and concentrate the element, thus allowing life to incorporate it? “Minerals, in a sense, are ways of pulling order out of chaos,” said Freeland. But, he adds, “all of this is swimming in an ocean of unknowns.”
If life requires only common minerals to form, those minerals would likely be available on another Earth-like planet. However, if life depends on rare minerals, then the chances of its emergence might be dicier. Mineralogical differences between planets might be “only of academic interest, or it could mean that they have differences in mineralogy that are profound,” said McKay.
Hazen’s team is now working to determine which minerals characterize Earth-like planets. Hazen thinks that the presence of many rare minerals might indicate that life emerged. For example, the interactions of different types of microbes with the soil create many specialized “microenvironments” where new minerals can form. And minerals might leave a more lasting imprint than cellular detritus.
Hazen’s team has also made predictions about minerals on Earth. The researchers found that the distribution of minerals — a few common ones and many rare ones — resembles the distribution of words in a text. A few words such as a and the appear frequently, but many words appear only sporadically. The team could therefore use models employed by linguists to analyze their data on minerals and extrapolate how many undiscovered minerals might exist on Earth. At least 6,394 minerals are present, which means that about 1,500 new ones could be found with current search techniques, the researchers estimate in a paper published in June in Mathematical Geosciences.
Many of these “missing minerals” likely escaped notice because they have dull colors or are unstable, the team notes in a paper scheduled to be published in October in American Mineralogist. But Hazen hopes to hunt some down. For instance, sodium minerals tend to be white or grey and might be found at Lake Natron in Tanzania, which contains huge deposits of white minerals.
Finding the missing minerals probably won’t add to our understanding of how life emerged. “I doubt we’re going to find some mineral that is the smoking gun for the origin of life,” said Hazen. But the work might help scientists make firm predictions about what new minerals might exist, rather than leaving their discovery to chance.
This article was reprinted on Wired.com.