Science fiction has long imagined alien worlds inhabited by silicon-based life, such as the rock-eating Horta from the original Star Trek series. Now, scientists have for the first time shown that nature can evolve to incorporate silicon into carbon-based molecules, the building blocks of life on Earth.

As for the implications these findings might have for alien chemistry on distant worlds, "my feeling is that if a human being can coax life to build bonds between silicon and carbon, nature can do it too," said the study's senior author Frances Arnold, a chemical engineer at the California Institute of Technology in Pasadena. The scientists detailed their findings recently in the journal Science.

Carbon is the backbone of every known biological molecule. Life on Earth is based on carbon, likely because each carbon atom can form bonds with up to four other atoms simultaneously. This quality makes carbon well-suited to form the long chains of molecules that serve as the basis for life as we know it, such as proteins and DNA. [The Search for Life on Mars in Pictures]

Still, researchers have long speculated that alien life could have a completely different chemical basis than life on Earth. For example, instead of relying on water as the solvent in which biological molecules operate, perhaps aliens might depend on ammonia or methane. And instead of relying on carbon to create the molecules of life, perhaps aliens could use silicon.

Carbon and silicon are chemically very similar in that silicon atoms can also each form bonds with up to four other atoms simultaneously. Moreover, silicon is one of the most common elements in the universe. For example, silicon makes up almost 30 percent of the mass of the Earth's crust, and is roughly 150 times more abundant than carbon in the Earth's crust. 

Scientists have long known that life on Earth is capable of chemically manipulating silicon. For instance, microscopic particles of silicon dioxide called phytoliths can be found in grasses and other plants, and photosynthetic algae known as diatoms incorporate silicon dioxide into their skeletons. However, there are no known natural instances of life on Earth combining silicon and carbon together into molecules.

Still, chemists have artificially synthesized molecules comprised of both silicon and carbon. These organo-silicon compounds are found in a wide range of products, including pharmaceuticals, sealants, caulks, adhesives, paints, herbicides, fungicides, and computer and television screens. Now, scientists have discovered a way to coax biology to chemically bond carbon and silicon together. 

"We wanted to see if we could use what biology already does to expand into whole new areas of chemistry that nature has not yet explored," Arnold said. [Facts About Silicon]

The researchers steered microbes into creating molecules never before seen in nature through a strategy known as 'directed evolution,' which Arnold pioneered in the early 1990s. Just as farmers have long modified crops and livestock by breeding generations of organisms for the traits they want to appear, so too have scientists bred microbes to create the molecules they desire. Scientists have used directed evolutionary strategies for years to create household goods such as detergents, and to develop environmentally-friendly ways to make pharmaceuticals, fuels and other industrial products. (Conventional chemical manufacturing processes can require toxic chemicals; in contrast, directed evolutionary strategies use living organisms to create molecules and generally avoid chemistry that would prove harmful to life.)

Arnold and her team — synthetic organic chemist Jennifer Kan, bioengineer Russell Lewis, and chemist Kai Chen — focused on enzymes, the proteins that catalyze or accelerate chemical reactions. Their aim was to create enzymes that could generate organo-silicon compounds. 

"My laboratory uses evolution to design new enzymes," Arnold said. "No one really knows how to design them — they are tremendously complicated.  But we are learning how to use evolution to make new ones, just as nature does."

First, the researchers started with enzymes they suspected could, in principle, chemically manipulate silicon. Next, they mutated the DNA blueprints of these proteins in more or less random ways and tested the resulting enzymes for the desired trait. The enzymes that performed best were mutated again, and the process was repeated until the scientists reached the results they wanted.

Arnold and her colleagues started with enzymes known as heme proteins, which all have iron at their hearts and are capable of catalyzing a wide variety of reactions. The most widely recognized heme protein is likely hemoglobin, the red pigment that helps blood carry oxygen. 

After testing a variety of heme proteins, the scientists concentrated on one from Rhodothermus marinus, a bacterium from hot springs in Iceland. The heme protein in question, known as cytochrome c, normally shuttles electrons to other proteins in the microbe, but Arnold and her colleagues found that it could also generate low levels of organo-silicon compounds. 

After analyzing cytochrome c's structure, the researchers suspected that only a few mutations might greatly enhance the enzyme's catalytic activity. Indeed, only three rounds of mutations were enough to turn this protein into a catalyst that could generate carbon-silicon bonds more than 15 times more efficiently than the best synthetic techniques currently available. The mutant enzyme could generate at least 20 different organo-silicon compounds, 19 of which were new to science, Arnold said. It remains unknown what applications people might be able to find for these new compounds.

"The biggest surprise from this work is how easy it was to get new functions out of biology, new functions perhaps never selected for in the natural world that are still useful to human beings," Arnold said. "The biological world always seems poised to innovate."

In addition to showing that the mutant enzyme could self-generate organo-silicon compounds in a test tube, the scientists also showed that E. coli bacteria, genetically engineered to produce the mutant enzyme within themselves, could also create organo-silicon compounds. This result raises the possibility that microbes somewhere could have naturally evolved the ability to create these molecules.

"In the universe of possibilities that exist for life, we've shown that it is a very easy possibility for life as we know it to include silicon in organic molecules," Arnold said. "And once you can do it somewhere in the universe, it's probably being done." [Best Places to Seek Alien Life in Our Solar System]

It remains an open question why life on Earth is based on carbon when silicon is more prevalent in Earth's crust. Previous research suggests that compared to carbon, silicon can form chemical bonds with fewer kinds of atoms, and it often forms less complex kinds of molecular structures with the atoms that it can interact with. By giving life the ability to create organo-silicon compounds, future research can test why life here or elsewhere may or may not have evolved to incorporate silicon into biological molecules.

In addition to the astrobiology implications, the researchers noted that their work suggests biological processes could generate organo-silicon compounds in ways that are more environmentally friendly and potentially much less expensive than existing methods of synthesizing these molecules. For example, current techniques for creating organo-silicon compounds often require precious metals and toxic solvents.

The mutant enzyme also makes fewer unwanted byproducts. In contrast, existing techniques typically require extra steps to remove undesirable byproducts, adding to the cost of making these molecules.

"I'm talking to several chemical companies right now about potential applications for our work," Arnold said. "These compounds are hard to make synthetically, so a clean biological route to produce these compounds is very attractive."

Future research can explore what advantages and disadvantages the ability to create organo-silicon compounds might have for organisms. "By giving this capability to an organism, we might see if there is, or is not, a reason why we don't stumble across it in the natural world," Arnold said.

The research was funded by the National Science Foundation, the Caltech Innovation Initiative program, and the Jacobs Institute for Molecular Engineering for Medicine at Caltech.

This story was provided by Astrobiology Magazine, a web-based publication sponsored by the NASA astrobiology program. Follow Space.com @Spacedotcom, Facebook and Google+. Story posted on Space.com. 

A Dragon returns, North Korea tests a rocket engine, a wheel breaks on Mars and a spacewalk streak begins — it's Space.com's top news stories of the week.

Transition begins

U.S. President Donald Trump signed a bill authorizing a $19.5 billion budget for NASA in 2017, setting its priorities for the year. Vice President Mike Pence also announced that he would head a re-established National Space Council, which will advise the president on space issues. [Full Story: President Trump Signs NASA Authorization Bill]

Dragon came back

SpaceX's Dragon spacecraft splashed down in the Pacific Ocean, bringing more than 2 tons of science experiments and gear back from the International Space Station. Officials retrieved the spacecraft and had time-sensitive scientific samples in the lab within 48 hours. [Full Story: SpaceX Dragon Capsule Returns to Earth with Ocean Splashdown]

Uncertainty engine

North Korea announced that they had performed a ground test of a powerful rocket engine suited to lifting satellites — or missiles — off the ground. [Full Story: North Korea Tests Rocket Engine: Is It for Satellites or Ballistic Missiles?]

Wear and tear

NASA's Curiosity Mars rover has two small breaks in the treads of one of its six heavy wheels. Curiosity has already lasted well beyond its original two-year mission lifetime, and it should be able to continue unhindered by this latest wear and tear. [Full Story: Breaks Found in Curiosity Mars Rover's Wheel]

(Tiny) thrusters, on!

New, dime-size ion thrusters could keep constellations of small satellites in orbit — or, working together, propel astronauts to Mars. The newly-developed "thruster chips" eject ions drawn from a salt-based propellant solution. [Full Story: Dime-Size Thrusters Could Propel Satellites, Spacecraft]

No laughing matter

The mysterious dark matter seems to make up a large percentage of the matter in the universe, but — perplexingly — early galaxies seem to not have very much of it. [Full Story: Lack of 'Dark Matter' in Early Galaxies Perplexes Astronomers]

Colorful cocoon

Three observant satellites, called Swarm, have mapped the complex magnetic field of Earth's crust and outer mantle, known as its lithosphere. The signal is tiny, but illuminating about the Earth's history. (And one city in Africa has a mysteriously sharp and strong magnetic spike.) [Full Story: Earth's Magnetic Cocoon Mapped in Extreme Detail]

Maybe no flow

Mars' famous dark streaks, thought to be caused by briny flows of water, may come from bone-dry landslides instead, new work suggests. [Full Story: Mars' Dark Streaks May Be Caused by Dry Landslides, Not Water]

But then again: Massive Martian Slopes May Harbor Ice

Going way back

A radio telescope that could probe the dawn of the universe's first stars and galaxies received a new grant, boosting its potential size from 240 huge antenna dishes to 350 — letting it look even further back in time. [Full Story: Telescope Eyeing Universe's First Stars Gets (Another) Big Boost]

Out there

Two astronauts completed a spacewalk Friday (March 24) to help prepare the International Space Station for future commercial spacecraft to dock and bring crew. It is the first of three spacewalks in a row, the third of which will be the station's 200th spacewalk. [Full Story: Space Station Astronauts Poised to Tackle Three-Spacewalk Streak]

Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com. 

Each week we uncover the most interesting and informative articles from around the world, here are 10 of the coolest stories in science this week.

Hardy microbes

A host of hardy microbes may be living miles beneath the ocean floor, new research suggests.

"Although we cannot pinpoint the exact origin of the organic matter, chemical analysis of the constituents resembles molecular signatures that could be produced by microbial life deep within or below the mud volcano," the researchers wrote in the study. [Read more about deep life.]

Elephantiasis

A rare condition called elephantiasis, which tends to strike people in tropical parts of the world, was long thought to occur due to a parasitic infection. But a new study shows that the condition can have another cause: sharp crystals found in certain soils.

But in 2014 and 2015, there were reports of two intense outbreaks of elephantiasis in western Uganda, an area that is not known to harbor these worms. [Read more about the rare condition.]

'Sterile' Neutrinos

Proposed elusive subatomic particles that only fleetingly interact with matter through gravity may not exist, at least if new data from a nuclear reactor is any indication.

"Among the possible explanations, the most exciting one is that we have a new piece of physics," such as sterile neutrinos, said Kam-Liu Bak, the spokesperson for the Daya Bay Collaboration. "That explanation is now unlikely." [Read more about the particles.]

Strange culture

Behind the scenes at the University of Aberdeen Museums, researchers are using high-tech 3D imaging software to reveal what is inside a 2,000-year-old Egyptian cat mummy and other ancient mummified artifacts in the museums' collections.

Household pets were commonly mummified and buried alongside their owners in ancient Egypt. Many were also preserved as sacred offerings to the gods, and an entire industry existed that is thought to have churned out more than 70 million animal mummies, BBC News reported. [Read more about cat mummies.]

Beauty and the Beast

The recent live-action movie "Beauty and the Beast" (Disney, 2017) is the latest retelling of the "tale as old as time" — a tender and life-changing romance between a human and a nonhuman animal that is ultimately made human by love — which has captivated audiences of all ages for hundreds of years.

What explains the enduring attraction of these myths and stories, and what might our affection for these bizarre pairings tell us about ourselves? [Read more about the fascination.]

Odd exhibit

In 1977, the Explorers Club in New York received an unusual gift. Unable to attend a whaling exhibition that the club was hosting, Mr. and Mrs. Frederick S. Schauffler sent their regrets in a note … along with a sperm whale foreskin, stuffed and mounted on an oak base.

According to a report by the Nantucket Historical Association, Sanderson was born in Ohio in 1874, but he lived his final years on the Massachusetts island of Nantucket. Sometime in the 1920s, he bought a historic Nantucket home and began filling it with whaling artifacts, such as "harpoons, boarding knives, cutting spades and bomb lances, as part of a careful interior restoration of that house," Aimee Newell wrote in the NHA report. [Read more about the strange artifact.]

Moon Enceladus

Saturn's icy moon Enceladus is looking more and more like a habitable world.

These reactions depend on the presence of molecular hydrogen (H2), which, the new study reports, is likely being produced continuously by reactions between hot water and rock deep down in Enceladus' sea. [Read more about the energy source.]

Lost art

They may now be underwater, but the oldest rock art paintings in southern Africa are about 5,000 years old, far more ancient than previously realized, a new study finds.

The researchers then used a novel technique to isolate the paint fragments before dating them, as well as 13 other fragments from rock art sites across southern Africa, including in Lesotho and South Africa. [Read more about the ancient rock art.]

Learning bots

When Microsoft released an artificially intelligent chatbot named Tay on Twitter last March, things took a predictably disastrous turn. Within 24 hours, the bot was spewing racist, neo-Nazi rants, much of which it picked up by incorporating the language of Twitter users who interacted with it.

The researchers experimented with a widely used machine-learning system called the Global Vectors for Word Representation (GloVe) and found that every sort of human bias they tested showed up in the artificial system. [Read more about artificial intelligence.]

Elon's odds

I've been imagining a network of electrodes clinging to my brain, trying, with increasing mechanical desperation, to unwind all of these thoughts and figure out what to do with them.

That's what tech guru Elon Musk wants: neural implants that would connect to the human brain and merge the mind with a machine. [Read more about brain hacking.]

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