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Tuesday, April 16, 2024

Did the Seeds of Life Ride to Earth Inside an Asteroid?

Billions of years ago, our solar system coalesced within an interstellar molecular cloud, a nursery made up of gas and dust that clumped together to form stars, asteroids, and planets—eventually, our own Earth. Somewhere along that cosmic timeline, the amino acids that preceded life appeared. These molecules chain together to form the proteins responsible for nearly every biological function. But where those amino acids come from has been an enduring mystery. Did these biological building blocks somehow arise from the prebiotic conditions of early Earth, or was our planet seeded with these ingredients from elsewhere in the universe? 

Some astronomers think life’s heritage must have begun off-planet, because amino acids have been discovered in meteorites, celestial time capsules composed of the same primitive materials from which our solar system formed. (A meteorite is a fragment of an asteroid or any other space rock that has fallen to Earth.) But despite their best efforts, scientists can’t pin down exactly how these molecules got there. Experiments in the lab can’t reproduce what’s found in nature. 

A team of researchers at NASA’s Cosmic Ice Laboratory set out to investigate this discrepancy by simulating the chemical activities of interstellar molecular clouds and asteroids, two places known to form amino acids.  While they didn’t solve the mystery, the results they published in early January hint that something complicated is happening to produce the distribution of materials found in meteorites. 

Knowing where these amino acids come from could say something about the possibility of life elsewhere in the cosmos, says Danna Qasim, an astrochemist at Southwest Research Institute who led the study. If they came from asteroids in our own solar system, it might mean these ingredients are unique to our region of the universe. But if they were birthed by our parent molecular cloud, Qasim says, “that tells us this cloud essentially has a frozen starter kit to life that’s been distributed to other solar systems—and potentially other planets.” 

Amino acids are easy enough to create. Past studies have shown that, under the right conditions, they arise when cosmic rays irradiate interstellar ice, and from the chemistry churning inside the bellies of asteroids. Short chains of amino acids can even spontaneously form on stardust. But other experiments prove that these molecules could have once been generated on our planet: inside ancient, deep sea hydrothermal vents, or when lightning struck the organic molecular soup of early Earth

Yet these molecules by themselves—and even the proteins they form—are not life, any more than a silicon wafer alone is a computer, says study co-author Jason Dworkin, an astrobiologist at NASA Goddard Space Flight Center. “That wafer is necessary if organized in a particular way, connected to a power supply, and encoded with software that permits it to do something,” he says. Similarly, the true seeds of life must be able to carry out characteristic functions like making energy, replicating, and passing down traits to offspring. 

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Nailing down the source of prebiotic amino acids, then, is a first step toward uncovering the processes that trigger biology. Still, it’s been hard to figure out which of these pathways—stardust or primordial soup, undersea vents or irradiated space ice—lead to life. “Getting amino acids is relatively straightforward,” says Dworkin. “But getting the amino acids used in biology is more of a mystery.” 

Nearly a hundred different types of amino acids have been observed in meteorites, but only a dozen of the 20 that are essential for life have been found. Biological amino acids also have a peculiarity that gives them away: They all have a “left-handed” structure, whereas abiotic processes create left- and right-handed molecules in equal measure. Several meteorites discovered on Earth have an excess of left-handed amino acids, Dworkin says—the only non-biological system ever observed with this imbalance. 

For this experiment, the team tested the theory that amino acids were first created within interstellar molecular clouds, then rode to Earth inside asteroids. They decided to recreate the conditions these molecules would have been exposed to at each stage in their journey. If this process produced the same assortment of amino acids—in the same ratios—as those found in recovered meteorites, it would help validate the theory.

The researchers began by creating the most common molecular ices found in interstellar clouds—water, carbon dioxide, methanol, and ammonia—in a vacuum chamber. Then they bombarded the ices with a beam of high energy protons, mimicking collisions with cosmic rays in deep space. The ices broke apart and reassembled into larger molecules, eventually forming a gunky residue visible to the naked eye: chunks of amino acids. 

Next, they simulated the interior of asteroids, which contain liquid water and can be surprisingly hot: between 50 and 300 degrees Celsius. They submerged the residue in water at 50 and 125 degrees Celsius for different lengths of time. This boosted the levels of some amino acids, but not others. The amount of glycine and serine, for example, both doubled. The alanine content stayed the same. But their relative levels remained consistent before and after the chunks were plunged into the asteroid simulation—there was always more glycine than serine, and more serine than alanine.

This trend is noteworthy, Qasim says, because it shows that conditions within the interstellar cloud would have had a strong influence on the makeup of amino acids inside the asteroid. But ultimately, their experiment ran into the same problem other lab studies have: The distribution of amino acids still didn’t match that found in real meteorites. The most notable difference was the excess of alpha-alanine over beta-alanine in their lab samples. (In meteorites, this typically occurs the other way around.) If there’s a recipe for creating life’s precursors, they hadn’t found it. 

That’s likely because their recipe was too simple, Qasim says: “The next experiments need to be more complicated—we need to add more minerals, and consider more relevant asteroid parameters and conditions.” 

But there’s another possibility. Maybe the meteoritic samples they’ve been using for comparison are contaminated. As the meteorites crash-landed, they could have been changed by their interactions with Earth’s atmosphere and biology, as well as centuries of geological activity that has melted, subducted, and recycled the planetary surface. 

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One way to test this is by using a pristine sample as the starting point: This September, NASA’s OSIRIS-REx mission will bring home something like a 200-gram chunk of the asteroid Bennu.  (That’s 40 times bigger than the last sample we got of untouched space rock.) A quarter of the sample will be analyzed for amino acids, which will help nail down the source of discrepancies between lab studies and meteorites. It could also uncover what other fragile materials are present in asteroids, but can’t survive the trip to our planet without the protection of a spacecraft. That information would help Qasim’s team perfect their recipe. 

The rest of the Bennu sample, like those from the Apollo mission 50 years ago, will be tucked away in airtight containers to give not-yet-born scientists a chance to analyze the asteroid with not-yet-invented techniques and technologies. “This is the legacy of sample returns,” says Dworkin, who is a project scientist for OSIRIS-REx. Lab experiments like these, he says—those simulating the conditions of space—are critical for interpreting these samples. A better understanding of asteroid chemistry will come in handy when analyzing the retrieved space rock, and help scientists figure out which of their theories best match up with nature. 

There’s also a third way to think about this issue: Maybe we are looking too far from home. Maybe the unique conditions that give rise to biology happened here, not in space. 

Yana Bromberg, a bioinformatician at Rutgers University, thinks the secret to life will be found in Earth-based biological records, rather than geological ones. “Rocks have a tendency to get ground up and cycled,” she says. “It’s hard to trace history this way.” Instead, Bromberg looks for the genetic blueprints for making cellular energy, a process that could have been invented by—and inherited from—ancient proteins created from Earth’s initial ooze. Last year, she published work showing similarities in the cores of modern proteins used by different organisms, hinting that they may trace back to the same ancestry.

But while she favors a planetary origin, Bromberg doesn’t think only Earth could give rise to life: “My suspicion is that you can make amino acids from any primordial soup, regardless of the planet you are on,” she says.

“Maybe there is this special, unique, niche environment that only existed in one place, and then things got spit out. That would be cool to know,” says planetary scientist Aaron Burton, who analyzes astromaterials at NASA’s Johnson Space Center to understand what chemical processes could have led to life. His gut tells him that biology emerged on Earth—but that’s not the impetus driving his research. “Wherever we think it started, how did it start there? That, for me, is the interesting question. And then we’ll answer ‘where’ along the way.” 

It’s possible that the answer to whether life started on Earth or in space is: both. Maybe in Earth’s case, “space was irrelevant except for the delivery of raw materials,” Dworkin says, and everything important subsequently happened here. But it’s also possible that the same chemical processes are also playing out in deep space—they do, after all, use the same ingredients. That could mean there are many environments brimming with the potential for life in our universe, both on the ground and in the heavens.

Update 2.21.2023 12:15 ET: This story was updated to correct an error about the prevalence of alpha-alanine over beta-alanine amino acids in the lab samples.

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