“I’ll toss my coins in the fountain,
Look for clovers in grassy lawns
Search for shooting stars in the night
Cross my fingers and dream on.”
Like most little kids, I was regaled with tales about genies and wizards who used their magical powers to grant people the desires of their heart. And for a time, I was obsessed with finding some way to make my own wishes become a reality, too. I blew on dandelions, hunted for four-leafed clover, tried to catch fairy insects and looked for shooting stars in the night sky. Unfortunately, nothing worked.
But, that didn’t mean that I gave up on my hopes and dreams. In time, I realized that sometimes my imagination outpaced reality.
I still have hopes and dreams today. Hopefully, they are more realistic than in than the ones I held to in my youth. I even have hopes and dreams about what I might accomplish as a scientist. All scientists do. It’s part of what drives us. Scientists like to solve problems and extend the frontiers of knowledge. And, they hope that they will make discoveries that do that very thing, even if their hopes sometimes outpace reality.
Recently, a team of biochemists turned to a meteorite—a small piece of a shooting star—with the hope that their dream of finding meaningful insights into the evolutionary origin-of-life question would be realized. Using state-of-the art analytical methods, the Harvard University researchers uncovered the first-ever evidence for proteins in meteorites.1 Their work is exemplary work—science at its best. These biochemists view this discovery as offering an important clue to the chemical evolutionary origin of life. Yet, a careful analysis of their claims leads to the nagging doubt that origin-of-life researchers really aren’t any closer to understanding the origin of life and realizing their dream.
Meteorites and the Origin of Life
Origin-of-life researchers have long turned to meteorites for insight into the chemical evolutionary processes they believe spawned life on Earth. It makes sense. Meteorites represent a sampling of the materials that formed during the time our solar system came together and, therefore, provide a window into the physical and chemical processes that shaped the earliest stages of our solar system’s history and would have played a potential role in the origin of life.
One group of meteorites that origin-of-life researchers find to be most valuable toward this end are carbonaceous chondrites. Some classes of carbonaceous chondrites contain relatively high levels of organic compounds that formed from materials that existed in our early solar system. Many of these meteorites have undergone chemical and physical alterations since the time of their formation. Because of this metamorphosis, these meteorites offer clues about the types of prebiotic chemical processes that could have reasonably transpired on early Earth. However, they don’t give a clear picture of what the chemical and physical environment of the early solar system was like.
Fortunately, researchers have discovered a unique type of carbonaceous chondrite: the CV3 class. These meteorites have escaped metamorphosis, undergoing virtually no physical or chemical alterations since they formed. For this reason, these meteorites prove to be exceptionally valuable because they provide a pristine, unadulterated view of the nascent solar system.
The Discovery of Proteins in Meteorites
Origin-of-life investigators have catalogued a large inventory of organic compounds from carbonaceous chondrites, including some of the building blocks of life, such as amino acids, the constituents of proteins. Even though amino acids have been recovered from meteorites, there have been no reports of amino acid polymers (protein-like materials) in meteorites—at least until the Harvard team began their work.
Figure: Reaction of Amino Acids to Form Proteins. Credit: Shutterstock
The team’s pursuit of proteins in meteorites started in 2014 when they carried out a theoretical study that indicated to them that amino acids could polymerize to form protein-like materials in the gas nebulae that condense to form solar systems.2 In an attempt to provide experimental support for this claim, the research team analyzed two CV3 class carbonaceous chondrites: the Allende and Acfer 086 meteorites.
Instead of extracting these meteorites for 24 hours with water at 100°C (which is the usual approach taken by origin-of-life investigators), the research team adopted a different strategy. They reasoned that the protein-like materials that would form from amino acids in gaseous nebulae would be hydrophobic. (Hydrophobic materials are water-repellent materials that are insoluble in aqueous systems.) These types of materials wouldn’t be extracted by hot water. Alternatively, these hydrophobic protein-like substances would be susceptible to breaking down into their constituent amino acids (through a process called hydrolysis) under the standard extraction method. Either way, the protein-like materials would escape detection.
So, the researchers employed a Folch extraction at room temperature. This technique is designed to extract materials with a range of solubility properties while avoiding hydrolytic reactions. Using this approach, the Harvard researchers were able to detect evidence for amino acid polymers consisting of glycine and hydroxyglycine in extracts taken from the two meteorites.3
In their latest work, the research team performed a detailed structural characterization of the amino acid polymers from the Acfer 086 meteorite, thanks to access to a state-of-the-art mass spectrometer that had the capabilities of analyzing low levels of materials in the meteorite extracts.
The Harvard scientists determined that a distribution of amino acid polymer species existed in the meteorite sample.The most prominent one was a duplex formed from two protein-like chains that were 16 amino acids in length, comprised of glycine and hydroxyglycine residues. They also detected lithium ions associated with some of the hydroxyglycine subunits. Bound to both ends of the duplex was an unusual iron oxide moiety formed from two atoms of iron and three oxygen atoms. Lithium atoms were also associated with the iron oxide moiety.
Researchers are confident that this protein-like material—which they dub hemolithin—is not due to terrestrial contamination for two reasons. First, hydroxyglycine is a non-protein amino acid. Secondly, the protein duplex is enriched in deuterium—a signature that indicates it stems from an extraterrestrial source. In fact, the deuterium enrichment is so excessive, the researchers think it may have formed in the gas nebula before it condensed to form our solar system.
If these results stand, they represent an important scientific milestone—the first-ever protein-like material recovered from an extraterrestrial source. A dream come true for the Harvard scientists. Beyond this acclaim, origin-of-life researchers view this work as having important implications for the origin-of-life question.
For starters, this work affirms that chemical complexification can take place in prebiotic settings, providing support of chemical evolution. The Harvard scientists also speculate that the iron oxide complex at the ends of the amino acid polymer chains could serve as an energy source for prebiotic chemistry. This complex can absorb photons of light and, in turn, use that absorbed energy to drive chemical processes, such as cleaving water molecules.
More importantly, this work indicates that amino acids can form and polymerize in gaseous nebulae prior to the time that these structures collapse and condense into solar systems. In other words, this work suggests that prebiotic chemistry may have been well under way before Earth formed. If so, it means that prebiotic materials could have been endogenous to (produced within) the solar system, forming an inventory of building block materials that could have jump-started the chemical evolutionary process. Alternatively, the formation of prebiotic materials prior to solar system formation opens up the possibility that these critical compounds for the origin of life didn’t have to form on early Earth. Instead, prebiotic compounds could have been delivered to the early Earth by asteroids and comets—again, contributing to the early Earth’s cache of prebiotic substances.
Does the Protein-in-Meterorite Discovery Evince Chemical Evolution?
In many respects, the discovery of protein species in carbonaceous chondrites is not surprising. If amino acids are present in meteorites (or gaseous nebula), it stands to reason that, under certain conditions, these materials will react to form amino acid polymers. But, even so, a protein-like material made up of glycine and hydroxyglycine residues has questionable biochemical utility and this singular compound is a far cry from the minimal biochemical complexity needed for life. Chemical evolutionary processes must traverse a long road to move from the simplest amino acid building blocks (and the polymers formed from these compounds) to a minimal cell.
More importantly, it is questionable if the amino acid polymers in carbonaceous chondrites (or in gaseous nebula) made much of a contribution to the inventory of prebiotic materials on early Earth. Detection and characterization of the amino acid polymer in the Acfer 086 meteorite was only possible thanks to cutting-edge analytical instrumentation (the mass spectrometer) with the capability to detect and characterize low levels of materials. This requirement means that proteins found in the Acfer 086 meteorite samples must exist at relatively low levels. Once delivered to the early Earth, these materials would have been further diluted to even lower levels as they were introduced into the environment. In other words, these compounds most likely would have melded into the chemical background of early Earth, making little, if any, contribution to chemical evolution. And once the amino acid polymers dissolved into the early Earth’s oceans, a significant proportion may well have undergone hydrolysis (decomposition) into constituent amino acids.
Earth’s geological record affirms my assessment of the research team’s claims. Geochemical evidence from the oldest rock formations on Earth, dating to around 3.8 billion years ago, makes it clear that neither endogenous organic materials nor prebiotic materials delivered to early Earth via comets and asteroids (including amino acids and protein-like materials) made any contribution to the prebiotic inventory of early Earth. If these materials did add to the prebiotic store, the carbonaceous deposits in the oldest rocks on Earth would display a carbon-13 and deuterium enrichment. But they don’t. Instead, these deposits display a carbon-13 and deuterium depletion, indicating that these carbonaceous materials result from biological activity, not extraterrestrial mechanisms.
So, even though the Harvard investigators accomplished an important milestone in origin-of-life research, the scientific community’s dream of finding a chemical evolutionary pathway to the origin of life remains unfulfilled.