Complexities of the quest for life

Astrobiologist Robert Hazen talks cosmic origins

Astrobiology is the field of science that strives to answer those questions we all ponder on long bus rides and sleepless nights: How did life originate? Are we alone in the universe? What does the future entail for humans?

These questions are as exciting to think about as they are difficult to answer. Astrobiologists are largely divided in their theories concerning all three. Take the first question – how did life begin? Scientists have proposed more recipes for life than there are for chicken soup. Some believe the first self-replicating molecules formed in a “primordial soup” of organic compounds. Others think microbes could have hitched a ride from Mars to Earth on an ancient meteorite. Still others suggest that life could have begun within cracks in the ice on Earth.

Beginnings of life

“All scientists see the origin of life through their own lens,” said Robert Hazen, an astrobiologist, mineralogist, and researcher at Carnegie Institution of Washington’s Geophysical Laboratory, when speaking at McGill on September 30. Hazen believes minerals played a key role in life’s beginnings.

The compounds necessary for life can form almost anywhere, he explained. Scientists have been able to synthesize organic molecules in conditions similar to Earth’s early atmosphere, deep sea hydrothermal vents, and even space. The unknown is not how these compounds were created – it’s how they came together as self-replicating molecules.

Although water is a requisite for the origin of life as we know it, it would have inhibited the formation of early, self-organizing molecules. Many organic compounds are unable to polymerize, or come together, in water. Though there are many ways these compounds could have formed on Earth, there must have been another factor involved that allowed them to come together and form polymers.

“What you need is some kind of surface that attracts, selects, and concentrates these compounds,” explained Hazen. “And this is where minerals come in and save the day.” Minerals allow for the polymerization of organic compounds, which cannot occur in solution.

These minerals may also explain another of life’s great mysteries. Like hands come in a left and right form, there are also molecules that come in right and left ‘handed’ configurations. Called chiral, they are the exact same, except that they are mirror images of each other. Although this may seem like an insignificant difference, it changes everything when it comes to reactivity. For example, one chiral form of the hydrocarbon limonene smells like lemons. The other smells of oranges. For some unknown reason, life selected only one ‘handed’ form of chiral compounds. Some believe this was but an accident. Hazen thinks otherwise, and many scientists agree with him.

“Most rock forming minerals display at least one common chiral surface,” explained Hazen in a 2006 article published in American Mineralogist. Left and right ‘handed’ molecules selectively adhere to mineral faces of different chirality. Thus, molecules of one chiral form would have concentrated on one face, while molecules of the other would have been attracted to different surfaces. Given this tendency, if life arose from the organic compounds adhered to minerals, its selection of only one chiral form seems completely natural.

So, given organic molecules in solution, minerals may have contributed to the formation of organic polymers and life’s chirality. This still leaves one question – how did life become self-replicating? The answer is not clear. “This is the biggest challenge to overcome and it will make the biggest headlines when solved,” said Hazen.

Nowadays, DNA requires proteins to replicate, and proteins require DNA to form, so it would have been impossible for one to form without the other, much like the problem of the chicken and the egg. As of yet, no plausible prebiotic molecule or cycle of molecules has been proposed, though there are some theories: one idea is that RNA, a macromolecule similar to DNA but single-stranded, could have replicated itself, while storing genetic information. However, many scientists assert that it would have been very improbable for a molecule like RNA to spontaneously form on the early Earth. The “metabolism first” theory posits that life might have come from a series of chemical reactions. The reactions and their components would have increased in complexity over time, eventually leading to a sort of self-replicating chemical system. Many scientists are critical of this hypothesis as well, arguing that it would have been very unlikely that the steps required to build this process would have occurred on the early Earth.

We simply don’t know how life began. However, we do know how it evolved. Given self-replicating molecules, natural selection is absolutely automatic, explained Hazen. “Molecules that work will be selected over molecules that don’t work.” Thus, survival of the fittest emerged, and 3.5 billion years later, here we are.

Life in other galaxies

Here we are. And naturally, the next big question of astrobiology rears its head — are we here alone? The answer, of course, is not clear. Missions to Mars, Europa and other bodies in our solar system could provide answers. These sites have the requirements for life as we know it— water, an energy source, and organic compounds.

However, it’s entirely possible that life could exist in a form entirely different from our own. Indeed, Hazen revealed a speculative and, as of yet, unpublished finding from the Deep Carbon Observatory, a global community of researchers who work together to study the Earth. The finding could indicate an entirely new domain of life right here on Earth. Researchers drilled into a black smoker, a type of hydrothermal vent, to allow water to pass from the 300 degrees Celsius interior through the hole and out into to the cold temperatures of the deep ocean. They lined this hole with chambers, each of which was a mineral surface, and after six months, returned to collect the findings. Within the chambers closer to the core of the vent, they found a sort of electrolytic gel that appeared to be capable of self-organization, growing laterally perhaps by templating itself.

“Is that something that’s alive and can evolve? Or is it just a coating of goo?” asked Hazen. The answer is – you guessed it – not clear.

“But what would be the fun of science if we knew everything?” he continued. You can’t have discovery if there are no unknowns, and every discovery leads to yet more questions. Such is the nature of science. And such is the nature of the human mind to pursue answers.

From atoms, the basic building blocks of the universe, arose the simplest of self-replicating molecules. From these grew a mind capable of questioning its own existence – and finding answers. In the words of Robert Hazen, “The universe is beginning to know itself.”