During a keynote speech at a NASA conference a decade ago on the search for extraterrestrial life an attendee shouted out: “We have no idea what’s out there!” One of NASA’s goals is to search for life on other planets like Mars, where there was once flowing water and a thick atmosphere, or moons of the outer solar system like Europa and Enceladus, where vast water oceans churn under thick layers of ice. But what if life on those worlds doesn’t use our DNA? How could we recognize it? A 2019 DNA breakthrough may be the key to answering these questions and many more.
In a research breakthrough funded by NASA, scientists have synthesized a molecular system that, like DNA, can store and transmit information. This unprecedented feat suggests there could be an alternative to DNA-based life, as we know it on Earth – a genetic system for life that may be possible on other worlds.
Re-thinking Life Beyond Earth
DNA is a complex molecule that stores and transmits genetic information, is passed from parent to offspring in all living organisms on Earth, and its components include four key ingredients called nucleotides – all standard for life as we know it. But, what about life on other worlds?
“Life detection is an increasingly important goal of NASA’s planetary science missions, and this new work will help us to develop effective instruments and experiments that will expand the scope of what we look for,” said Lori Glaze, acting director of NASA’s Planetary Science Division.
Alternative chemical systems to DNA that support Darwinian evolution
One way to imagine the kinds of foreign structures found on other worlds is to try to create something foreign on Earth. A team of researchers, led by Steven Benner at the Foundation for Applied Molecular Evolution in Alachua, Florida, successfully achieved the fabrication of a new informational molecular system that is like DNA, except in one key area: The new molecule has eight informational ingredients instead of four.
The synthetic DNA includes the four nucleotides present in Earth life – adenine, cytosine, guanine, and thymine – but also four others that mimic the structures of the informational ingredients in regular DNA. The result is a double-helix structure that can store and transfer information.
Benner’s team, which collaborated with laboratories at the University of Texas in Austin, Indiana University Medical School in Indianapolis, and DNA Software in Ann Arbor, Michigan, dubbed their creation “hachimoji” DNA (from the Japanese “hachi,” meaning “eight,” and “moji,” meaning “letter”). Hachimoji DNA meets all the structural requirements that allow our DNA to store, transmit and evolve information in living systems.
This new molecular system, which is not a new life form, suggests scientists looking for life beyond Earth may need to rethink what they are looking for. Crystal structure of a hachimoji double helix shown below is built from four naturally-occurring bases, G (green), A (red), C (blue), T (yellow), and four synthetic bases, B (cyan), S (pink), P (purple), and Z (orange). Notable is the geometric regularity of the pairs, a requirement for evolution. Credit: Millie Georgiadis, Indiana University School of Medicine.
“By carefully analyzing the roles of shape, size and structure in hachimoji DNA, this work expands our understanding of the types of molecules that might store information in extraterrestrial life on alien worlds,” said Benner.
NASA’s “Earth-Centric” Life Detection Strategies
“Nearly all of the life detection strategies that NASA uses are unimaginatively “Earth-centric”, remarked Steven Benner at the Foundation for Applied Molecular Evolution in an email to The Daily Galaxy. “This hinders NASA In its performance of one of its essential mission functions: To seek out alien life.”
“About 10 years ago, in a book entitled “Life, the Universe, the Scientific Method”, we offered five general strategies to remove NASA’s blinders,” Benner notes in his email. “One of these involves creating alternative chemical systems that can support Darwinian evolution. Our “hachimoji”, or “eight letter” DNA is one of these. Eric Kool at Stanford has another. Shuichi Hoshika at FfAME has several more, including ‘skinny’ and ‘fatty’ alternatives. Any one of these could support alien life on Mars, Europa, and elsewhere where water is found.”
Universal Features in Genetic Molecules
“Importantly, the creation in the laboratory of alternative genetic molecules also identifies structural features that cannot change in genetic molecules,” Benner explains. “These features will be found in informational molecules in life universally, including in the water on Mars. As one of these features, genetic molecules must universally have a repeating backbone charge. Human DNA has a repeating backbone negative charge. Repeating positive charges are possible as well.
“Simple instruments can concentrate very small amounts of such genetic molecules from very large amounts of water,” Benner concludes in his email. “This fact creates the possibility for a truly “agnostic life finding” (ALF) mission. Unfortunately, NASA still has not moved to incorporate such ALFs into its missions to Mars. We are hoping to persuade Elon Musk at SpaceX to incorporate an ALF as part of his operation to mine water to make fuel for a return mission.”
Alien Environments Might be Teeming with Exotic Life
Scientists have much more to do on the question of what other genetic systems could serve as the foundation for life, and where such exotic organisms could be found. However, this study opens the door to further research on ways life could structure itself in environments that we consider inhospitable, but which might be teeming with forms of life we haven’t yet imagined.
“Incorporating a broader understanding of what is possible in our instrument design and mission concepts will result in a more inclusive and, therefore, more effective search for life beyond Earth,” said Mary Voytek, senior scientist for Astrobiology at NASA Headquarters.
“The discovery that DNA with eight nucleotide letters is suitable for storing and transmitting information is a breakthrough in our knowledge of the range of possibilities necessary for life,” said Andrew Serazin, president of Templeton World Charity Foundation in Nassau, The Bahamas, which also supported this work.
This research was supported by NASA’s Astrobiology Program through the Exobiology Program.
Avi Shporer, formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL) currently an observational astronomer with the MIT Kavli Institute for Astrophysics and Space Research via Steven Benner and NASA/Astrobiology
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