New research demonstrates that asteroids and meteorites can give scientists keen insight into when and how molecules initially formed on Earth.
Nicholas Hud, the director of the NSF-NASA Center for Chemical Evolution at the Georgia Institute of Technology, believes that asteroid “time capsules” show what molecules originally existed in the solar system, giving scientists the starting point they need to reconstruct the complex pathway that got life started on Earth.
Hud presented his findings as a panelist at the “Asteroids for Research, Discovery, and Commerce” press briefing on Feb. 17 during the 2018 Annual Meeting of the American Association for the Advancement of Science (AAAS) in Austin, TX. He said that finding molecules in asteroids provides the strongest evidence that such compounds were present on Earth prior to life being formed.
Knowing exactly what molecules were present could help establish the initial conditions that led to the formation of amino acids and related compounds that came together to form peptides—small protein-like molecules that may have kicked off life on Earth.
“We can look to the asteroids to help us understand what chemistry is possible in the universe,” Hud said in a statement. “It’s important for us to study materials from asteroids and meteorites, the smaller versions of asteroids that fall to Earth, to test the validity of our models for how molecules in them could have helped give rise to life.
“We also need to catalog the molecules from asteroids and meteorites because there might be compounds there that we had not even considered important for starting life,” he added.
For several years, NASA scientists have been analyzing compounds found in asteroids and meteorites.
The Miller-Urey experiment, conducted in 1952 to simulate conditions believed to have existed on the early Earth, discovered more than 20 different amino acids, organic compounds that are the building blocks for peptides.
“If you model a prebiotic chemical reaction in the laboratory, scientists can argue about whether or not you had the right starting materials,” Hud said. “Detection of a molecule in an asteroid or meteorite is about the only evidence everyone will accept for that molecule being prebiotic. It’s something we can really lean on.”
Since the Miller-Urey experiment—where researchers discovered that water, methane, ammonia and hydrogen all likely existed in the atmosphere when the Earth was very young—scientists have demonstrated the feasibility of other chemical pathways to amino acids necessary for life.
Hud used cycles of alternating wet and dry conditions to create complex organic molecules over time. Under such conditions, amino acids and hydroxyl acids—compounds that differ chemically by just a single atom—could have formed short peptides that led to the formation of larger and more complex molecules.
“We now have a really good way to synthesize peptides with amino acids and hydroxy acids working together that could have been common on the early Earth,” Hud said. “Even today, hydroxy acids are found with amino acids in living organisms – and in some meteorite samples that have been examined.”
According to Hud, there are many possible ways that molecules could have been formed.
“What we find is that these compounds can form molecules that look a lot like modern peptides, except in the backbone that is holding the units together,” Hud said. “The overall structure can be very similar and would be easier to make, though it doesn’t have the ability to fold into as complex structures as modern proteins.
“There is a tradeoff between the simplicity of forming these molecules and how close these molecules are to those found in contemporary life,” he added.
A better understanding of early Earth conditions could give scientists a stronger foundation for hypothesizing what could have taken place during the planet’s early days, while offering hints to other pathways that may not have been considered yet.
“There are probably a lot more clues in the asteroids about what molecules were really there,” Hud said. “We may not even know what we should be looking for in these asteroids, but by looking at what molecules we find, we can ask different and more questions about how they could have helped get life started.”