Imagining early Earth as an exoplanet may help us search for extraterrestrial life, scientists say


What if we considered our ancestors to be an extraterrestrial life form?

Such an exercise could be one way to test our hypotheses about how habitable conditions arise and are maintained, and perhaps even how life evolved from simple microbes to something more complex, have heard from reporters during a May 18 online presentation held by the American Geophysical Union as part of its astrobiology-focused AbSciCon conference.

“Old Earth is a kind of exoplanet,” said Aaron Goldman, a biology professor at Oberlin College who studies the emergence of life. He said the comparison works because early Earth was so different from what we know today.

Related: The Earth’s atmosphere is full of microbes. Could they help us find life on other worlds?

Examining Earth’s life history, he said, provides an accessible framework for understanding how microbes may behave on other planets, though such avenues may not tell us all. the story. But through his research on evolution, Goldman and his collaborators try to understand the scenarios by which life can arise.

Life today, he says, is cellular and DNA-based, but neither cells nor DNA were present in the earliest life forms on Earth. By association, “these are things that must have evolved early in evolutionary history,” he said. “So understanding how and why these features of life evolved on early Earth may indicate whether or not we expect the same things to be present in extraterrestrial organisms.”

However, assuming that life evolved elsewhere similar to that on Earth is just one way scientists approach astrobiology. Heather Graham, an organic geochemist at NASA’s Goddard Space Flight Center in Maryland, focuses instead on molecules.

One methodology by which her group performs these studies, she said, is to look at molecular complexity and then extend it to populations (or assemblies) of molecules. “We can say what the probability was that this molecule or this assembly came from a living system,” Graham said.

It’s a big universe, but how many worlds could support life? (Image credit: NASA/JPL-Caltech)

“We’re also working on how to use scaling laws, which we think are very universal in life,” she said. Elemental stoichiometry, which focuses on the balance between different elements, is one possible tool that allows scientists to see how organisms build molecules and, in turn, learn about metabolism. The technique, she said, can help “break away” from assumptions about how life began.

Adrienne Kish, a microbiologist at the National Museum of Natural History in Paris, said we also need to remember how life can arise in places on Earth that are completely inhospitable to humans. An example is hydrothermal vents deep in the ocean.

“I’m in the lab and learning exactly the molecular machinery of what the living environment is that the human body just can’t handle,” she said. “We look very deeply at the mechanics of life at a very basic level, and then try to imagine now: Could something similar have happened on another planet? What does that tell us about our own planet and the potential for life elsewhere?

Hydrothermal vents, although intolerable to humans, are home to crabs, tube worms and many other types of life. (Image credit: Ralph White via Getty Images)

Betül Kaçar, a biologist at the University of Wisconsin-Madison, pointed out that life is innovative and finds “really creative ways to use what’s in its environment.” Kaçar’s lab seeks to create experimental tools to recreate ancient DNA.

“It’s like thinking about our planet’s past as an utterly alien environment,” Kaçar said, but pointed out that our lack of understanding at the molecular level is holding us back.

“We like to think of ourselves as molecular time travelers. We resuscitate ancient DNA inside modern organisms, to reprogram modern insects to behave as they once did. So we really want to understand this dance between life itself and the environment.”

Being a biologist, Kaçar said, is a vocation where you can study life across the universe. “Astrobiologists are going to change that,” she said of the field of biology. “And that makes our job really, really special.”

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