There are a lot of good ideas floating around about the conditions necessary for life to develop, said Peter Girguis, professor of organismic and evolutionary biology in the Faculty of Arts and Sciences and co-director of the Harvard Microbial Sciences Initiative.
What’s not known — and perhaps not provable — is whether life began on Earth, or if it arrived here from somewhere else. It’s an idea known as panspermia.
It’s a “far out” hypothesis Girguis acknowledged, and one that makes a lot of scientists uncomfortable. It’s theoretically possible that some proto-microbe or bacterial spore hitched a ride to Earth on a meteorite billions of years ago. But science requires reproducibility, and with a data point of one, it’s difficult to prove either way.
“We’re running into the very edges of our ability to use science to address some of these questions,” he said.
Without a time machine, it’s unlikely we’ll prove panspermia one way or the other. But the broader question — whether life exists beyond Earth — is perhaps easier to tackle. In September, NASA reported that a Mars rover discovered chemical compounds that could be evidence of microbial life from billions of years before. It’s the closest sign to date of life on other worlds, but it’s far from a smoking gun.
“I can’t assert that life exists on another planet because I have no data to support that,” Girguis said. “But as a curious and open-minded person, I would strongly argue that I have no scientific data to refute that, either. So if we’re talking about life elsewhere in the solar system or in the universe, I personally lean towards, ‘Yeah, maybe.’”
Also interested in the question of life’s origin is David Kring, the 2025-2026 Edward, Frances, and Shirley B. Daniels Fellow at the Harvard Radcliffe Institute and a principal scientist at the Lunar and Planetary Institute of the Universities Space Research Association.
Kring is a principal author of the impact origin of life hypothesis, which suggests that heavy asteroid bombardment about 4 billion years ago created hydrothermal environments rich in the kinds of chemicals that could lead to life. His team found the remnants of a microbial ecosystem in a hydrothermal system beneath the floor of the Chicxulub impact crater in Mexico.
But, he said, it’s one of several reasonable hypotheses.
“I don’t champion the idea. That’s a notion in science that I find offensive: It means you no longer have an open mind. I just want to know what the right answer is; whatever nature did is going to be interesting.”
Between about 4.4 and 3.8 billion years ago, Earth and other inner solar system planets experienced a period of bombardment heavy enough, in some cases, to vaporize entire oceans. The impacts would have churned the Earth’s crust and created subsurface hydrothermal systems and chemical environments conducive to early life.
Unfortunately, Kring explained, “That period in Earth history is largely erased from the geologic record on our planet.” But there may be another way to get some answers: The same asteroids that bombarded Earth also did a number on the moon.
“By collecting samples of impact craters or impact basins on the moon, we can determine their age and the types of asteroids or comets hitting the Earth-moon system,” Kring said. “Because those impacts jettisoned pieces of early Earth toward the moon, we also have an opportunity, if our minds are open to it, to find bits and pieces of early Earth history there.” Kring has his eye on NASA’s 2028 Artemis IV mission, which is expected to send a crew to the surface of the moon for the first time in more than 50 years.