Did Life on Earth Come from Space?

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Matt O'Dowd (1:03)

What if the first genesis of life, the A biogenesis, is so unlikely that it only happened once in the entire galaxy, and that once was not on Earth? What if primitive life arrived on Earth after having traveled vast distances across the Milky Way?

Some scientists think this may be the case. This is the panspermia hypothesis.

There's something odd about the first appearance of life on Earth. The oldest fossils are now dated to only a few hundred million years after the moment Earth first became habitable.

Is it really reasonable to imagine that evolution turned an unliving chemical soup into the first true living cells in that geological eye blink? Well, maybe, but it's also possible that life on Earth didn't start on Earth at all. Perhaps it started on a distant world somewhere in the Milky Way and somehow survived a long journey through the void to colonize the early Earth. The plausibility of panspermia comes down to whether or not any living organism can survive its three deadly stages. Ejection from an origin world, travel to the new world, and entry to that world's biosphere. Let's start with the exciting parts, the beginning and the ends of the journey. One way for a budding microbial astronaut to travel the stars is via lithopanspermia, basically be attached to a rock that travels between planets. Now, we know that debris from planetary surfaces can be ejected into space during asteroid or comet impacts. Hundreds of meteorites have been found on Earth that have compositions suggesting they originated from the moon or Mars. Many chunks of Earth have also been ejected into space, most of them from the largest impacts, like the one that killed the dinosaurs. Some of that debris would have contained life, but could hitchhiking microbes have survived that ejection? To escape Earth's gravitational field, a chunk of impact debris has to be kicked to a minimum of 11.2 kilometers per second. That's the escape velocity at Earth's surface. That requires an acceleration of up to hundreds of thousands of Gs and hundreds of thousands of Earth atmospheres in shock pressure. Similar forces apply when it smacks down at the other end of the journey. Temperatures in these rocks can rise to several hundred Kelvin in the impacts and during re-entry. That all sounds pretty unsurvivable, but scientists have engaged in various forms of microbe abuse to test this. This includes shooting high-velocity projectiles loaded with colonies, spitting them up in an ultra-centrifuge, attempting to splat colonies with extreme pressure impact plates. And if that's not realistic enough, microbe-laced rocks have been dropped from space, strapped to the outside of re-entry vehicles. Many life forms have been tested from common bacteria that you find everywhere in the soil, to primitive cyanobacteria, to heat-resistant extremophiles, and even fungus spores and lichens. While survival rates may be low, at least a small fraction of bugs of any of these types can survive the pummeling of high-pressure acceleration.

Exposure to extreme temperature, for example, in atmospheric re-entry, is more problematic. Some extremophiles can survive at temperatures above 100 Celsius, but the surfaces of rocks during impact or re-entry will be much, much hotter than this. And this is where we identify our first microbial contender for panspermia. Endoliths.

These are organisms that live deep within rocks. Deep enough that they're protected from the extremes of temperature.

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When you need to build up your team to handle the growing chaos at work, use Indeed Sponsored Jobs. It gives your job post the boost it needs to be seen, and helps reach people with the right skills, certifications, and more. Spend less time searching and more time actually interviewing candidates who check all your boxes. Listeners of this show will get a $75 sponsored job credit at indeed.com/podcast. That's indeed.com/podcast. Terms and conditions apply. Need a hiring hero? This is a job for Indeed Sponsored Jobs.

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The best part of waking up? A full cup of Folger's Coffee and music on full blast.

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Matt O'Dowd (6:00)