We’ve spent billions of dollars trying to find evidence of life on Mars, but have so far come back empty handed. We shouldn’t consider those billions a waste, though. The research the money has funded keeps validating the conviction that we’re right to keep searching—and that we’re likely to soon find what we’re looking for. The latest such affirmation comes from a study just published in Nature, where researchers report the oldest evidence for life on Earth. Life may have existed as early as 4.3 billion years ago, merely 200 million years after the Earth’s crust was thought to have been formed. Quartz has you covered for everything you need to know about the discovery. What’s more intriguing, however, is how the finding bolsters the case for searching for life on Mars, and may even narrow down the kinds of places on the red planet where we should look. Hot Earth, Cool Mars Conditions on early Earth and Mars would’ve been like a scene from hell. For hundreds of millions of years after the planets were formed, they would’ve been regularly bombarded with asteroids and comets much bigger than the one we believe destroyed the dinosaurs on Earth. In April 2016, researchers suggested that though Mars would’ve been colder than Earth because it’s farther than the sun, bombardment of comets would’ve created periods of high temperature that would’ve led to habitats favorable for life. In any case, after the period of heavy bombardment was over, liquid water (probably brought by comets) would’ve existed in abundance both on Mars and Earth. The evidence that liquid water existed on Mars in the past is seen today in channels carved by floods and valleys of ancient river networks. NASA even believes there may be briny liquid water on the red planet even today. What’s more is that along with the right temperature and liquid water, Mars also had two other ingredients needed for life as we know it: the right elemental nutrients, namely carbon, hydrogen, oxygen, nitrogen, and phosphorus; and an atmosphere that protected the surface from harsh cosmic radiation and solar flares. “Mars was warmer and wetter than Earth at the time,” says Crispin Little, a researcher at the University of Leeds and co-author of the March 1 Nature study. “The likelihood of life existing on Mars would have been higher.” Crispin is not the first to float this idea. Steve Benner of the Westheimer Institute of Science and Technology has gone a step further. He believes life may have first evolved on Mars and blasted off to Earth by an asteroid impact. We know that life can be incredibly hardy, so it’s not impossible that living things survived deep space travel on asteroids. The foundation for Benner’s theory is the planetary distribution of two other elements: boron and molybdenum. Based on what we know about life today and walking backwards to guess at what would have been required to trigger early life, Benner says these two elements would’ve been needed under the right chemical conditions to cajole lifeless molecules into creating life. His analysis of Martian meteorites suggest that the right forms of boron and molybdenum were only available on Mars and not on Earth. Hence, life first evolved on Mars and then came to Earth. “It’s lucky that we ended up here nevertheless, as certainly Earth has been the better of the two planets for sustaining life,” Benner told Space.com. “If our hypothetical Martian ancestors had remained on Mars, there might not have been a story to tell.” Needle in a haystack Dominic Papineau, a researcher at the University College London who led the recent study that found the oldest fossil on Earth, doesn’t agree with Benner’s theory. “Microorganisms that are known to survive such conditions are quite advanced,” Papineau says. “These pathways are, I think, a little bit too complex to have arisen so early and have resisted space and then seeded Earth.” In other words, life could not have been hardy from the get go. Even if Martians never traveled to Earth on asteroids, Papineau’s discovery on Earth helps us limit what kind of rocks we should look at on Mars in our search for life there. The oldest fossils on Earth have been found in minerals formed in hydrothermal vents, which are giant structures created on the seafloor when fissures open to release some of the chemicals from the planet’s hot, iron-rich interior. Scientists believe these sorts of vents may have also existed on Mars. Papineau thinks we should be searching for rocks on Mars that once belonged to such vents. He believes they should be easy to spot, maybe even with the cameras that Mars rovers are carrying. After all, he spotted the rock that contained the oldest fossils on Earth while simply walking past them. The only “eyes” with the resolution needed to spot such rocks we have on the red planet are those belonging to NASA’s Opportunity and Curiosity rovers. For a planet that has a surface area nearly 10 times that of Russia, any help narrowing down the type of rock we are looking for would go a long way in speeding up the rovers’ job. Additional reporting by Michael Tabb.