In Nautilus, Tim Folger writes about how scientist are still debating whether organic and inorganic materials found on Martian meteorite ALH84001 contain evidence that life existed on Mars before it existed on Earth. If it did, then life could have spread to Earth from meteorites, which could make human beings ─ and other Earthly life ─ descended from Martians.

While many scientists consider liquid water to be the most essential ingredient for life, Earth may once have harbored too much water. “The best evidence we have suggests that early Earth was completely covered by oceans,” says Kirschvink. Without some dry land, he says, it would have been difficult for the basic chemical ingredients of life to form. “The reason is very simple … if you link two amino acids together to make a protein, you have to remove water.” And that would have been impossible if the amino acids were immersed in an ocean. Life needed some land—literally a beachhead—to get started. Ancient Earth might not have had any dry land, but Mars certainly did.

“All this is controversial since we’re talking about a world 4 billion years ago,” says Kirschvink. “But it’s very clear that Mars had southern highlands, and what is looking more and more like a north polar ocean basin. If you’ve got volcanic terrain sticking up, with rainfall and streams and rivers—if life had managed to get started there, it would have thrived.” That scenario, which seems very likely to Kirshvink, has some remarkable implications: Life, after its genesis on Mars, might have spread from there to Earth, borne here by meteorites. And that would make us—and every other living thing on Earth—the descendants of spacefaring microbes from Mars. According to Kirschvink, we won’t find our first ETs on some other world—we just have to glance in a mirror. “I really think we’re Martians,” he says. For Kirschvink, life on Mars is unlikely to represent the second genesis that McKay is looking for.

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