That would make it more likely that life originated on our planetary neighbor, Benner says. Martian microorganisms could have reached Earth on meteorites, flung away from the Red Planet's surface by cosmic impacts.
Benner's hypothesis "is a neat idea, but not yet proven," says biochemist William Bains of the Massachusetts Institute of Technology in Cambridge. Some theories for the origin of life do not need molybdenum at all, Bains says, and scientists don't know for sure whether early Earth was completely covered in water while early Mars was not.
Astrobiologist Paul Davies of Arizona State University, Tempe, agrees that Benner's argument "greatly strengthens the case" for Mars as the first home of terrestrial life. But, he adds, "It comes down to probabilities. The case is suggestive but not overwhelming." Even if early life existed on Mars, he says, it would be hard to prove that those life forms planted the seeds of our own existence. "In fact, because the traffic of [meteoritic] material between Earth and Mars is so prolific, once life gets going on one it will be transferred to the other very quickly, making the place of origin almost impossible to discern."
Astrochemist Pascale Ehrenfreund of George Washington University in Washington, is a bit more optimistic about resolving the issue. Laboratory experiments under conditions that resemble early Mars might lead to realistic answers, she says. But she doesn't find Benner's "interesting idea" convincing.
Benner himself concedes that scientists may never know how and where life emerged. "We will likely need to be satisfied with answers to a more indirect question: How might life have emerged?" Finding martian life, either extant or extinct, could help by revealing information about ancient martian biochemistry. "This could lead to an 'Aha!' moment that opens new thinking relevant to the historical question."