As for molybdate, it only forms in the presence of oxygen. The atmosphere of the early Earth appears to have been nearly oxygen-free.

At the moment, Mars looks more promising to Dr. Benner. The evidence gathered by satellites and rovers suggests that both oceans and continents existed early in the planet’s lifetime. Under those conditions, borate might have formed.

Just this June, some more evidence emerged that supports this idea. Studying a meteorite from Mars, scientists at the University of Hawaii reported that it contained high levels of boron, a component of borate.

The atmosphere of early Mars also shows signs of having contained oxygen, enabling molybdate to form. With a supply of both borate and molybdate, Mars might have been a favorable place for RNA to emerge, and for life to start. A giant impact on the Red Planet could then have kicked up microbe-laden rocks, which later fell to Earth.

In his recent lecture, Dr. Benner did not present this argument as proof that we are Martians. Instead, he offered a way to organize our thinking about the origin of life. One of his lecture slides was entitled “A Logic Tree.” It displayed a series of linked questions that scientists should ask themselves.

The first question is, did life start out as RNA? If the answer is no — and some scientists believe that to be the case — then they have to grapple with a different set of challenges to explain the origin of life.

For scientists who do accept an RNA-based origin of life, however, they need to find chemistry to produce it. Dr. Benner has one hypothesis. If scientists don’t like it, then it’s up to them to find an alternative — which other scientists are indeed doing.