— Early Earth may have had too much water for life to take hold.
— Conditions for life may have been better on Mars, which had dry spots.
— Cycles of water and drying may have been needed to give the molecular building blocks for life a chemical toehold.
Given the same raw materials, Mars would have been a better host for life to arise than Earth, which some scientists believe was too flooded for the chemistry of life to gain a toehold.
Without at least occasional dry land, the chemistry needed to get life started doesn't work very well because the molecules to support genetics, such as RNA, are chemically unstable in many ways, particularly in water.
That raises a problem, because life, at least as we know it today, seems to require water.
"How is it possible that the chemicals that we now have supporting modern life, which is so unstable in water, could have arisen in water?" biochemist Steven Benner, head of the Foundation For Applied Molecular Evolution in Gainsville, Florida, told Discovery News.
The answer could be that life evolved in places that occasionally dried out.
"You can get RNA and its building blocks to be stable in an Earth-like environment, provided you put them into some environment that is deficient in water," Benner said, pointing to a place like Death Valley, where there is intermittent rainfall to provide organic compounds from the atmosphere as well as cycles of dryness.
"If you get building blocks for RNA, you get genetics and you're off to the races. You've got life," Benner said.
But there's a catch.
Scientists who model what early Earth was like believe the planet had no dry spots. It was a water world, similar to what was portrayed in the Kevin Costner movie of the same name.
"If Earth had two or three times the amount of water that it has now, there'd be no dry land sticking up," geophysicist Norman Sleep with Stanford University, told Discovery News.
The nearest place that fits the bill is Mars. Though dry today, Mars is believed to have had liquid surface water in the past, albeit never in the amounts found on Earth.
"On Mars, you can have all of the chemistry that we would have wanted to have without having to worry about Mars being a waterworld," Benner said.
Life also could have evolved more quickly there than on Earth because Mars is smaller, cooled faster and didn't have its surface vaporized in a massive collision that formed a moon. It also was more protected than Earth from asteroid bombardments.
"Mars was open for habitation before the 'For Rent' sign goes up on the Earth," Sleep said.
Rocks from Mars regularly fall onto the surface of Earth, providing a possible transport mechanism between the two worlds.
"The old geological record from 4 billion years ago is still present on Mars, so ironically life originating on Mars is more testable than anything on the Earth," Sleep said.
Benner's latest research on the subject is being published in the Journal of American Chemical Society.