Did life begin on Mars and then travel to Earth for its blossoming?
A long-debated and often-dismissed theory known as "panspermia" got new life in the past week, as two scientists separately proposed that early Earth lacked some chemicals essential to forming life, while early Mars likely had them.
First came Steven Benner, an iconoclastic and highly regarded origins-of-life chemist with the Westheimer Institute of Science and Technology in Gainesville, Florida.
Last week, during a keynote talk at the Goldschmidt conference for geochemists in Florence, Italy, Benner said that two elements that allow the precursors of life to form were almost certainly unavailable on early Earth but were likely present on early Mars.
"Basically, we went looking on Mars because the origins-of-life options on Earth just aren't looking very good," Benner said.
One of the stumbling blocks to life starting on Earth is the fact that water is almost universally accepted as necessary for the onset of life. Yet RNA—which many consider to be the earliest expression of genetic replication and another essential precursor to life as we know it—falls apart if you try to build it in water.
What keeps that from happening, Benner has found over years of study, is the presence of a form of the element boron. While geologists say boron was too scarce on early Earth to support any widespread creation of RNA, it was seemingly more abundant on early Mars. One sign of its presence on the red planet is that at least one meteorite has delivered some Martian boron to Earth.
Benner has also found in his lab that if a form of the element molybdenum is added to the mix, the boron-steadied compounds are rearranged to form a stable version of ribose—the "R" in RNA.
Again, the element was far more available on early Mars than early Earth. (See "Naked Science: Finding the Origin of Life.")
So the question arises: Did RNA on Mars lead to actual DNA-based life? And did those lifeforms then travel to Earth on rocks kicked up when a meteorite struck Mars?
"The Phosphate Problem"
A few days after Benner's talk on August 29, a paper appeared in the journal Nature Geoscience that made a similar argument about phosphorus compounds, which form the backbone of RNA, DNA, and proteins.
While phosphates were present on early Earth, said lead author Christopher Adcock of the University of Nevada, Las Vegas, they were most frequently found in a solid state, in which they are most stable. Yet biology is understood to have started in water, which would have contained little of the phosphates on early Earth.
"This has long been called 'the phosphate problem,'" Adcock said in an interview. "There are theories out there about how it might have worked [on early Earth], but there's no consensus.
"That played a part in getting us interested in Mars," he said.
On Mars, Adcock's team concluded, the phosphate problem appears to be much smaller. Adcock and his colleague Elizabeth Hausrath synthesized the two types of phosphates known to be on both early and current-day Mars, compounds that have also been delivered to Earth via meteorites.
Those Martian phosphates turned out to be far more soluble in water and also more abundant. So when it came to essential phosphates, at least, Mars appears to have been a better nursery for life.
Answering the Big Cosmic Question
The reemergence of the theory of panspermia is intertwined with progress (or lack of progress) in a long-term scientific quest to find out how life began on Earth, a question that synthetic biology experts such as Benner have been working on for decades. Despite some advances, the field has come up against chemical walls that are proving impossible to climb.
For instance, Benner said, the organic—meaning carbon-based—compounds understood to have come together to form life in a "prebiotic soup" do not behave in the lab in a way that would indicate they led to the formation of life on early Earth.
When these compounds are energized by heat or light, instead of producing early RNA they create tar—hardly the stuff from which we would all evolve. Yet discoveries over the past decade on Mars have pointed to a planet that was once warmer and wetter than it is now.
No living or fossil organisms have been found on Mars. But the science team working with the rover Curiosity concluded earlier this year that they had drilled into an ancient lake bed that had all that was needed to support life—and consequently that the planet had been habitable. (See "NASA's Mars Rover Makes Successful First Drill.")
That doesn't mean it ever was inhabited, but scientific signs are beginning to point, however hesitantly, in that direction.
Does this mean Benner or Adcock sees panspermia as a likely beginning for life on Earth? Not exactly.
Benner says that "it's yet another piece of evidence which makes it more likely life came to Earth on a Martian meteorite." But it's more of a changing of probabilities than it is scientific proof.
"A panspermia solution, after all, produces another panspermia problem," he said. "If a Martian microbe did make it from Mars to Earth, maybe it would be as if it landed in Eden. But just as likely, it would quickly die."