New evidence presented by Professor Steven Benner at The Westheimer Institute for Science and Technology in Florida suggests that, billions of years ago, Mars was a much better place for the first cells to have formed compared to Earth. This gives more weight to the theory that life may have started on the Red Planet and then found its way to Earth aboard a meteorite.
Life on Earth
The fossil record tells us that life first appeared on our planet around 3.5 billion years ago, but we know very little about how the first cells came to be.
Scientists theorize that, as the Earth slowly cooled down, simple organic compounds (monomers) slowly formed and combined into more complex compounds (polymers). Then, as sea currents pooled these large molecules by "hotspots" like oceanic shores and hydrothermal vents on the sea floor, they may have eventually combined to form the first protocells.
However these building blocks combined, there is growing evidence that the first cells replicated using RNA instead of DNA, and that the switch to DNA replication, which is stabler but harder to achieve, happened only much later in the history of evolution. (One convincing piece of evidence for this is that many of the critical components of cells, which evolve the slowest, are composed mostly or entirely of RNA.)
According to Benner, if the RNA hypothesis is true then the story just doesn't add up.
Scientists believe that when life first appeared on Earth our planet was completely submerged in water and very low in dioxygen. Benner says life couldn't have originated under these conditions because borate and molybdate, two crucial catalysts to the formation of RNA, would have been extremely rare.
"If early Earth really was a 'water world,' then borate concentration would have been difficult," Prof. Benner tells Gizmag. "For molybdate, the problem is that it is highly oxidized (four oxygen atoms for every molybdenum atom), and ancient Earth probably did not have much dioxygen in the atmosphere."
When simple organic molecules dissolved in water are given a source of energy but no borate or molybdate are present, the end result is tar, not RNA. Borate minerals help simple organic molecules form carbohydrate rings, and molybdenum then rearranges these rings to form ribose, a crucial building block of RNA. Both these minerals would have been extremely scarce on early Earth, but abundant on Mars.
Life on Mars
Out of the 120 Martian meteorites we have collected so far, some contain promising evidence to support Benner's theory.
"Analysis of a Martian meteorite recently showed that there was boron on Mars," says Benner. "We now believe that the oxidized form of molybdenum was there too. In addition, recent studies show that these conditions suitable for the origin of life may still exist on Mars."
The prospect of extremophile bacteria surviving the trip to Earth inside a meteorite isn't as far-fetched as it may seem.
"We spend much time on 'planetary protection' so that a launch to Mars does not carry Earth bacteria to forward contaminate Mars, but we find that many bacteria (like radiodurans) can survive the trip, especially if tucked inside of the craft (or, by analogy, within the meteorite)," Benner says.
While there is no "smoking gun," the evidence presented by Prof. Benner gives more credibility to the theory that life may have first started on Mars rather than Earth. It also gives us more hope of finding life back on the Red Planet, and perhaps even elsewhere in the solar system.