Without the Moon, there would have been no life on Earth.
Four billion years ago, when life began, the Moon orbited much closer to us than it does now, causing massive tides to ebb and flow every few hours. These tides caused dramatic fluctuations in salinity around coastlines which could have driven the evolution of early DNA-like biomolecules.
This hypothesis, which is the work of Richard Lathe, a molecular biologist at Pieta Research in Edinburgh, UK, also suggests that life could not have begun on Mars.
According to one theory for the origin of life, self-replicating molecules such as DNA or RNA emerged when small precursor molecules in the primordial "soup" polymerised into long strands. These strands served as templates for more precursor molecules to attach along the templates, creating double-stranded polymers similar to DNA.
But the whole theory fails without some way of breaking apart the double strands to keep the process going, says Lathe. It would take some external force to dissociate the two strands, he says.
As an analogy, he points to PCR, the technique used to amplify DNA in the lab. DNA is cycled between two temperatures in the presence of appropriate enzymes.
At the lower temperature of about 50 °C, single DNA strands act as templates for synthesising complementary strands. At the higher temperature of about 100 °C, the double strands break apart, doubling the number of molecules. Lower the temperature, and the synthesis starts again. Using this process, a single DNA molecule can be converted into a trillion identical copies in just 40 cycles.
Lathe believes that thanks to the Moon, something similar happened during Earth's early years. Most researchers agree that the Moon formed five billion years ago from debris blasted off Earth in a giant impact.
A billion years later when life is thought to have arisen, the Moon was still much closer to us than it is now. That, plus the Earth's much more rapid rotation, led to tidal cycles every two to six hours, with tides extending several hundred kilometres inland, says Lathe. Coastal areas therefore saw dramatic cyclical changes in salinity, and Lathe believes this led to repeated association and dissociation of double-stranded molecules similar to DNA.
When the massive tides rolled in, the salt concentration was very low. Double-stranded DNA breaks apart under such conditions because electrically charged phosphate groups on each strand repel each other.
But when the tides went out, precursor molecules and precipitated salt would have been present in high concentrations. This would have encouraged double-stranded molecules to form, since high salt concentrations neutralise DNA's phosphate charges, allowing strands to stick together.
These unrelenting saline cycles would have amplified molecules such as DNA in a process similar to PCR, says Lathe. "The tidal force is absolutely important, because it provides the energy for association and dissociation [of polymers]."
Many researchers do not believe DNA and RNA were the first replicating molecules. Graham Cairns-Smith of the University of Glasgow, UK, thinks much simpler "genetic" material formed first, from the crystallisation of clay minerals.
But he says Lathe's idea deserves attention. "Whatever the replicating entities were that started the evolutionary process, it would be significant that they lived in an environment in which the conditions were changing."
If the theory is right, life could not have evolved on Mars, says Lathe. Phobos, the larger of Mars's two Moons, is so small that the tidal forces it generates are just one per cent of those generated by our Moon. "Even if there was water on Mars, life could not have evolved there because these polymers could not have replicated," he says.
Journal reference: Icarus (vol 168, p 18)
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17:45 21 August 2008
11:17 21 August 2008
