Tides
The following passage is adapted from an article that originally appeared in Scientific American (© 2009).
- The ocean tides mirror life
- itself. Their ebb and flow pay
- homage to the cyclic nature
- of the cosmos. But is life itself
- also ultimately a fluke of the
- tides? If so, life may ultimately
- owe its origins to our
- serendipitously large moon. At
- an average distance of 235,000
- miles, the moon is currently
- receding from Earth at a rate of
- 1.5 inches per year. As it does,
- Earth’s own spin rate is slowing.
- And, in the process, roughly
- 1020 joules of gravitational
- energy is shed into the oceans
- annually.
- Over the eons, all that energy
- has had an evolutionary impact.
- The oceans’ tidal flow helps
- transport heat from the equator
- to the poles,” says Bruce Bills, a
- geodynamicist at the NASA Jet
- Propulsion Laboratory in
- Pasadena, Calif. “Without the
- lunar tides, it’s conceivable that
- climate oscillations from the ice
- age to the interglacial would be
- less extreme than they are.
- Such glaciations caused
- migrations of animal and plant
- species that probably helped
- speed up speciation.”
- Peter Raimondi, an ecologist at
- the University of California, says
- the tools of evolution are also
- driven by the tides’ influence on
- these intertidal regions. In a
- rocky intertidal area, it’s very
- clear there are strong
- evolutionary pressures brought
- on by a changing environment
- over a short spatial scale.
- Without our moon, our marine
- environment would have far
- fewer species. But is the
- influence of the lunar tides
- actually responsible for life
- itself? If life originated around
- deep ocean vents, then the
- lunar tides played a minor role,
- if any, says James Cowen, a
- biogeochemical oceanographer.
- If, however, life originated in
- tidal waters, then tidal cycles
- could have played a major role.
- Both DNA and RNA—the
- messengers of life as we know
- it—almost certainly were
- selected and evolved from a
- large diverse group of
- protonucleic acid molecules.
- But for DNA and RNA to
- evolve, first they had to be
- able to replicate. That involved
- organizing their copying via
- cyclic assembly and dissociation.
- ” A lot of origin-of-life reactions
- involve getting rid of water,”
- says Kevin Zahnle, a planetary
- scientist at the NASA Ames
- Research Center. “So you look
- for means to concentrate your
- solutions. One way to do that is
- to throw water up on a hot rock,
- then have the waters recede
- and evaporate.” Molecular
- biologist Richard Lathe contends
- that some 3.9 billion years ago,
- fast tidal cycling caused by the
- influence of our moon enabled
- the formation of precursor
- nucleic acids. Lathe says that a
- 12-hour Earth day would have
- produced high tides a little less
- than every six hours. These
- lunar tides would have moved
- far inland and onto a vast
- sandscape.
- Today, this sort of ocean cycling
- pervades the sandy flats
- surrounding France’s famed
- tidal island of Mont-Saint-
- Michel. In the early Earth
- environment, such fast lunar
- tidal oscillations would have
- resulted in the highly saline
- low-tide environment that
- protonucleic acid fragments
- would have needed to
- associate and assemble
- molecular strands. Having
- bonded in pairs at low tide,
- these newly formed molecular
- strands would then likely
- dissociate at high tide, when
- salt concentrations were
- reduced, providing what
- Lathe terms a self-replicating
- system. Lathe believes that
- DNA would ultimately have
- arisen from such protonucleic
- acids.
- If the lunar tides were a crucial
- part of evolution on our own
- planet, what of other
- ocean-bearing terrestrial
- planets without benefit of a
- significant lunar neighbor?
- Would their prospects for life
- be diminished due to lack of
- tides? Odds of nucleic acids
- forming on Earth without the
- lunar tides would be much
- lower. By this accounting,
- Mars, with its two puny moons,
- could not have formed life.
- Within our own solar system,
- the moons of Jupiter have
- turned the idea of tidal
- influence on its head. On
- Jupiter’s icy moon Europa,
- tidal heating is believed to
- maintain a large liquid water
- ocean below its frozen surface.
- But even with strong tides,
- any evolutionary ambitions of
- microbes on Europa would
- soon be stymied by their
- harsh habitat.
- Furthermore, our moon’s
- gravitational influence also
- helped ensure that Earth’s
- spin axis and climate remained
- stable over long timescales.
- That’s arguably just as
- important as our oceans’
- tidal ebb and flow. Still, as
- paleobiologist Bruce
- Lieberman points out: “I
- suspect that eventually life
- would have made land
- without the tides. But the
- lineages that ultimately gave
- rise to humans were at
- first intertidal.”