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The following passage is adapted from an article that originally appeared in Scientific American (© 2009).

  1. The ocean tides mirror life
  2. itself. Their ebb and flow pay
  3. homage to the cyclic nature
  4. of the cosmos. But is life itself
  5. also ultimately a fluke of the
  6. tides? If so, life may ultimately
  7. owe its origins to our
  8. serendipitously large moon. At
  9. an average distance of 235,000
  10. miles, the moon is currently
  11. receding from Earth at a rate of
  12. 1.5 inches per year. As it does,
  13. Earth’s own spin rate is slowing.
  14. And, in the process, roughly
  15. 1020 joules of gravitational
  16. energy is shed into the oceans
  17. annually.
  18. Over the eons, all that energy
  19. has had an evolutionary impact.
  20. The oceans’ tidal flow helps
  21. transport heat from the equator
  22. to the poles,” says Bruce Bills, a
  23. geodynamicist at the NASA Jet
  24. Propulsion Laboratory in
  25. Pasadena, Calif. “Without the
  26. lunar tides, it’s conceivable that
  27. climate oscillations from the ice
  28. age to the interglacial would be
  29. less extreme than they are.
  30. Such glaciations caused
  31. migrations of animal and plant
  32. species that probably helped
  33. speed up speciation.”
  34. Peter Raimondi, an ecologist at
  35. the University of California, says
  36. the tools of evolution are also
  37. driven by the tides’ influence on
  38. these intertidal regions. In a
  39. rocky intertidal area, it’s very
  40. clear there are strong
  41. evolutionary pressures brought
  42. on by a changing environment
  43. over a short spatial scale.
  44. Without our moon, our marine
  45. environment would have far
  46. fewer species. But is the
  47. influence of the lunar tides
  48. actually responsible for life
  49. itself? If life originated around
  50. deep ocean vents, then the
  51. lunar tides played a minor role,
  52. if any, says James Cowen, a
  53. biogeochemical oceanographer.
  54. If, however, life originated in
  55. tidal waters, then tidal cycles
  56. could have played a major role.
  57. Both DNA and RNA—the
  58. messengers of life as we know
  59. it—almost certainly were
  60. selected and evolved from a
  61. large diverse group of
  62. protonucleic acid molecules.
  63. But for DNA and RNA to
  64. evolve, first they had to be
  65. able to replicate. That involved
  66. organizing their copying via
  67. cyclic assembly and dissociation.
  68. ” A lot of origin-of-life reactions
  69. involve getting rid of water,”
  70. says Kevin Zahnle, a planetary
  71. scientist at the NASA Ames
  72. Research Center. “So you look
  73. for means to concentrate your
  74. solutions. One way to do that is
  75. to throw water up on a hot rock,
  76. then have the waters recede
  77. and evaporate.” Molecular
  78. biologist Richard Lathe contends
  79. that some 3.9 billion years ago,
  80. fast tidal cycling caused by the
  81. influence of our moon enabled
  82. the formation of precursor
  83. nucleic acids. Lathe says that a
  84. 12-hour Earth day would have
  85. produced high tides a little less
  86. than every six hours. These
  87. lunar tides would have moved
  88. far inland and onto a vast
  89. sandscape.
  90. Today, this sort of ocean cycling
  91. pervades the sandy flats
  92. surrounding France’s famed
  93. tidal island of Mont-Saint-
  94. Michel. In the early Earth
  95. environment, such fast lunar
  96. tidal oscillations would have
  97. resulted in the highly saline
  98. low-tide environment that
  99. protonucleic acid fragments
  100. would have needed to
  101. associate and assemble
  102. molecular strands. Having
  103. bonded in pairs at low tide,
  104. these newly formed molecular
  105. strands would then likely
  106. dissociate at high tide, when
  107. salt concentrations were
  108. reduced, providing what
  109. Lathe terms a self-replicating
  110. system. Lathe believes that
  111. DNA would ultimately have
  112. arisen from such protonucleic
  113. acids.
  114. If the lunar tides were a crucial
  115. part of evolution on our own
  116. planet, what of other
  117. ocean-bearing terrestrial
  118. planets without benefit of a
  119. significant lunar neighbor?
  120. Would their prospects for life
  121. be diminished due to lack of
  122. tides? Odds of nucleic acids
  123. forming on Earth without the
  124. lunar tides would be much
  125. lower. By this accounting,
  126. Mars, with its two puny moons,
  127. could not have formed life.
  128. Within our own solar system,
  129. the moons of Jupiter have
  130. turned the idea of tidal
  131. influence on its head. On
  132. Jupiter’s icy moon Europa,
  133. tidal heating is believed to
  134. maintain a large liquid water
  135. ocean below its frozen surface.
  136. But even with strong tides,
  137. any evolutionary ambitions of
  138. microbes on Europa would
  139. soon be stymied by their
  140. harsh habitat.
  141. Furthermore, our moon’s
  142. gravitational influence also
  143. helped ensure that Earth’s
  144. spin axis and climate remained
  145. stable over long timescales.
  146. That’s arguably just as
  147. important as our oceans’
  148. tidal ebb and flow. Still, as
  149. paleobiologist Bruce
  150. Lieberman points out: “I
  151. suspect that eventually life
  152. would have made land
  153. without the tides. But the
  154. lineages that ultimately gave
  155. rise to humans were at
  156. first intertidal.”