Reading 35

Tides

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.”