The coevolution of life and environments

James Hutton, the late eighteenth century father of Geology, recognized the goodness of fit between organisms and the environments in which they live. To Hutton, this presented a conundrum, because simple observation showed that the Earth is constantly changing. Trees and shrubs grow on mountainsides, but erosion strips sediments from highlands and carries them to the sea. Clams and seaweeds live in bays and estuaries, but the sediments eroded from mountains are continually delivered to the ocean, where they fill in those embayments. How can species be maintained in the face of such flux? Hutton’s solution, presented in his seminal Theory of the Earth, was elegant in its simplicity: environmental constancy is maintained dynamically (Hutton 1788). Through time, the uplift of sedimentary basins approximately balances erosion, perpetuating the mountains and bays for which species are manifestly well designed. Half a century later, Darwin (1859) stood this logic on its head. Species are no more constant than the Earth, reasoned Darwin; populations adapt via natural selection to the changing biological and physical circumstances of their environment. The idea that planetary surfaces not only change through time, but also change directionally first appeared in the writings of an astronomer, Percival Lowell, in a description of Mars. Lowell (1908) famously, or infamously, claimed that he could discern linear features on the Martian surface, and he interpreted these as canals built by technologically advanced Martians to carry seasonal melt water from the glaciated poles to parched populations at low latitudes. From this, Lowell concluded that Mars was once much wetter than it is today, a view widely accepted by present day planetary scientists although decidedly not for the reasons advanced by Lowell. A century after its publication, however, Lowell’s general point seems strikingly modern: planetary surfaces provide potentially habitable worlds that change systematically through time. Now that we have the