Abstract
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Skipping undergraduate school at Michigan State, I arrived as a graduate student at MIT in 1967, soon finding the plate tectonic revolution in full swing. Marine scientists presented their results on the North Atlantic at Woods Hole just before fall classes started. It was obvious that seafloor spreading is basically correct but also that little was known about how it worked. I did not realize until many years later that I had found my scientific niche in the nascent field of geodynamics. There was plenty of low-hanging scientific fruit on the modern Earth. Planetary science was reserved for the senior scientists on the Apollo Program. I never thought that my work would eventually involve astrobiology.
Moving into academia, I obtained a junior faculty position at Northwestern in 1973 by default in that I was the lone applicant who expressed interest in the slow tectonics of the continental platform. Pat Hurley at MIT had already introduced me to global geochemical cycles. Bob Garrells and Fred MacKenzie were actively trying to integrate plate tectonics into the field. Graduate student Tom Wolery and I began to investigate the effects of hydrothermal circulation at ridge axes. Submarine vents teeming with life were discovered by the end of the decade. It was obvious that this abode had existed since the Earth's surface cooled enough for liquid water, but we did not give biology serious attention. Bryan Windley visited Northwestern, and we began to interpret the Archean geological record in terms of plate processes and geodynamics.
At this time, NASA recruited earth scientists into planetary activities in contrast to the closed system of Apollo investigators. I became involved in the large Basaltic Volcanism project organized by Tom McGetchin and the much smaller Tharsis Province of Mars project organized by Roger Phillips. Tasks included the early histories of the Earth and Mars. Again biology really did not come up. I moved to Stanford in 1979, continuing with the geodynamics and global geochemical cycles.
Much to my surprise, I became what was then an exobiologist by being appointed to the Space Science Board Committee on Planetary Biology and Chemical Evolution in 1985. I was to provide expertise in planetary dynamics and global chemistry. The committee was to advise on planetary quarantine and provide insight on the origin and distribution of life. By then, scientists had strongly linked the end-Cretaceous extinction to an asteroid impact. Kevin Zahnle and Jay Melosh, in my office, thrashed out how returning ejecta ignited global firestorms after the impact, but my involvement with this extinction was minimal.
The question arose as to whether impacts coincident with the Late Heavy Bombardment of the Moon were a threat to terrestrial life. I do not remember when I first heard it raised, but the topic was common in casual conversations once the end-Cretaceous impact horizon was known. It was clear that impacts comparable to the pinprick end-Cretaceous event were no threat to terrestrial life as a whole. Two facile viewpoints vaguely prevailed on Late Heavy Bombardment impacts: (1) The bombardment of hundred-kilometer-diameter objects was terrible, and nothing could survive; (2) The lethal effects of impacts were local to the crater and the ejecta field, and the rest of the Earth was basically safe for microbial life. Impacts were infrequent compared to the time for organisms to recolonize the planet, so there was no threat to life as a whole.
At the committee's urging and from contacts at NASA Ames, I started to investigate what large impacts really did to life that happened to be around. Fortunately for a geophysicist, the likely killing mechanism was heat, and the relevant heat scale is the energy to boil the ocean. The kinetic energy of a projectile is simply related to its approach velocity and mass and to the gravitational potential of the Earth. The lunar record indicated that there were at most only a few objects large enough to boil the ocean.
The biological distinction between a last common ancestor (bottleneck survivor) and first common ancestor had not sunk in to my thinking. Although Hy Hartman and Norm Pace were on the NRC committee, I still had no inkling of the value of molecular genomics. It was clear with heat as the killing mechanism that thermophilic organisms would be the last to check out, and I convolved this conclusion into the hypothesis that life originated in thermophilic environments—then popular following the discovery of hydrothermal vents.
I submitted in 1987 a letter to Nature with high hopes. The editors sent the paper to traditional paleontologists who missed its main problem with evolutionary bottlenecks. Neither of them looked at the physical arguments. One reviewer wanted evidence of life before 3.8 Ga, as the Hadean was defined as lifeless. The other considered the life requirements, including temperature for modern organisms, to be irrelevant in the deep past and demanded discussion of thermophilic organisms, although I had discussed “high-temperature” organisms. I pointed out the dogma and impossible expectations to the Nature editors, but to no avail.
I had had enough dogmatic rejections to realize that quantification is a good antidote: by a holdout against plate tectonics on the thermal subsidence of Atlantic margins, by a marine heat flow measurer who thought the subsurface thermal structure of ridge axes was irrelevant because one could not insert a heat flow probe in the basalt exposed at the axis, and by a holdout against hydrothermal circulation at the ridge axis. I continued working with people at NASA Ames, including Kevin Zahnle, Jim Kasting, and Hal Morowitz. This collaboration was quite productive for constraining the heat balance of large impacts and what happens when the ocean partially or completely boils. It also helped to obtain a much better constraint with explicit statistics on the implications of the lunar record to terrestrial impact flux. Hal Morowitz brought in the importance of considering ecosystems, rather than species. This time Nature accepted a full article with only minor changes.
Something unexpected had happened to me in the then sparsely populated field of astrobiology over the course of 4 years. I had gone from being vaguely aware that the field existed to being relatively senior on the basis of coauthorship of one paper with what now seems to be simple calculations. Yet I was hooked on considering physical processes and geochemical cycles relevant to the origin and early evolution of life. Later, I would learn that the genome and cellular chemistry of extant organisms sequesters information of the environments of their distant ancestors of the early Earth. I would appreciate that myriads of potentially habitable environments exist in the universe. Perhaps ironically, the hypothesis that terrestrial life originated on Mars is more testable than its beginning on the Earth. The ancient Mars record exists; it is just hard to sample.
Conversely, I now realize that life has a significant effect on hard crustal rocks and even the Earth's mantle. These biological changes are large enough that putative prebiotic environments must be vetted to see if they could really exist before life. This realization causes me to reflect deep into my childhood. The first scientific discussion that I can remember was between my mother and my uncle on the analogous problem of whether the tall-grass prairie, a putative wilderness, was really human-made. My grandfather LeRoy Harvey researched this topic early in the previous century.