Experiencing an international childhood, and an innate curiosity of learning the origin of himself and his surroundings, Steve Mojzsis set out to try his best to make sense of the universe he was born into. This quest will never be fulfilled, but it doesn’t prevent him from trying. The ultimate prize is to figure out how the biosphere came into being. The path towards learning began with adventures mostly in North America and Central and Western Europe. To find himself still further meant going, to Africa, Asia and Australia, with expert guidance from the best mentors along the way. Ever the bridge builder, he was not satisfied with staying within one narrow discipline of science for very long before trying a hand at something else, albeit related. In this way he stumbled into biogeodynamics and geoastronomy; we will probably not see spectacular developments in either of these nascent fields until the end of 21st century or long after he is returned to some mineral salts, CO2 and H2O. Back on his home world, it is evident that in its first 500 Myr (q.v. Hadean eon) Earth’s crust co-existed with liquid water; it was affected by late accretion bombardments and harboured chemically diverse hydrothermal systems. Globally active volcanism powered by a hotter mantle, impact generated topography and inherent differential buoyancy of the crust means that scattered emergent land masses existed irrespective of different estimates for plausible Hadean ocean volumes. Consequently, land surfaces in the first few hundred million years – with diverse subaerial and (sub-)aqueous environments – could bring prebiotic chemical ingredients to reactive concentrations. This active chemistry led to the first life. Yet, the inferred complexity for even the minimum biological entity probably means that both operative and persistent biology in a planetary environment is the most difficult developmental stage to reach. Soon after Earth’s formation, its surface had the capacity to host fully fledged (i.e. living) biochemistry. While life emerged at the dynamic interface of the planet’s primordial geosphere, hydrosphere and atmosphere, nobody knows whether any Hadean environment was ideally suited, or merely good enough, to facilitate its emergence. Given that we do not have time machines to go back and see what happened on Earth, or anywhere else for that matter, we are compelled to tell stories that fit the data. That said, comfortable fictions about what “might” have happened, just because somebody said something (without evidence), ought to be taken with a grain of salt. If we ever are to come up with a plausible pathway to a life giving world, it will be more metaphorical than literal. All of this will likely change if we find an independent origin of life, either on purpose or, more likely in Mojzsis’ opinion, by accidental discovery of something strange in the data.
The long term evolution of Earth’s atmosphere and climate has been an active topic of investigation for at least the last 60 years. My own participation in this investigation goes back more than 45 years, and this monograph relates that story from my personal perspective. One major thread concerns the rise of atmospheric O2 from near-zero levels initially to the 21 percent mixing ratio that we observe today. Photochemical models developed by me and my students, along with some close colleagues, have helped to better constrain the prebiotic O2 concentration and to interpret the constraints imposed by the record of mass independent fractionation of sulfur isotopes. Most geochemists now agree that a so called Great Oxidation Event (GOE) occurred between 2.4 and 2.2 Ga and that the atmosphere has been O2-rich since that time. However, the exact level of O2 during the ensuing Proterozoic Era remains controversial, as do the timing and magnitude of subsequent O2 increases. The corresponding development of the ozone layer is also of interest because of its moderating influence on surface solar UV fluxes and their effect on biological evolution. This can also be studied with photochemical models. A second thread concerns the gradual decline in atmospheric CO2 in response to slowly increasing solar luminosity. The early Earth would have been frozen had the atmosphere not contained high concentrations of greenhouse gases, most importantly CO2. A negative feedback in the carbonate-silicate cycle that controls CO2 over long time scales has ensured that Earth’s surface has remained habitable during most of Earth’s history, despite occasional forays into Snowball Earth conditions. Evidence from palaeosols provides support for this hypothesis. CH4 is an additional greenhouse gas that may have supplemented surface warming prior to the GOE. The increase in O2 at that time may have caused CH4 to decrease, possibly triggering the Huronian glaciations. The same feedback mechanism that controls long term CO2 evolution on Earth could operate on Earth-like planets orbiting other stars, increasing the probability that some of them may harbour life. Large direct imaging space telescopes currently under development may eventually allow us to test this hypothesis and to learn whether we have company in this part of our galaxy.
Thwarted in pursuit of a career in aviation, academic underachiever Mark Harrison then kicked around the world for a couple of years.Doing so he stumbled into a series of geological technician jobs in the southern hemisphere that motivated a return to school in Canada to prepare for a research career.A series of inspirational mentors at Australian, Canadian, and American universities imbued him with an outsider's perspective that encouraged intellectual grazing across several fields, including geochronology, tectonics, and early Earth evolution. The connective tissue between these disciplines was the development of thermochronology - the release of temperature history information stored in minerals to infer geophysical mechanisms acting in the distant past.Since most geodynamic processes involve heat flow discontinuities, this new science could image ancient events that might otherwise go undetected. A second unifying theme, reflective of both Harrison's contrarian nature and his visionary mentors, was a pronounced mistrust of received wisdom. His early kinetic calibrations established Ar-40/Ar-39 thermochronology as a key tool in documenting epeirogenic histories. Of particular note, development of the multi-diffusion domain model required a natural test bed with the highest possible dynamic range of geologic rates, taking him first to Tibet and then the Himalaya. Together with UCLA colleague An Yin, they developed a widely emulated, hybridised application of geochemistry and tectonics to address longstanding questions in the evolution of that mountain system.On hand as a graduate student in Australia to witness development of the high sensitivity ion microprobe, Harrison and colleagues commissioned the first high sensitivity ion microscope, which could determine both in situ ages and stable isotope compositions. This unique capability permitted them to find microscale evidence that life likely emerged prior to 3.8 billion years (Ga) ago - 300 million years (Ma) earlier than previously thought. Harrison's research groups, both at UCLA and the Australian National University, set out to create an unprecedented archive of >4 Ga zircons from which they documented evidence most simply interpreted as reflecting Hadean eon oceans, continental crust, plate boundary interactions, and the emergence of terrestrial life as early as 4.1 Ga. These interpretations challenged the longstanding paradigm that Earth had not evolved stable continental crust or life over the first 500 to 1000 million years of its history. That dialectic led Harrison to ponder the intellectual and philosophical underpinnings of historical geology, which he sees as underdeveloped. Ultimately, the field needs to adopt a multiple working hypothesis credo along with the scientific humility to acknowledge the limitations imposed by the fragmental and biased rock record. Doing so shouldn't dampen our enthusiasm for understanding Earth history but instead only make its pursuit more intriguing, challenging, and communally rewarding.
