23 May 2015

Interesting developments in thinking about the History of Life on Earth

I should like to briefly share with you some very interesting points made by Ward and Kirshvink in New History of Life.

First, they take very seriously the idea that life might have originated on Mars and been transported to Earth sometime after the Late Bombardment, roughly -3.8 GA. They explain that one of the most “difficult” processes in the origin of life was the synthesis of ribose (essential to the theory of the origin of life), and consequently of RNA and later DNA. These substances cannot have spontaneously arisen in water. But the Earth of that era was almost certainly entirely covered by oceans, with no land surface at all (there was about four times as much water as at present, and the continental crusts had not yet differentiated). So where did the ribose come from?

Experiments have shown that ribose can self-synthesize in the presence of boron salts, which can arise as a result of chained evaporitic lakes in severe deserts. The best contemporary example is the deserts of the Amargosa watershed (Death Valley). But it is quite plausible that something like this could have occurred in impact crater lakes on Mars of this era. Mars is believed to have had a thicker atmosphere and seas and lakes at that time, but to have been mostly rather severe desert.

Couple this with the fact that over 1 billion tonnes of material has found its way as the result of meteorite strikes to Mars, from Mars to Earth, since that era. It thus becomes quite plausible to suppose that ribose and “RNA world” life could have evolved on Mars, where, as a result of desiccation and UV sterilization it subsequently became extinct. But not before “contaminating” the previously sterile Earth with … life!

Another issue they discuss in some depth is the near-complete abandonment of the former paradigm of uniformitarianism. It is now pretty well established that there were at least two, and possibly as many as four, planetwide “Snowball Earth” glaciations, each of which lasted tens of millions of years, before the eventual emergence of “complex life” in the latest Proterozoic, setting the stage for the “Cambrian explosion.”

As they explain (the reasoning based on chemistry is complex), it is highly likely that the first, and in fact only  time oxygen producing photosynthesis ever evolved (the emergence of cyanobacteria) occurred just prior to, and in fact triggered, the first of these Snowball Earth episodes (water-breaking photosynthesis drew down previously high levels of greenhouse gases methane and CO-2). The emergence of an oxygen rich atmosphere depended on these “catastrophes,” as did the emergence of complex life and probably even of nucleated cells (Eukarya). This is completely contrary to former views that evolution of photosynthesizing life, then nucleated cells, then complex, multicellular life, was all a smooth process of increasing complexification.

Previously, it was always assumed that “Snowballs” could not have happened, because if the Earth ever had surface ice all the way to the equator, because of ice albedo effect, that condition would be permanent ---at least until the ever-brightening sun finally melted the ice, in around 1 GA from now, although by then it would be too late for complex life to evolve, since the melting of the ice would be followed almost immediately by the complete loss of the oceans to space; which is what will happen around that time in any case. If a “Snowball” were to occur in the future, in fact, that is what would happen, because volcanism has declined over time and there would be no effective mechanism for melting the ice. (Indeed, until the 1970s even the fact that, as a typical main sequence star, the Sun has been brightening continuously throughout its life and will continue to do so was scarcely appreciated, and the effects it had on the evolution of habitability on our planet was not thoroughly analyszed or understood. This fact, and its implications, are not well known to most educated people even today).

It is now understood that the ancient Snowballs eventually came to an end because there was nothing to remove CO-2 from the atmosphere, so it built up, from volcanic release, primarily. Indeed, CO-2 levels may well have reached 15% or even higher, before finally reaching a high enough level to increase air temperatures sufficiently to cause the ice to melt. But with such high levels of CO-2, the Earth’s climate shifted suddenly to ultra-Greenhouse, and temperatures reached extreme levels. Only when the deposition of carbonate rock (which reached maxima in these post-Snowball eras) finally brought CO-2 levels down, did the climate stabilize. The whole process was a close call, however. Had the Earth been only a little further from the Sun, it’s believed that the temperature at the poles during Snowball episodes would have been below the freezing point for dry ice. Had that occurred, the CO-2 from the volcanism would have precipitated at the poles, and the Earth would indeed have remained ice-covered until the brightening sun finally melted the ice, some 1 GA from now.

All of this shows that previous uniformitarian models are clearly wrong, and a form of pre-Lyellian catastrophism is actually closer to the truth. It also supports a growing consensus that the complex life of our planet is the result of quite a series of relatively unlikely evolutionary and geo-system events, some of which life barely managed to squeak past, including the origin of RNA (which may have depended on importation of life itself from Mars!), the evolution of oxygen-producing photosynthesis, which happened only one time and in the context of global environmental catastrophe, and the origin of endosymbiotic nucleated cells, which also apparently only happened one time.

Heady stuff.

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