A New History of Life Read Online Free PDF

photosynthesis? It is very difficult to produce oxygen in nonbiological ways, but one way that works is through photochemical reactions involving ultraviolet radiation, the same UV that causes sunburns on unprotected skin. UV radiation hitting water and CO 2 molecules in the atmosphere will generate trace levels of O 2 and other chemicals. Today, solar UV radiation is mostly blocked by a layer of ozone high in the atmosphere, far above the layers with water vapor (that freezes out). But early in Earth history there was no oxygen and thus no ozone, and hence no UV screen. Thus very strong ultraviolent radiation from thesun blasted the Earth, creating a tiny number of oxygen molecules. Unfortunately, reactions similar to those that generate the oxygen quickly snuff it out, making it unlikely to survive long enough to generate a biological effect, particularly as this is all happening in a UV radiation bath that is very good at scrambling DNA and sterilizing anything it hits. What is needed is a mechanism that allows the oxygen to be separated from the other products (hydrogen and CO in particular) before it is snuffed out.
    Two processes are known that can do this. First, if water gets high up in the atmosphere, a significant fraction of the hydrogen atoms liberated from the UV light will be traveling faster than Earth’s escape velocity and can be lost to space. That will leave a small trickle of oxygen, ozone, and hydrogen peroxide diffusing down from above (which are too heavy to escape), but it is really only a trickle. Reducing gases produced by biological activity and from volcanic eruption will easily squelch these oxic compounds long before they could reach the biosphere. The second process occurs on the Earth’s surface—but on the surface of a glacier! In Antarctica today the “ozone hole” permits a wider spectrum of UV radiation to reach the surface, where it can blast water molecules apart, eventually generating H 2 gas and H 2 O 2 (hydrogen peroxide). That peroxide gets locked in the ice, separated from the H 2 gas. Working with a graduate student at Caltech, Danny Liang, we calculated that up to 0.1 percent of the ice during a Precambrian glaciation could be made of H 2 O 2 , which, when the glacier melts, would be converted into O 2 and water. 4 Although this is not enough to breathe, it is enough to cause life with its powerful tool kit we call evolution to react. As noted below, we think the first oxygen-releasing cyanobacterium evolved during one of these Precambrian glacial intervals, and it certainly must have had evolved protection from oxygen.
    In a 2008 paper, one of the most experienced researchers about life and the early Earth, Roger Buick of the University of Washington, broke down the alternatives to the “when” of oxygenation as follows: First, oxygenic photosynthesis (such as is common today in all green plants) evolved hundreds of millions of years before the atmospherebecame significantly oxygenated, because it took eons to oxidize the continued production of reduced volcanic gases, hydrothermal fluids, and crustal minerals. Second, it arose at ~2.4 billion years ago in what we refer to in these pages as the great oxidation event, causing immediate environmental change. Third, oxygen production from photosynthesis or any other means began very early in Earth’s history, before the start of the geological record, leading to an Archean (greater than 2.5GA) atmosphere that was highly oxygenated. To choose between these alternatives, let’s look at the record as we now know it, because this is so critical to a good understanding of the history of life, and indeed there is a great deal that is “new” about this history in terms of our knowledge.
GEOLOGICAL CONSTRAINTS ON THE GREAT OXYGENATION EVENT
    Despite the widespread agreement that the evolution of the cyanobacteria was the most profound biological event on this planet (even more so than the evolution of the eukaryotic cell, and then