Read A New History of Life for Free Online
Authors: Peter Ward
science is about doubting and questioning. Let us jump almost ten years, to 2008, and look at another paper on this subject, with one of the coauthors being the same J. Brocks who was senior author of the 1999 Science paper mentioned above. Here are the salient two sentences here: “The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78–1.68 billion years ago and around 2.15 billion years ago, respectively. Our results eliminate the evidence for oxygenic photosynthesis at about 2.7 billion years ago and exclude previous biomarker evidence for the long delay (circa 300 million years) between the appearance of oxygen producing cyanobacteria and the rise in atmospheric oxygen 2.45–2.32 billion years ago.”
Quite a difference! So what happened between 1999 and 2008 causing this abrupt scientific volte-face?
The original biomarker studies from the late 1990s were criticized on several fronts, including the fact that many ancient biochemical pathways that do not use oxygen are known to have been “updated” after the great oxygenation event to incorporate enzymes that do. However, the real problem with the biomarker studies was the methodology used to get the samples, not the analyses of what was in the samples. The investigators were finding the precious biomarkers, all right. But when, exactly, did the biomarkers get into the cores? Rocks are not the impermeable, hard, and durable objects we usually take them for, but actually exist often in environments where chemical changes—and later contamination—occur. In the late 1990s there was not yet sufficient appreciation of the intense need for testing for—and eliminating—the chance of younger contamination in these ancientsamples, particularly when the putative biomarkers are present in concentrations less than that of the surrounding air.
Thus it was to the horror of the mainstream biomarker community that one of their rising stars—Jochen Brocks of the Australian National University—suddenly changed his tune in 2005 (ultimately leading to the 2008 article we have cited from above), arguing that his own thesis work documenting the presence of Archean biomarkers was confounded by contamination! That, in turn, led one of the major geobiology funding agencies (the Agouron Institute) to support a critical repeat of the biomarker scientific drilling projects, with new means of testing for contamination. The result (as of this writing in mid-2014) is that no biomarkers were found. In fact, at a meeting late in 2013 the source of the contamination was revealed to be a stainless steel saw blade that had been made “stainless” (by the manufacturer) by high-pressure impregnation with petroleum products! As of this writing, the biomarker community has not developed intellectually rigorous tests to prove that the organic biomarkers in Archean rocks—any of them—date back from the time the sediments accumulated.
Another big picture in the great debate about the origin of molecular oxygen in Earth’s atmosphere was framed using a new kind of Earth history tool: comparing the concentrations of sulfur isotopes. We have already seen (and will see again, in the sections on mass extinction) that comparing the compositions of the isotopes of carbon is useful for studying life, and was even used to try to decide when the first life appeared on Earth, since living cells favor specific isotopes of the same kinds of atoms (such as carbon or oxygen, and as we show here, sulfur) over the others of that same element. In normal chemical reactions, light isotopes move through reaction series slightly faster than heavy isotopes because the lighter elements have slightly weaker chemical bonds that can be made and broken faster, producing higher reaction rates, and because of this, plants prefer the lightest isotopes of carbon and oxygen over their slightly heavier, more massive sister isotopes. James Farquhar, Mark Thiemens, and colleagues at the
Quite a difference! So what happened between 1999 and 2008 causing this abrupt scientific volte-face?
The original biomarker studies from the late 1990s were criticized on several fronts, including the fact that many ancient biochemical pathways that do not use oxygen are known to have been “updated” after the great oxygenation event to incorporate enzymes that do. However, the real problem with the biomarker studies was the methodology used to get the samples, not the analyses of what was in the samples. The investigators were finding the precious biomarkers, all right. But when, exactly, did the biomarkers get into the cores? Rocks are not the impermeable, hard, and durable objects we usually take them for, but actually exist often in environments where chemical changes—and later contamination—occur. In the late 1990s there was not yet sufficient appreciation of the intense need for testing for—and eliminating—the chance of younger contamination in these ancientsamples, particularly when the putative biomarkers are present in concentrations less than that of the surrounding air.
Thus it was to the horror of the mainstream biomarker community that one of their rising stars—Jochen Brocks of the Australian National University—suddenly changed his tune in 2005 (ultimately leading to the 2008 article we have cited from above), arguing that his own thesis work documenting the presence of Archean biomarkers was confounded by contamination! That, in turn, led one of the major geobiology funding agencies (the Agouron Institute) to support a critical repeat of the biomarker scientific drilling projects, with new means of testing for contamination. The result (as of this writing in mid-2014) is that no biomarkers were found. In fact, at a meeting late in 2013 the source of the contamination was revealed to be a stainless steel saw blade that had been made “stainless” (by the manufacturer) by high-pressure impregnation with petroleum products! As of this writing, the biomarker community has not developed intellectually rigorous tests to prove that the organic biomarkers in Archean rocks—any of them—date back from the time the sediments accumulated.
Another big picture in the great debate about the origin of molecular oxygen in Earth’s atmosphere was framed using a new kind of Earth history tool: comparing the concentrations of sulfur isotopes. We have already seen (and will see again, in the sections on mass extinction) that comparing the compositions of the isotopes of carbon is useful for studying life, and was even used to try to decide when the first life appeared on Earth, since living cells favor specific isotopes of the same kinds of atoms (such as carbon or oxygen, and as we show here, sulfur) over the others of that same element. In normal chemical reactions, light isotopes move through reaction series slightly faster than heavy isotopes because the lighter elements have slightly weaker chemical bonds that can be made and broken faster, producing higher reaction rates, and because of this, plants prefer the lightest isotopes of carbon and oxygen over their slightly heavier, more massive sister isotopes. James Farquhar, Mark Thiemens, and colleagues at the