Geoscience Reference
In-Depth Information
of present levels, and indeed, to levels probably much greater than this.
There you have it. But not so fast; some important questions still remain.
For example, we still haven't explained what caused the GOE. Nor have
we discussed how high oxygen concentrations actually rose during the
GOE. Also, was there any obvious influence on biology? We'll focus
on the first of these questions in this chapter, the second of them in the
next, and the last in
chapter 10.
So what caused the GOE? To be honest, lots of ideas have been pro-
posed, and I will not discuss them all. I will, however, try to give an
overview of the most promising avenues of thought. The null hypothe-
sis, and the simplest, is that the GOE represents the evolution of cyano-
bacteria. Simple as that. This idea is promoted most strongly by Joe
Kirschvink of the California Institute of Technology. Joe is well known
for thinking outside the box and he has contributed some of the most
creative ideas in modern Earth Science.
3
In this GOE debate, Joe plays
devil's advocate and specifically acknowledges that he holds the “skep-
tical” viewpoint. He approaches the problem with the following ques-
tion: “hen does geologic evidence demand the presence of oxygenic
photosynthesis?” He takes on nearly all of the evidence we have dis-
cussed so far for cyanobacteria before the GOE, but he invests most of
his energy on the sterane evidence for an oxygen-containing environ-
ment that we explored in
chapter 6.
Recall that as far as we know, steranes have an absolute requirement
for oxygen in their synthesis, and therefore, finding them is a good sign
of oxygen in the ancient environment. We also noted that contamina-
tion is a big concern. Joe raises this flag too, but his major argument is
that while the known biochemical pathway of sterane formation has
steps requiring oxygen, this was not necessarily the case through all of
Earth history. He points out that the aerobic steps in sterane synthesis
may have potential anaerobic counterparts that did not require oxygen.
No living organism is known to conduct the anaerobic (oxygen-free)
synthesis of steranes, but Joe argues that this is because the anaerobic
pathways were replaced by oxygen-requiring pathways when oxygen
became available. Joe's hypothesis could be correct, but it is difficult
to defend without evidence. Perhaps it's a matter of style, but I prefer
to side with explanations of the geologic record that rely on well-
documented pathways and processes. Having said this, I appreciate
