OXYGEN LEVELS AT 2700 Ma.

This is a very important paper, presenting evidence that the organic molecules called steranes occur in rocks around 2700 m.y. old, together with other "biomarkers" called alpha-methylhopanes. These methylhopanes are only made by cyanobacteria, and since the rocks under study include BIF, that is interesting but not surprising. However, the steranes can only have come from the breakdown of complex sterols, and sterols are only made today by eukaryotes. The simplest interpretation is therefore that eukaryotes were living at 2700 Ma, about 500 million years earlier than anyone had expected, and about 600 m.y. before we see the first eukaryotic fossil, Grypania.

It gets better (or worse): the chemical pathways that are used to make sterols require molecular oxygen, so the evidence now requires that there was sufficient oxygen available at 2700 Ma for these biochemical processes. Yet many other lines of evidence show that Earth's free oxygen levels were very low until perhaps 2300 or 2200 Ma, when the Oxygen Revolution took place. There is then a geochemical problem as well as a biological one.

OK, we are scientists, and our job is to explain the data that we find. How do we deal with this one?

The evidence of a significant oxygen increase around 2300 or 2200 Ma is very solid. Yet we cannot reasonably throw out the new information. Here is my two cents (remember, you read it here first!):

The samples studied by Brocks et al. came from organic-rich shales that were interbedded with BIF, and they found abundant evidence of molecules from cyanobacteria as well as eukaryotes. So the oxygen-requiring eukaryotes were apparently living in the same environments as the oxygen-producing cyanobacteria. I do not think this is a coincidence.

Please read again the section I wrote on Stromatolites as Evolutionary Forcing Houses. Extending that thought even further, here is my proposed interpretation of the new data. First, there is no real evidence of a global rise in oxygen at or near 2700 Ma. Any levels of oxygen high enough to allow the evolution of eukaryotes are local, probably in or around stromatolites.Whatever else it requires, the evolution of a eukaryote (which is treated in Chapter 3), requires oxygen. So it makes sense that the first eukaryote should have evolved in a stromatolitic setting.

One can object that oxygen would quickly have diffused into the general ocean, leaving eukaryotes without a supply. That is true as a general principle, but there are at least three mechanisms to soften any oxygen crisis.

• First, many eukaryotic cells living today can simply shut down in an oxygen-poor season (as in winter, or during a storm that kicks up sediment and shuts off sunlight for a while). Eukaryotic cells need oxygen for growth and reproduction, but it does not have to be permanently present.
• Second, cyanobacteria are usually surrounded by their own slime, for many reasons; and the slime can and does trap bubbles of oxygen that therefore do not diffuse away quickly or easily. Therefore, as long as eukaryotes were able to live very close to cyanobacteria, their oxygen supply would have been more secure.
• Third, since closeness is the key to a secure oxygen supply, direct contact between cyanobacteria and eukaryotes would have been encouraged. I expect that as soon as it became possible, an early eukaryote experimented by incorporating cyanobacteria inside it in a symbiotic relationship (thus becoming the first alga), and making the oxygen supply as secure as it could be. (More in Chapter 3).

So then, there is not necessarily a contradiction in the paper by Brocks et al. (1999). What I think it does is set rather precise limits on the timing, situation, and geochemical environment of eukaryotic evolution. Eukaryotes, I suspect, MUST have evolved in "oxygen islands" (associated with abundant stromatolites), and could ONLY have lived there until global oxygen levels rose enough. And that seems to have taken 500 million years.

Note also Bernhard, J. M., et al. 2000. The Santa Barbara basin is a symbiosis oasis. Nature 403: 77-80. Joan Bernhard (UC Davis Geology alumna, by the way) and colleagues have suggested that the deep water of the Santa Barbara Basin is much like the 2200-m.y. old environment in which eukaryotes first evolved. Quite possibly, in terms of its oxygen level. But perhaps not in its depth. One has to go deep today to find seriously oxygen-low water in the open ocean; that would not have been the case at 2200 Ma.

I have placed this mini-essay also in the notes for Chapter 3, since I deal there with the origin of eukaryotes.

Written August 1999.

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