During this “trek through time,” I’ve been keeping a copy of the International Chronostratigraphic Chart close at hand.
I do like all the colors, and I’m slowly getting a grip on the cacophony of names, but it’s also striking to think about the way time gets divided up on the chart. It’s not linear. Just one-ninth of Earth’s history gets three big columns, while Earth’s first 4 billion years - the Precambrian - gets just a few inches in the fourth column. It was hazier territory back then, without any big critters crawling around, and fewer preserved rocks. The Precambrian record pales beside the Phanerozoic. On my end, I have thousands of detailed email records of my life since my college years, but before then I have spotty physical records — a letter or postcard here and there, deplorable high school poetry, a few faded drawings from Kindergarten, maybe a few locks of blond baby hair. Same for my memory: before age three or four things are decidedly Archean. Hard to devise a mental picture.
As such, when wandering through the vast expanses of Earth’s history, especially during the relatively quiet Precambrian, I find myself looking for signposts like the Great Oxygenation Event. But of course sometimes these “events” are not events in the traditional sense — unless you can imagine a birthday party that lasts a hundred million years. Here’s what Andrew Knoll at Harvard has to say on the matter:
Still, as Knoll himself points out, the shift from an Earth without much available oxygen at all to an Earth with ANY available oxygen was a big one. In a sense, you could divide the history of Earth into three big eras: No oxygen, a little oxygen, lots of oxygen. (Somehow those names don’t sound adequately scientific. Gotta work on that.)
Anyway, back to the GOE, and the Earth 2.4 billion years before the present. Although an increase in oxygen was undoubtedly toxic for many anaerobic bacteria and archaea, I like the way Knoll characterized it in his book, Life on a Young Planet: “rather than considering the early Proterozoic as a time of environmental transition, it may be more profitable to think of it as an interval of environmental expansion — one that enabled the Earth to support an unprecedented diversity of life.”
Here in the mid-Siderian, after all, oxygen-free waters still persist, so not all the anaerobic microbes die out. The paleo-Proterozoic Earth has a little free oxygen in its atmosphere and surface oceans, but anoxic waters down in the deep seas. You could think of these as different habitats. With more habitats and diverse environmental constraints, life will continue to diversify.
That's the cornerstone of the world of Geobiology, which has become a catch-phrase in the Earth Sciences of late. Planet shapes life, life shapes planet. Round and round we go. The more diverse habitats, the more diverse life. At least generally speaking.
If you twist your mind around a little, you could think of modern-day Superfund sites in a similar way. Sure, toxins kill most life as we know it, but who knows what new microbes will evolve and emerge from the muck.
In Cambridge, I don't think we can count our slushy, blackening snowbanks as sites of profound evolutionary innovation, but I wonder what microbes might be eeking out a living in there. Time to read about snow and extremophiles...