Cambridge is getting another foot of snow today. It’s swooping down in white waves.
In Earth Time, we’re still 3.5 billion years before the present day, but we can be pretty sure that there is life on the planet Earth.
Where is it? Most attention has been lavished upon the Pilbara Craton of Western Australia, which over the course of 3.5 billion years will ride around on the planet but not metamorphose as much as other places. The rocks will change, but perhaps not utterly. In other words, the Pilbara will give future geobiologists a fair crack at finding fossils. They’ll find spheres, and elongate structures, and wonder: is this Earth’s earliest life?
They can’t say for sure. Andrew Knoll, at Harvard, suggested to me that many hydrologic processes can give you shapes that look like fossils. So geologists end up with a game of animal, vegetable or mineral. Biogenicity is the word scientists use: did life make this fossil? With the Pilbara, it’s hard to say.
Still, Knoll reassures, there are other ways we can be fairly sure there was early life: stromatolites, and the carbon-12 record.
Stromatolites aren’t microbe fossils per se. They are sedimentary structures left behind - we think - by stacks of microbial mats. We haven’t found convincing microbial fossils within them yet. But their existence strongly suggests that microbes, changing the chemistry of the water, caused accretion that led to the buildup of these structures over time. I think of the snow piling up in layers along my window, and in gentle mounds in the baseball field on Fulkerson Street. I remember my first day downhill skiing when I was 17, and I fell into and broke open a smooth mogul on a Vermont slope. Stromatolites, the moguls of the Archean.
Layer after layer, the record is made.
There are also traces in the rocks not of fossils but of chemistry left behind by living things. Living things prefer lighter carbon isotopes, so patches of rock with less of the heavy carbon, and more of the light, might indicate an Archean cemetery.
So the Earth is about a billion years old, and if we have microbes, surely they’ve been around a while? Or at least, long enough to have evolved from simpler cells? And given the difficulty of surviving during the molten Hadean or the ridiculous Late Heavy Bombardment, are we suggesting that life essentially emerged as soon as it was possible to do so? In other words, life on Earth happened very quickly.
I’d like to dig into this a bit more in the next few days, as it has obvious implications for astrobiology, and our sense of how special life is in the universe.
For now, I’ll wander home through the maelstrom and try not to get too much snow in my socks, which don’t match. What a difficult planet this is.