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This Month in the Earth Year: April April 26, 2011

The last bits of snow have disappeared from the piles we plowed this winter and the daffodils have poked up through the defrosting topsoil, so it’s relatively easy to get a sense of time passing in our day-to-day lives. And at this time of year, when some readers may think that, as is often said, the only things certain in life are death and taxes, I’d like to add to that expression the existence of an ever-evolving Earth. For if we take the calendar year as a metaphor for the age of this planet, using January 1 as the date of its formation, by the end of April only 1.5 billion years of Earth history will have transpired. That may seem like quite a bit of time but if we gauge the passing of geologic time by looking for milestones in Earth history, it’s easy to see that Earth time is deep.

April is an auspicious period in the planet’s metaphorical history. The Archean. If you time-traveled to April of the Earth year it’s likely you wouldn’t recognize the Earth as the same planet we inhabit today. The Earth’s crust would have cooled enough so that continents had begun to form, but the similarities to today’s Earth would have ended there. To sustain a visit to the Archean, you’d need to have brought with you a supply of oxygen, for the atmosphere would have been unbreathable. Consisting of methane and ammonia, among other gases, the Archean atmosphere would have been toxic to most of the life that exists on our planet today.

Nonetheless, the first life that appeared on Earth—cyanobacteria— lived in the Archean. In fact, the oldest known fossils date to this slice of time.  The bacteria that grew in the Archean seas left behind large layered mounds called stromatolites that formed as the colonies trapped sediment and secreted calcium carbonate. Though stromatolites don’t commonly develop in today’s seas–because too many other organisms are around to eat them–those simple bacteria are still here, like death and taxes.