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This Date in the Earth Year October 11, 2009

As indicated in an earlier post (September 14), using a calendar year as a metaphor for the 4500 million years of Earth history and employing January 1, New Year’s Day, as the Earth’s birthday, I calculate the current date’s location in the Earth Year and detail what was happening paleontologically at that moment in Earth history.

Today, is day 284 out of 365 days in this (non-leap) year. Not even an entire month has elapsed since my last post on this subject, yet in geologic time, October 11 represents 999 million years ago.  Though we are still in the Proterozoic eon, at this point in earth history single-celled organisms have begun to live together in colonies. Protozoans living in colonies would have occupied more space than protozoans living alone and might have been less vulnerable to challenges of daily living in the late middle Proterozoic. Furthermore, colonial living would have enabled some cells to specialize in certain tasks such as reproduction or locomotion. As a result, colonial protozoans might have had a metaphorical ‘leg-up’ in the late middle Proterozoic over their relatives who lived alone!

Evidence of these ancient colonial protozoans occur as fossilized stromatolites (pictured above). Found in numerous places on earth today, stromatolites are the main sedimentary features of carbonate rocks from earliest Earth history. They are the products of sediment trapping by mat-building microorganisms, known most widely as cyanobacteria. Living stromatolites also occur today but are quite rare because they are subject to predation.