Last week a reader presented me with this wonderful photo of an Archean stromatolite from northern Canada. Stromatolites are the remains of long-lived colonies of bacteria ("blue-green algae"). They're rare in rocks from the last half-billion years, but for about 2 billion years before that stromatolites were the most complex life form on Earth. See a close-up of this specimen in the Stromatolite Picture Gallery.
Photo courtesy Hans-Jürgen Kirstein, all rights reserved