Charles Darwin had a problem: with a poor fossil record preceding the Cambrian explosion, his theory of evolution could not account for life in its earliest stages. Darwin speculated that the first life forms began in a “warm little pond with all sorts of ammonia and phosphoric salts, lights, heat, electricity etc. present (where) a protein compound was chemically formed.” We now know that Darwin was right, and moreover that such life does leave a trace in the fossil record in the form of stromatolites. 

Stromatolites first appeared in the fossil record during the Archean Eon, 3,481,000,000 years ago. That's 3.4 billion years, nearly two-thirds of Earth's existence. Photosynthetic cyanobacteria living in shallow pools of water formed large colonies, taking in light and carbon dioxide and producing mucus and calcium carbonate. Each layer of the microbial mat built upon the last, forming an intricate laminate structure, growing into fascinating patterns of limestone that would stand through the ages as evidence of the lives of these microscopic organisms.

Cyanobacteria have existed for an extremely long time and the traces of their early existence are one of the best records of the evolution of life and the environment of early Earth. Their age leads to plenty of variations in the appearance of stromatolites, with some patterns growing miles wide. They can also be found in many different shapes with flat, domical, branching, and conical formations. These formations are still produced today in a handful of environments where conditions are just right, like Shark Bay in western Australia.

Not only are stromatolites traces of some of the earliest life forms, they provided the conditions that allowed more complex life to generate. Since cyanobacteria are photosynthetic, they raised the oxygen levels in the atmosphere from about 1% to 20%, effectively terraforming the Earth and making it habitable for their descendants. Evolutionary forces do not simply respond to the environment, rather it is a comingling of the two spheres as they shape and form each other.

Stromatolites still form today but there is a marked difference between the contemporary and ancient specimens. Modern formations do not boast nearly as many shapes or sizes, with far smaller diameters in the columnal formations. Part of this has to do with competition: when stromatolites first evolved, they had none. Those that exist now have to contend with food competition with other aquatic organisms that stromatolites allowed to evolve in the first place, a generational struggle that stromatolites are on the losing side of.

As with all studies of early life on Earth, stromatolites have their application in the search for extraterrestrial life, specifically on Mars. Were a stromatolite found on the red planet, it would be major evidence to suggest Mars once maintained the conditions needed to foster life, although even this would not be definitive. Stromatolites are such basic formations of life that they can also be produced by similar non-living abiotic processes. While there are some candidates found by unmanned missions to Mars, nothing definitive has yet been discovered.