You’re strolling along the shoreline of Lake Michigan combing the beach for interesting stones and driftwood or perhaps beach glass. You find a common gray beach stone and admire it for the smooth way it feels in your hand, ground down by the wind, wave and sand action of the big lake. It even smells of the fresh outdoors. But upon a closer look, you can see layers of striations interesting and beautiful. When wet, they suddenly pop out and there’s no mistaken this is not an ordinary mineral rock. It’s a stromatolite.
What Are Stromatolites?
For us laymen, simply put, they’re fossils of bacteria. You need a firm understanding of biology, geology and chemistry to fully understand them. Nevertheless, I will attempt to delve into their fascinating formation.
Forming in water, scientists today generally agree stromatolites are layered structures formed by cyanobacteria, single-cell microorganisms capable of photosynthesis producing their own food. Cyanobacteria are prokaryotic cells (the simplest form of modern carbon-based life) in that they lack a DNA nucleus. Bacteria, including the photosynthetic cyanobacteria, were the only form of life on Earth for the first two billion years that life existed on Earth.
Forming The Layers
The stromatolite bacteria live in between thin sheets of filament bound together by a sticky substance. Photosynthesis in the bacteria depletes carbon dioxide in the surrounding water making it less acidic and initiating the release of calcium carbonate. Calcium carbonate and other minerals and grains of sediment settle, then get trapped on the outside sticky layer. The cyanobacteria thus rises to the top of the stromatolite structure over the sediment and the layers recycle repeatedly building the solid structures that can take several forms such as mounds, sheets or columns which appear like giant mushrooms.
While the microbes that construct the layered mats generally are not preserved, the wrinkled calcium carbonate, mineral rich layers remain in the fossilized forms.
Stromatolites are the oldest discovered fossils dating as far back as 3.5 billion years. First appearing during the Archean Eon, their hay day was during the Upper Proterozoic Eon long before multi cellular Cambrian creatures evolved.
Modern stromatolites were first discovered growing in the salty waters of Shark Bay, Australia in 1956. Before then, scientists believed they were extinct. Other locations discovered around the globe include the shallow waters of Yellow Stone, Mexico, Canada, Brazil, Oregon and most uniquely, Bahamas. Stromatolites lost out when animals such as snails evolved that ate them. Modern stromatolites thus live in water too salty or hot for those predators, except in the Bahamas.
Studies of modern stromatolites have shone they are not uniform in shape and form, but also host a variety of bacteria and archaea (bacteria like microorganisms). Archaea usually live in extreme, often very hot or salty environments such as hydrothermal vents or mineral hot springs, ie Yellow Stone. In any event, various biological environmental conditions may attribute to the differences in their make-up and shapes. Some form a round ball or lumpy mass. The example below shows one of these such forms found on a Lake Michigan beach, red in colored from iron infused sediment.
Some of the most ancient stromatolite fossils found are in the 3.35 billion year old Strelley Pool chert of Western Australia, part of a fossilized ocean reef. Seven different types were identified, so there was already a variety of stromatolite shapes even back then. The stromatolite fossils found in Michigan are typically younger, dating from 2.2 billion years ago. During the great ice age 10,000 years ago, glaciers cut the Great Lakes digging up time-buried layers of sediment containing many varieties of fossils we find on the beaches today. This could explain how I picked up the stromatolite fossil pictured above on the beach in Southwestern Michigan.
Why Are Stromatolites Important To All Life?
Cyanobacteria that make up stromatolites were ultimately responsible for one of the most important global changes that the Earth has undergone. Being photosynthetic, cyanobacteria produce oxygen as a by-product. Photosynthesis is the only major source of free oxygen gas in the atmosphere. As stromatolites became more common 2.5 billion years ago, they gradually changed the Earth’s atmosphere from a carbon dioxide-rich mixture to the present day oxygen-rich atmosphere. This major change paved the way for the next evolutionary step, the appearance of life based on the eukaryotic cell (cell with a nucleus).