Microbial mats, sheets of microorganisms held together by foam, dominated the planet before multicellular organisms evolved. Fossil evidence dates back 3.5 billion years, and the mats were initially chemoautotrophs before evolving photosynthesis. The first multicellular organisms may have lived attached to the mats, and the Cambrian Period marked the end of the mats’ dominance. They are now only found in areas devoid of other life forms.
Microbial mats are multilayered sheets of microorganisms – primarily bacteria and archaea (another bacterium-sized domain of microbes) that dominated much of the planet for billions of years, before the evolution of multicellular organisms, which readily ate those microbes as soon as they arrived on site. Microbial mats are often found at the interface between two substances, especially in wet or submerged environments, such as the seabed. These microbial mats are held together by extracellular polymeric substances, also known as foam, which reinforce their structure and adhere them to the substrate.
Fossil evidence of microbial mats dates back 3,500 million years, serving as the first direct evidence of life on Earth. Initially, the microbes in the mats were chemoautotrophs, meaning they obtained their energy and carbon by combining chemicals found primarily in hydrothermal vents on the ocean floor. So, about 2,600 million years ago, microbes evolved photosynthesis, could have expanded from the “hydrothermal ghetto” into a much wider range of environments, especially the upper 100 – 300 m (328 – 985 ft) of the column water, known as the pelagic zone, and any stretch of seabed with available light.
Microbial mats are the context within the first evolved multicellular organisms. Some scientists claim that the first multicellular organisms, the Ediacaran fauna, lived attached to the mats and acquired their energy through symbiosis with algae, distributed throughout the body. At least, that’s what seems plausible, since Ediacaran organisms lack a gut or obvious feeding apparatus. In what is called “life on the mats,” the mobile organisms initially evolved as burrowers in the mats, digging shallow horizontal holes through them, some of which are preserved to this day.
During the Cambrian Period, about 542 million years ago, there was a “bioerosion revolution” that occurred as mobile animals expanded in size, complexity, and range of behaviors. In fact, complex vertical tunnels – absent in the preceding Ediacaran period – are part of the official definition of the beginning of the Cambrian, together with the appearance of ubiquitous organisms known as the small shell faunas. These complex burrows marked the beginning of the end for microbial mats, which could no longer afford to exist outdoors as such a concentrated food source. Today, microbial mats are found only in areas devoid of most other life forms, such as thin crusts in deserts, very brackish water or deep on the ocean floor.
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