Life › The microbial world › Bacteria & archaea: the microbial world
Stromatolites
3.48 billion years ago (Pilbara, Australia)· Fossil or site· Microbes & cells
Strong evidenceGood evidence; dates or details still being refined.
Layered rock mounds built by mats of microbes, among the oldest widely accepted evidence of life.
See it on the Life map →What happened
In shallow, sunny water, sticky microbial mats trap sand and help minerals crystallize. As sediment buries them, the microbes grow up through it and form a new layer, so domes and columns build up over centuries. The oldest widely accepted examples, 3.48 billion years old, are in the Pilbara region of Western Australia. They stayed common for billions of years, then declined after about 1 billion years ago. Living ones survive at Shark Bay, Western Australia.
Why it matters
They are visible, rock-solid evidence that life was well established by 3.5 billion years ago; later cyanobacterial mats helped oxygenate the planet.
Think of it like
Like a building that adds a new floor each time the ground level rises around it.
The evidence (fossils, genes, rocks)
Fossil shapes and microscopic layering, organic carbon in the rock, and close comparison with living Shark Bay stromatolites, studied since the 1950s.
Still debated
Some layered mounds can form without life, so telling biological from purely chemical layering in the oldest examples is still debated.
Inside this (1)
Connections (4)
▸Earliest claimed traces of life would predate StromatolitesDebated
The connection
The oldest widely accepted stromatolites are 3.48 billion years old, in Australia. In 2016 a team reported 3.7-billion-year-old cone-shaped stromatolites at Isua, Greenland, which would beat them by about 220 million years. A 2018 re-examination argued the cones are squeezed rock, not microbial structures.
When and how
Allen Nutman and colleagues announced the Isua find in 2016; Abigail Allwood and colleagues challenged it in 2018.
How sure are we?
The challenge used 3-D views and chemistry showing the shapes formed by deformation. Most geologists now doubt the Isua stromatolites.
Debates and open questions
Whether any true stromatolites survive in Earth's few rocks of that age, almost all of which have been heated and squashed.
▸Bacteria & archaea: the microbial world built StromatolitesWell established
The connection
Stromatolites are built by layered communities, not single species: sunlight-using microbes on top, others living off their waste below. Their sticky slime traps grains and helps minerals form, and the community keeps growing up through each new layer of sediment.
When and how
From at least 3.48 billion years ago in the Pilbara, Western Australia; still forming today at Shark Bay, Western Australia, studied since the 1950s.
The evidence
Living Shark Bay stromatolites grow this way and resemble ancient ones; fossil examples preserve microscopic layering and organic matter.
Debates and open questions
Which microbes built the earliest ones, and how to tell microbial layers from layers that minerals can form on their own.
▸Apex Chert 'microfossils' controversy offers disputed evidence alongside StromatolitesDebated
The connection
The Apex Chert lies in the same Pilbara region as the 3.48-billion-year-old stromatolites and the 3.43-billion-year-old Strelley Pool reefs. The stromatolites are now the stronger evidence for life at that time; the Apex threads are a disputed extra line. Together they show how carefully early-life evidence must be weighed.
When and how
Pilbara region, Western Australia; stromatolite work by Abigail Allwood (2006) and others; the Apex debate from 1993 and 2002.
How sure are we?
Stromatolites are accepted because shape, layering and chemistry agree; the Apex threads lack that agreement, so the pairing is as much contrast as support.
Debates and open questions
Whether the Apex dispute will ever be settled, and whether Strelley Pool microfossils, reported in 2011, now hold the title of oldest accepted cells.
▸Cyanobacteria built many later StromatolitesWell established
The connection
Once cyanobacteria evolved, they became the main builders of stromatolites, forming sunlit mats in shallow water. Stromatolite variety peaked about 1.25 billion years ago and then declined, perhaps as grazing animals and competing algae appeared. Today's Shark Bay stromatolites are also largely built by cyanobacteria.
When and how
From about 3 billion years ago (debated) to the present; Shark Bay, Western Australia, is the best-known living site.
The evidence
Cyanobacteria dominate living stromatolite mats, and fossil cyanobacteria appear in stromatolite-bearing rocks such as Canada's 1.88-billion-year-old Gunflint Chert.
Debates and open questions
Whether the oldest stromatolites, 3.48 billion years old, were built by cyanobacteria or by earlier microbes using photosynthesis that made no oxygen.