What is the first evidence of life on Earth?
The biological structures found in the Dresser Formation are known as stromatolites, which are the preserved remains of 'microbial mats' stacked on top of one another. These mats form when communities of bacteria and other microbes secrete sticky substances that bind them together.
Over time, complex structures can develop as new mats grow on top of old ones. The mats nearest the top of these formations tend to contain photosynthetic organisms, while lower mats have different methods of sourcing energy such as the extraction of chemical energy.
The stacking of these mats can trap minerals between them, forming distinct structures that can be preserved. These structures, which can be observed in stromatolites formed today by living microbial mats, can then be used to identify similar forms in the fossil record.
'Dome-shaped structures with layers thickening toward the crest of the dome are characteristic of photosynthetic growth,' Keyron explains. 'Microbes do not grow at the same rate throughout a microbial mat, and these structures indicate growth towards a source of nutrients, which in this case is the Sun.'
'The other objects visible in the stromatolites are column-like structures separated by flat layers which appear very similar to a microbial growth texture called palisade structure. These are the result of upward microbial growth and are a major structural element of these layers.'
These distinctive characteristics, together with an ensemble of microscopic structures, can be used to identify potential life in the past even where no organic material remains.
A general lack of organic material in the Dresser Formation stromatolites has meant that their interpretation as biological structures is controversial. Other scientists have proposed that the structures could form non-biologically or have concluded that there is not enough evidence to decide either way.
The new paper uses a variety of techniques, including microscopy, chemical analysis and 3D scanning, to build a case that these structures have structural characteristics that could only represent evidence of life.
'If an archaeologist finds the foundations of a ruined city, they would know that it was something built by people as it would have all the hallmarks of having been built by people, such as bricks and doorways,' Keyron says.
'Similarly, stromatolites have structural elements that indicate their construction by microbes. This allows us to be archaeologists in deep time, even if the architects of the structures we study are much smaller.'