New traces of the first life on Earth have been discovered in Saudi Arabia

Stromatolites are the oldest geological record of life on Earth. These strange biotic structures are made of carpets of algae growing towards the light and precipitated carbonates. After their first appearance 3.48 Ga ago, stromatolites dominated the planet as the sole living carbonate factory for almost three billion years.

Stromatolites are also partially responsible for the Great Oxygenation Event, which drastically changed the composition of our atmosphere by introducing oxygen. This oxygen initially obliterated competition from stromatolites, allowing them to become established in Archean and early Proterozoic environments. However, as more life forms adapted their metabolism to an oxygenated atmosphere, stromatolites began to decline and appear in the geologic record only after mass extinctions or in difficult environments.

“The bacteria are still around, but they usually don’t have the opportunity to form stromatolites,” explains Volker Vahrenkamp, ​​author of the new study Geology. “They’re pretty much outmatched by the corals.”

In modern times, stromatolites are relegated to marginal extreme environments such as hypersaline marine environments (e.g. Shark Bay, Australia) and alkaline lakes. Until recently, the only known modern analogue of the biologically diverse, open shallow marine environment where most Proterozoic stromatolites developed was the Exuma Islands in the Bahamas.

That is, until Vahrenkamp discovered living stromatolites on Sheybarah Island, on the northeastern shelf of the Red Sea in Saudi Arabia. Vahrenkamp was studying tipi structures — domes of salt crust that can be seen from space — when he stumbled upon an unassuming stromatolite field. The discovery was surprising, but fortunately Vahrenkamp is one of the few people who have seen stromatolites in the Bahamas before.

“When I stepped on them, I knew what they were,” explains Vahrenkamp. “It’s 2,000 km of coast with a carbonate plateau, so basically it’s a desirable area to look for stromatolites… but then it’s the same in the Bahamas, and yet there’s only one small area where you can find them.”

Sheybarah Island is intertidal to shallow intertidal with regularly alternating wetting and drying conditions, extreme temperature fluctuations between 8°C and >48°C and oligotrophic conditions – similar to the Bahamas. As similar environmental conditions are widespread on the Al Wajh carbonate platform, other stromatolite fields may exist nearby. Vahrenkamp and his team have begun this exploratory work, but the stromatolites are small, about 15 cm in diameter, making them difficult to detect until one gets very close.

There are several hundred stromatolites in the Sheybarah Island area. Some are well-crafted, perfect textbook examples. Others are rather leafy, with low relief. “Maybe they could be juveniles,” posits Vahrenkamp, ​​”but we don’t know what a juvenile stromatolite looks like. They have to start small, but we don’t know.”

Part of the problem is that we don’t know how fast stromatolites grow. Dating them is very difficult because they contain two different carbonate components that are virtually impossible to separate: newly precipitated by microbes, which is interesting, and carbonate sand present in the environment, which is misleading. Currently, Vahrenkamp’s team monitors the field monthly to note any visual changes. There may soon be an attempt to transfer some of the stromatolites from Sheybarah Island to an aquarium and grow them there – an exciting experimental prospect.

Vahrenkamp’s discovery provides us with an opportunity to better understand the formation and growth of stromatolites. This will provide insight into early life and the development of oceans on Earth and may even help us in our search for life on other planets such as Mars. What would life on Mars look like and how would we know it? Looking at stromatolites, which were the first life forms on Earth, before our planet even had an oxygenated atmosphere, is the most promising path.