In a large shallow embayment on the Australian west coast lies one of the oldest life forms on Earth.
They look unspectacular, but these stone-like objects hold information that reveals how life evolved billions of years ago. The organisms, located in Shark Bay, a vast marine area on the Australian west coast, are colonies of algae that have formed hard, dome-shaped deposits. For three-quarters of the entire history of life, they dominated the Earth’s ecosystem. Quite simply, they ruled the world. Those left today are living samples of how Earth once was.
The Shark Bay area, listed by UNESCO in 1991, is famous for other things than its ‘living fossils’. The shallow waters, which have an average depth of nine metres, are twice as salty as what is normal, with the salient value increasing towards the south of the bay. These conditions have given birth to seagrass meadows totalling 4,800 square kilometres (1,850 square miles), making them the largest and richest in the world.
This seagrass provides food and shelter for numerous marine species, entrapping shells, skeletons and debris moving with the tidal flow. Geographically, the bay is also well protected from outside disturbances. These factors have attracted rich biodiversity and fauna, including 240 bird species, 320 fish species and more than 800 types of plant. In the water, there are humpback- and southern right whales, bottlenose dolphin and a large ray population. Green turtles nest on the beaches. And yes – it has sharks too.
Yet biologically and historically, the highlight is the stony organisms thriving in the shallow waters. The fossils, called ‘stromatolites’, are made of simple single-cell organisms called ‘prokaryotes’. For three billion years, these were the basis of higher life forms on Earth. Nothing else existed. Back then, they created extensive reefs along the floors of the early seas. They have since hardened into rock-like fossils, revealing to us how the Earth functioned in its earliest stages.
“Surprisingly, one way we know this is from the fossil record,” says Ken McNamara, senior lecturer in Earth sciences at the University of Cambridge. “While the term ‘fossil’ generally conjures up the idea of dinosaur bones, or strange shells of long-extinct animals, cells can also sometimes get fossilised. But more remarkably, perhaps, is the ability of some groups of simple prokaryotic cells to make rock. They literally have the ability to cause the crystallisation of calcium carbonate that over hundreds or even thousands of years very slowly builds cauliflower-like mounds of rock.”
These are the stromatolites; the stony objects gracing the seafloor. They can grow as slowly as five centimetres per century. The examples in Shark Bay are no more than 2,000 to 3,000 years old, but they are identical to those formed in ancient times. They thrive in Shark Bay because the water is so salty: none of its predators can survive in such conditions, leaving the stromatolites in peace. Their specific location is a seabed called the Hamelin Pool.
There is another reason why they thrive here. The Hamelin Pool is shallow, which enables the stromatolites to feed off sunlight. And that, says McNamara, is important. “What is also remarkable about these cells is that, like plants, they photosynthesise, obtaining their energy from the sun, absorbing carbon dioxide, converting it into rock and giving off oxygen,” he explains. “Indeed it has been suggested that the oxygen we breathe today originally came from the activity of such prokaryotic cells billions of years ago.”
Such discoveries demonstrate why these organisms are important. UNESCO says they “greatly assist in the understanding of the nature and evolution of the Earth’s biosphere until the early Cambrian [time period]”. According to McNamara, similar organisms exist elsewhere in Western Australia. Some have formed quite recently; others as far back as 3,460 million years. “We therefore have this ideal situation in Western Australia of being able to observe how these structures form today, and use this information to help us understand the early Earth and the conditions under which life first gained a foothold on the planet,“ he says.
Photos: Janelle Lugge, jlarrumbe, Rob Bayer [all via Shutterstock.com].