One of the unanswered existential questions is how life on earth started. It’s probably one of the most significant and intriguing mysteries in science. While there is no definitive answer, several scientific theories attempt to explain how life came about on our planet.
The abiogenesis theory proposes that life arose from non-living matter through natural processes. The early Earth had a very different environment compared to that of today, with a reductive atmosphere containing simple molecules such as methane, ammonia, water vapour and hydrogen. Under specific conditions, these molecules could have interacted with each other to form more complex organic compounds, including amino acids and nucleotides, the building blocks of proteins and DNA/RNA, respectively. The famous Miller-Urey experiment in the 1950s demonstrated that it was indeed possible to generate amino acids under conditions that simulated the early Earth’s atmosphere and energy sources like lightning.
Other theories postulate that life may have originated elsewhere in the universe and that it arrived here on earth via comets or asteroids – or was even brought here by little green men from an advanced extra-terrestrial civilization. As far-fetched as some of these speculations are though, they still don’t explain how life first came about. The actual process of life’s origin remains elusive, and so scientists continue to explore this fascinating topic. The study of life’s origin is interdisciplinary and combines fields like biology, chemistry, geology, and astronomy to gain a better understanding of the early Earth’s conditions and the processes that might have given rise to life.
What could the first life forms on earth have looked like? We can get some clues by looking at structures called stromatolites that still exist today.
Stromatolites are living fossils (bear with me while I explain this seeming oxymoron) and the oldest living lifeforms on our planet. The name derives from the Greek, stroma, meaning “layer”, and lithos, meaning “rock”. Stromatolite literally means “layered rock”. The existence of these ancient rocks extends three-quarters of the way back to the origins of the Solar System.
Photo: Economic Times
Stromatolites are fascinating structures that have been critical in the evolution of life as we know it. They started to form from aggregates of cyanobacteria (also known as blue-green algae).
Cyanobacteria are among the oldest known organisms on Earth, with evidence of their existence dating back more than 3.5 billion years. They are thought to have evolved from ancient photosynthetic bacteria through a process called endosymbiosis, in which a host cell engulfed a photosynthetic prokaryote (small single-celled organism that lacks a nucleus and other organelles) and then formed a symbiotic relationship with it.
Cyanobacteria are important because they were the first organisms capable of oxygenic photosynthesis, which involves the use of water as an electron donor and the release of oxygen as a by-product. This process had a profound impact on the Earth’s atmosphere, leading to the oxygenation of the planet and paving the way for the development of aerobic life forms.
The oxygen released by the aggregates of cyanobacteria caused precipitation of a layer of calcium carbonate around them. Sediment then covered this layer in shallow water, and bacteria grew over it, binding the sedimentary particles and building layer upon millimetre-thick layer until the layers became mounds. The process continued – more cyanobacteria grew on top of the mounds, causing more calcium carbonate to precipitate, gradually forming larger and larger stromatolite structures.
The stromatolites’ empire-building brought with it their most important role in Earth’s history. They breathed out oxygen. Using the sun to harness energy, they produced and built up the oxygen content of the Earth’s atmosphere to about 20%, giving the kiss of aerobic life to all that was to evolve.
What happened to the stromatolites? Most of them are now only evident as fossils. As oxygen became available, the biodiversity of the oceans increased, including micro-organisms that love to eat stromatolites. These micro-organisms destroy the typical layered appearance and turn them into structures known as thrombolites. In addition, more advanced life forms like sea snails relish a snack of the blue-green algae found in the stromatolites.
Despite these challenges, stromatolites do continue to form today, but only in a few lagoons or bays where the salt content is too high for their predators. They grow in greatest abundance in Shark Bay in Western Australia. These mat-like layers of blue- green algae are able to grow in the shallow waters of the bay because evaporation causes high salt concentrations that discourage snails and other organisms from eating them.
Living stromatolites can also be seen in Southern Africa’s Lake Makgadikgadi in Botswana. The lake is part of the Makgadikgadi Pans, a large salt pan complex. These modern stromatolites offer a unique opportunity to study the growth and development of these structures in a contemporary setting.
Closer to home, fossils of stromatolites can be found in the Namaqualand schist, specifically near the town of Springbok.
Fossils of the earliest known stromatolites – about 3.5 billion years old – are on record. With Earth an estimated 4.5 billion years old, it’s staggering to realise that we can get an idea of how the world looked at the dawn of time when the continents were forming. Before plants. Before dinosaurs. Before humans.
Photo and text from the BBC:
“And then I saw them. There were thousands of pumice-hued stromatolites quasi-camouflaged beneath the ripples, submerged like migrations of ancient turtles holding their breaths under the slightly opaque water. I was awestruck. Blocking out the peripheral surrounds and imagining the sky methane orange from volcanic activity, this is what life looked like at the beginning of time.”