What is the most ancient fossil ever discovered?

While fossils like Sue the Tyrannosaurus rex often steal the spotlight, being the most complete T. rex ever unearthed and a remarkable 67 million years old, they pale in comparison to the oldest known fossils. These ancient relics, called stromatolites, are more than 50 times older than Sue, with an age of 3.45 billion years [source: University of Munster].

Though it’s difficult to grasp such vast spans of time, J. William Schopf, the paleobiologist who first uncovered these fossils in 1993, offers a perspective in his book "Cradle of Life: The Discoveries of Earth's Earliest Fossils." He explains that if Earth's entire history were compressed into a single 24-hour day, humans would show up in the final minute. In contrast, stromatolites would have existed for more than 18 hours, and remarkably, these ancient formations are still occurring today.

Unlike fossils like dinosaur skeletons, stromatolites were never part of a living organism. Instead, they develop in a way similar to how a cast is made from a statue, preserving the form of an object without containing it. In stromatolites, the 'cast' consists of thin layers of sediment and calcium carbonate—the same material that makes up limestone—that accumulate around colonies of cyanobacteria (also known as blue-green algae) and other single-celled organisms. Stromatolites form very slowly, keeping a record of thousands of years of life. Fortunately, stromatolites from every geological period still exist. By carefully studying these formations, scientists can uncover some of the last remaining clues about Earth’s first life forms.

When Earth first formed nearly 4.5 billion years ago, it was completely inhospitable. The surface was likely a molten mass for the first 800 million years, not exactly a suitable environment for life [source: Waggoner]. Once the planet cooled and solidified into continental plates, the first microorganisms emerged. Cyanobacteria were among the most important of these, thriving in shallow saltwater basins where they were protected from the sun's harsh rays but still close enough to the surface to engage in photosynthesis. Over time, a wide variety of stromatolites developed around these cyanobacteria colonies and other early life forms. The remarkable complexity of these structures is evidence that they were once full of life. These formations, despite enduring billions of years of geological upheaval, were discovered by Schopf in 1993 in Western Australia.

The discovery of these particular stromatolites was incredibly significant for many reasons. When Darwin introduced the theory of evolution, he acknowledged that gaps in the fossil record posed challenges to his idea that all life shared common ancestors. At the time, technology wasn't advanced enough to identify fossils older than a few million years, and for over a century, that remained the case. Some believed that the so-called 'missing link' between modern life and the earliest life forms would never be found, likely having been destroyed by eons of earthquakes and erosion.

Schopf’s discovery, however, was a game-changer. Biologists now had definitive evidence of when and what kinds of life first inhabited Earth, offering a clearer understanding of how life evolved. For example, during the Archaean period, when cyanobacteria and other life first appeared, Earth’s atmosphere consisted of methane, ammonia, and gases that would be lethal to most life today. Scientists now believe cyanobacteria played a crucial role in producing oxygen through anaerobic respiration. Who knows what other secrets stromatolites may reveal about the ancient Earth?

Sadly, while stromatolites still grow in a few places like Yellowstone National Park and parts of the Bahamas, they are much rarer today. If we fail to protect these remarkable formations, we could lose one of the planet’s most reliable and precise record keepers forever.