The search for the earliest evidence of life on Earth offers glimpses into the genesis of biological processes. Fossils serve as windows into this deep past, allowing scientists to piece together the history of our planet’s biosphere. Pinpointing the “oldest fossil” pushes the timeline of life’s origins further back, revealing how quickly life may have emerged on a young Earth. This quest also informs the search for life beyond Earth.
Discovery of the Oldest Known Fossil
The current frontrunner for the oldest known fossil comprises tiny hematite tubes and filaments, remnants of ancient bacteria, discovered in the Nuvvuagittuq Supracrustal Belt (NSB) in Quebec, Canada. These microfossils were found encased within layers of quartz, within some of Earth’s oldest sedimentary rocks. The NSB likely formed part of an iron-rich deep-sea hydrothermal vent system, providing a suitable environment for early life.
These microscopic structures are believed to be the fossilized remains of iron-oxidizing bacteria, similar to those found around modern hydrothermal vents. Researchers from University College London announced their discovery in 2017. The estimated age of these microfossils ranges between 3.77 and 4.28 billion years old, placing them remarkably early in Earth’s history.
Dating Billion-Year-Old Rocks
Determining the age of ancient geological samples relies on radiometric dating. This method measures the decay of unstable radioactive isotopes within rocks, which transform into stable “daughter” atoms at a predictable rate. Scientists do not directly date the fossils themselves; they date the igneous or volcanic rock layers surrounding the fossil-bearing sedimentary rocks.
Uranium-lead (U-Pb) dating is a widely used radiometric dating technique for ancient samples. This method is often applied to zircon crystals, minerals that incorporate uranium atoms into their structure but reject lead during their formation. As uranium within the zircon decays into lead, the ratio of remaining uranium to newly formed lead allows scientists to calculate the rock’s age. By dating volcanic ash layers or igneous intrusions above and below the sedimentary layer containing fossils, researchers can establish a minimum and maximum age, or “bracket,” the fossil’s age. This provides a reliable timeframe for when the organisms lived and were preserved.
Distinguishing Life from Geology
With specimens billions of years old, a challenge involves definitively proving whether a structure is truly a fossil (biogenic) or simply a product of non-biological geological processes (abiogenic). The relatively simple morphologies of early microbial life can often resemble mineral formations or alterations caused by heat and pressure. Scientists employ several criteria to establish biogenicity, including analyzing the morphology, size, distribution, and chemical composition of the structures.
For example, researchers look for cell-like shapes, evidence of internal organic matter, and patterns of growth characteristic of biological activity, rather than random mineral accumulation. Other famous contenders for the “oldest fossil” title include the 3.7-billion-year-old stromatolites from the Isua Supracrustal Belt in Greenland. While initially proposed as microbial mats, these structures have faced debate, with some scientists suggesting they could be mineral formations mimicking life, highlighting the rigorous and often contentious nature of this field.
What Ancient Microbes Reveal About Early Earth
The discovery of evidence for life appearing so early in Earth’s history suggests that life may emerge quickly under suitable conditions. If the Nuvvuagittuq microfossils are indeed biological, they indicate that life was present on Earth between 3.77 and 4.28 billion years ago, not long after the planet had cooled enough for oceans to form. This pushes back the known timeline for life’s origins, implying that the conditions necessary for abiogenesis—the process by which life arises from non-living matter—were established rapidly.
Such early evidence suggests that primordial Earth, perhaps around deep-sea hydrothermal vents, possessed the chemical ingredients and energy sources to support a nascent biosphere. These ancient microbes provide insights into the early Earth’s environment, which was likely very different from today, possibly iron-rich with a less oxygenated atmosphere. The implications of life emerging so early on Earth also extend to the search for extraterrestrial life, suggesting that life might be more common in the universe than previously thought.