Evidence of some of Earth’s oldest life forms may exist in 3.47 billion-year-old rocks from Northwestern Australia. Scientists believe these rocks contain evidence of sulfate-reducing bacteria, pushing back the known existence of these organisms by 750 million years. This discovery positions sulfate-reducing bacteria among the earliest life forms on Earth. The key to this finding lies in the analysis of sulfur isotopes within the rocks.
Sulfur Isotopes and Bacterial Preference: A Telltale Sign
The evidence doesn’t lie in fossilized bacteria, but in the ratio of sulfur isotopes within the rocks. Isotopes are variations of an element with the same number of protons but differing numbers of neutrons. When compared to sulfur 32, sulfur 34 has more neutrons, making it heavier. This subtle difference in weight influences the bacteria’s dietary preferences.
Sulfate-reducing bacteria prefer to consume lighter sulfur isotopes, like sulfur-32, especially when sulfate is abundant. The resulting sulfide waste product is also lighter. Analysis of the Australian rocks revealed that the sulfide contained significantly less sulfur-34 than the sulfates – 12 parts per thousand less. This suggests the lighter sulfur-32 was preferentially consumed by bacteria.
Challenging the Bacterial Hypothesis: Hydrothermal Vents
While the isotopic ratios strongly suggest bacterial activity, alternative explanations exist. Some argue that high-temperature geological processes, like those around hydrothermal vents, could also separate sulfur isotopes. However, the researchers argue the rocks never reached temperatures above 300 degrees Celsius, the threshold required for non-biological isotope separation. The presence of minerals like barite, believed to have originated as gypsum, further supports the theory of a cooler environment suitable for bacterial life.
Opponents of the bacterial hypothesis suggest that hydrothermal vents, emitting fluids at temperatures between 175 and 250 degrees Celsius, could have caused the observed sulfate reduction. Analysis of a third sulfur isotope, sulfur-33, by another research team supports this hydrothermal vent theory. They argue that the specific isotopic ratios observed can only be explained by gas-phase reactions occurring in such environments.
Supporting Evidence and the Bigger Picture: A Primitive Ecosystem
Despite the counterarguments, the researchers maintain that hydrothermal and atmospheric processes alone can’t fully explain the rock’s characteristics. They point to additional evidence of biological activity, such as stromatolites found directly above the analyzed rocks. These layered sedimentary structures are formed by microbes in shallow saltwater pools.
The presence of sulfate-reducing bacteria 3.5 billion years ago suggests a diverse early Earth teeming with microorganisms and a basic food chain. Sulfate reducers require dead organic matter and a source of sulfate, implying the existence of other primary producing organisms and potentially anoxygenic photosynthesizers. This points to a simple yet complete ecosystem. Further research on these ancient rocks, similar formations in South Africa, and even on Mars, could solidify these findings and potentially uncover even older signs of life.
