layers was suggestive of biological activity having occurred there. "Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present." In August 2016, an Australia-based research team presented evidence that the Isua Greenstone Belt contains the remains of stromatolite microbial colonies that formed approximately 3.7 billion years ago. However, their interpretations are controversial. If these structures are stromatolites, they predate the oldest previously known stromatolites, found in the Dresser Formation in western Australia, by 220 million years. The complexity of the stromatolites found at Isua, if they are indeed stromatolites, suggest that life on Earth was already sophisticated and robust by the time of their formation, and that the earliest life on Earth likely evolved over 4 billion years ago. This conclusion is supported in part by the instability of Earth's surface conditions 3.7 billion years ago, which included intense asteroid bombardment. The possible formation and preservation of fossils from this period indicate that life may have evolved early and prolifically in Earth's history. The stromatolite fossils appear wavy and dome-shaped, are typically high, and were found in iron- and magnesium-rich dolomites that had recently been exposed by melting snow. The surrounding rocks suggest that the stromatolites may have been deposited in a shallow marine environment. While most rocks in the Isua Greenstone Belt are too metamorphically altered to preserve fossils, the area of stromatolite discovery may have preserved original sedimentary rocks and the fossils inside them. However, some geologists interpret the structures as the result of deformation and alteration of the original rock. The ISB sedimentary layers containing the possible stromatolites overlay volcanic rocks that are dated to 3.709 billion years old and are capped by dolomite and banded iron formations with thorium-uranium zircons dated to years old. All layers, including those bordering the stromatolites, experienced metamorphism and deformation after deposition, and temperatures not exceeding . The identity of the ISB features as stromatolites is controversial, because similar features may form through non-biological processes. Some geologists interpret the textures above the putative stromatolites as sand accumulation against their sides during their formation, suggesting that the features arose during the sedimentary process, and not through later, metamorphic deformation. However, others suggest that the rocks are so altered that any