Over 3,770m years ago, the Earth looked very different. There were no plants, no animals, the sky was not blue. The surface would have resembled a bare rocky wasteland.
Yet it was around this time that we think the first life appeared, deep in the ocean around hot fissures in the seabed known as hydrothermal vents. Here, hot fluids circulate through the rocks on the seafloor, carrying iron and other elements out of the rocks and into the surrounding water. The chemicals and energy in these environments make them look like the perfect place for life to start.
To test this theory, my colleagues and I studied an ancient group of rocks in north-east Canada called the Nuvvuagittuq belt, dated to be between 4,280m and 3,770m years old. Preserved within this belt are iron formations formed in settings analogous to hydrothermal vents today. And in it we found microfossils that we believe to be 300m years older than the previous oldest known microfossils from rocks in western Australia dated to be around 3,500m years old. That makes these the oldest known fossils and possibly the oldest known evidence for life on Earth.
To uncover the fossils, we cut slices of the rocks so thin you could see through them and study them with a microscope. In doing so, we found microscopic filaments and tubes of iron, ranging in size from 5-10 microns in diameter, less than half the width of human hair, and up to half a millimetre in length. The tubes and filaments we saw were very detailed features that shared remarkable similarities with fossils of microbes in younger rocks and also modern microbes.
Features of these ancient filaments, such as their attachment to clumps of iron, are similar to those found in modern microbes, which use these clumps to hold themselves to rocks. These iron-oxidising microbes trap iron coming out of underwater vents, which they use in a reaction to release chemical energy. They then use this energy to turn carbon dioxide from the surrounding water into organic matter, allowing them to grow.