There was a time when the Earth was an easy target. The meteorites hit it recurrently, violently altering its surface. 3.26 billion years ago, a gigantic space rock larger than four Mount Everests—and up to 200 times larger than the one that extinct the dinosaurs— crashed into our planet and left a footprint so monumental that scientists are able to trace it to this day. But it did not manage to put an end to life, which was beginning to awaken in the form of single-celled beings. On the contrary, it strengthened it, according to a new study.
The research, published this Monday in the scientific magazine PNAS, outlines some of the consequences that this meteorite – between 30 and 60 kilometers in diameter, and named S2 – had for planetary dynamics. The main author of the study is Nadja Drabonearly Earth geologist at Harvard University (USA), who followed the meteorite trail to the Barberton Greenstone Belt in South Africa. Drabon explains that “there are only a few places in the world where we have rocks that old. This is because plate tectonics constantly destroys the Earth’s crust. Barberton is one of those few places left, and the rocks are remarkably well preserved.”
The first signs of the impact were found in the form of spherules the size of grains of sand. When such impressive blows occur, the meteorites and part of the Earth evaporate. This cloud of rock vapor goes around the world and these spherical particles are formed. The researchers found a similar layer associated with the impact that wiped out the dinosaurs, but that layer is less than a centimeter thick, while that of S2 is more than 15 centimeters.
The impact of that gigantic meteorite triggered a tsunami that stirred the ocean and dragged debris from the seabed towards coastal areas. The heat from the collision caused the upper layer of the sea to evaporate and, as a result, the atmosphere warmed. A thick cloud of dust covered everything, obscuring the planet and stopping any photosynthetic activity that was occurring.
In principle, the idea of a meteoric impact so powerful it can suggest that the entire Earth was razed and life was reduced to ashes. But, in reality, it was an impulse, as Drabon points out: “Until recently, the impacts were thought to be disastrous for evolution. However, this way of thinking is changing and it is now believed that life was not only resilient, but may have benefited from such violent events.”
A fertilizer pump
The researchers’ analysis reveals bacterial life recovered rapidly, causing a sharp increase in populations of single-celled organisms. Even bacteria need to feed, and the meteorite cooked them a perfect recipe. It is likely, the article details, that the tsunami produced by the impact dragged the iron lodged in the depths of the ocean into shallow waters, and that both the space rock itself and the increase in soil erosion added phosphorus to the Earth’s surface.
Scientists have no doubt that the meteorite probably had an initial negative effect on any life forms that lived on land or in shallow waters. But after that first blow, life quickly recovered. Drabon details it: “Before the impact, Earth’s early oceans were probably biological deserts due to a lack of nutrients and electron donors such as iron. “The crash released essential nutrients on a global scale.”
In fact, one of the students who participated in the field investigations described the meteorite as “a fertilizer bomb.” What the study highlights, according to its main author, is that this violence would have had benefits for life, which was in its beginnings. “It allowed it to flourish,” says Drabon.
Juli Pereto, professor of Biochemistry and Molecular Biology at the University of Valencia, points out that, before the meteorite, ecosystems suffered from a shortage of these elements, which limited the expansion of life. “After the impact, a kind of global fertilization occurred, making those limiting chemical elements more accessible and giving a boost to the diversification and propagation of microorganisms,” says this researcher who is not involved in the study.
Like the god Janus of Roman mythology, S2 had two faces. On the one hand, it could have been catastrophic for part of the nascent biosphere: those organisms that could not withstand the temporary warming of the sea or the darkening of the atmosphere, which could last for decades, perished. But it also represented an extraordinary evolutionary opportunity for microorganisms that until then had seen their expansion limited. “We can see it as further proof of life’s extraordinary capacity for adaptation,” summarizes Pereto.
This process, according to Jesús Martínez Frías, an expert in meteorites, planetary geology and astrobiology at the CSIC and president of the Spanish Network of Planetology and Astrobiology, is similar to what happens with large volcanic eruptions. “They have a destructive role, affecting environments and species, but also constructive because they rebuild marine contexts and continental regions that are affected by the catastrophe,” he points out.
The study that has just been published opens a new line of research to unravel the mysteries of the early Earth. “We are exploring how other microbes reacted to the impact, for example those that metabolize sulfur,” details Drabon. His team is also analyzing environmental changes after other high-impact events in our planet’s early history and how early living things took advantage of them.
