At that time, our planet’s magnetic field wobbles – and this would have critical consequences, they argue. Normally this field protects life on the surface by repelling cosmic radiation and charged particles emitted by our sun. But by 550 million years ago, it had fallen to a fraction of its current strength – before sharply regaining its strength. And in the wake of this planetary reboot, Earth witnessed the sudden proliferation of complex multicellular life on its surface. This was the Cambrian explosion, when most major groups of animals first appeared in the fossil record. Now scientists have linked it to events at the center of the Earth. graphic Our planet is made of spheres. There is a layer of rock 5-70 km thick that covers the Earth like an eggshell. This is called the crust, and below that is the mantle, which consists of a 3,000-kilometer-long layer of silicates. Further down, there is the outer core, of molten iron, and within it is another sphere – of solid iron. It has a diameter of over 2,000 kilometers and is growing by about a millimeter a year. “Earth’s magnetic field is created by the swirling of iron in the outer core,” said John Tarduno, a professor of geophysics at the University of Rochester in New York. “Before the Cambrian explosion the core was completely molten and its ability to generate a magnetic field collapsed.” Analysis of crystals in rocks in Quebec by Tarduno’s team showed that Earth’s magnetic field was less than 10% of its current strength and would provide poor protection from cosmic and solar radiation. The dynamo driving the Earth’s magnetic field was probably losing power due to the rapid loss of heat from the core, it has been argued. Then the core began to solidify at its center, which had profound consequences. It essentially made turbocharger motions in the outer core, restoring strength to the planet’s magnetic field. “Our research shows that the formation of the inner core started about 550 meters ago, and this happened just before the Cambrian explosion happened,” Tarduno said. Why and how the inner core was born was a mystery. From its tiny beginnings half a billion years ago, it has evolved into a moon-sized sphere of solid iron. It is the most metallic place on Earth and has had a significant impact on surface conditions. View of snowy Stolby Nature Reserve, Krasnoyarsk, Russia. These rock pillars date from the Cambrian period, more than 600 meters ago, to the Carboniferous period. Photo: Anadolu Agency/Getty Images More importantly, it gave our world a magnetic field. Observations of other worlds – where these fields have disappeared – reveal the dramatic consequences of this loss. An example is Mars, which lost its magnetic field 4 billion years ago. Without protection from the solar wind – the constant stream of protons and electrons pouring from the surface of the sun – the Martian atmosphere was blown into space, leaving its surface dead and arid. “Earth wouldn’t have evolved like Mars, but it certainly would have lost more water than it does today if it hadn’t restarted its magnetic field,” Tarduno added. “It would certainly be a much drier planet than the one we live on today.” However, the geophysicist was reluctant to speculate exactly how the regeneration of the Earth’s magnetic field would have affected the evolution of life. “I don’t think that the return of the Earth’s magnetic field and the subsequent explosion of life on Earth can be unrelated. But we cannot say what the exact pattern of events has been so far. This needs more study.”
