Six hundred million years ago, something strange happened. For billions of years, life on Earth had included only microbes, and simple soft-bodied organisms. Then, within just a few tens of millions of years, a variety of complex aquatic animals appeared.
Because these new animals had hard body parts, their fossils were more abundant than anything before. Among them, paleontologists have found ancestors of all major groups of animal life on Earth today. This burst of evolutionary change is called the Cambrian explosion. It was a critical event in life’s evolution, but paleontologists don’t understand why it happened.
Some geophysicists think that one culprit could have been Earth’s core, which is made of molten metal, swirling around a central sphere of solid iron. Currents in the molten metal act like a dynamo to generate Earth’s magnetic field. This magnetic field is critical to life on Earth, because it shields the planet from harmful space radiation.
Earth’s interior remains molten hot because of radioactive materials, and leftover heat from the planet’s formation. It has cooled as radioactivity decayed, and heat was lost at Earth’s surface. Geophysicists used mathematical models and studies of ancient rocks to determine that this cooling weakened Earth’s dynamo, and thus its magnetic shield. The cooling would eventually cause the innermost part of Earth’s core to become solid.
The models predict that this would turbocharge dynamo motion in the still molten outer core, renewing the strength of Earth’s magnetic shield. Studies of magnetic traces in Cambrian age rocks confirm that this magnetic field boost really happened—at the same time as the Cambrian explosion. The resulting improved radiation shielding might explain why the Cambrian explosion happened.