Complex life emerged on Earth 591 million years ago, now scientists think they figured out how it happened

Around six hundred million years ago, our planet almost lost the invisible shield that keeps its surface hospitable. Earth’s magnetic field, generated by whirling molten iron deep below our feet, faded until it was so weak it was barely detectable.

At the same moment, oxygen flooded the oceans and atmosphere, and curious soft-bodied creatures began sliding across the seafloor. The coincidence has fascinated geologists and biologists for decades.

Fresh measurements from ancient crystals now reveal just how close Earth came to a magnetic stall. They also strengthen the suspicion that a waning field set the stage for the rise of complex life.

The new data trace a dramatic drop in magnetic intensity and help explain why the field later rebounded, saving Earth from becoming a dry, radiation-blasted rock like Mars.

Earth’s ancient magnetic field

To read the planet’s magnetic diary, researchers collected grains of pyroxenite and gabbro from South Africa, Brazil, and Canada.

Each mineral contains microscopic iron needles that froze in alignment with the ambient field when the rocks cooled.

By reheating single crystals in a carefully shielded furnace, the team coaxed out tiny magnetic signals that record field strength at the time of eruption.

After analyzing hundreds of crystals, University of Rochester geophysicist John Tarduno and colleagues saw a clear pattern.

“In general, the field is protective. If we had not had a field early in Earth history, water would have been stripped from the planet by the solar wind (a stream of energized particles flowing from the Sun toward Earth),” Tarduno explained.

“But in the Ediacaran, we had a fascinating period in the development of the deep Earth when processes creating the magnetic field … had become so inefficient after billions of years that the field almost completely collapsed.”

Very weak magnetic field

Crystals dated to 591 million years ago indicate a magnetic field about 30 times weaker than today – the most feeble long-term value ever measured.

Follow-up samples show the enfeebled state persisted for at least 26 million years.

Before and after that window, rocks more than 2 billion years old and younger than 565 million years record a field as sturdy as the modern one, revealing just how exceptional the Ediacaran lull was.

During the low-field stretch, solar wind could punch far deeper into the atmosphere. Charged particles stripped away lightweight hydrogen, a process that left relatively heavy oxygen behind.

The timing lines up with multiple geochemical markers that show a sharp increase in oxygen levels, supporting the idea that the weakened shield helped push Earth past a threshold needed for the development of large, mobile animals.

Thinner armor, richer air

The atmospheric link also finds support from geobiologist Shuhai Xiao of Virginia Tech.

“The magnetosphere shields the Earth from solar wind, thus holding the atmosphere to the Earth. Thus, a weaker magnetosphere means that lighter gases such as hydrogen would be lost from the Earth’s atmosphere,” Xiao explained to CNN.

As hydrogen escaped, oxygen concentrations rose, allowing metabolism-hungry organisms to branch out.

Tarduno agrees that other processes – such as photosynthetic microbes – were certainly at work.

“We do not challenge that one or more of these processes were happening concurrently. But the weak field may have allowed oxygenation to cross a threshold, aiding animal radiation (evolution),” he said.

Magnetic field and Earth’s core

Rock records also solve a puzzle about when Earth’s inner core began to form. Once field strength hit its nadir, it bounced back quickly.

“The observations appear to support the claim that the inner core first nucleated soon after this time, pushing the geodynamo (the mechanism that creates the magnetic field) from a weak, unstable state into a strong, stable dipolar field,” noted Peter Driscoll of the Carnegie Institution for Science.

A solid seed at the center released extra heat, turbocharging convection in the outer core and restoring the magnetic screen.

Strange seafloor pioneers

While the field flickered, the Ediacaran seas hosted life forms unlike anything alive today.

Disk-shaped Dickinsonia spread as wide as 4.6 feet (1.4 meters) across the mud; quilted fronds waved gently in the currents; and slug-like Kimberella scraped microbial mats for food.

These pioneers vanished before the Cambrian period began, around 539 million years ago, but their brief reign proved that multicellular bodies could flourish once oxygen climbed to breathable levels.

What if it happens again?

Not every expert is convinced the magnetic dip directly influenced the course of evolution.

“It is hard for me to evaluate the veracity of this claim because the influence that planetary magnetic fields might have on climate is not very well understood,” Driscoll cautioned.

Untangling all the variables – solar activity, mantle chemistry, biological innovation – will take years of careful detective work.

For now, the crystal record points to a simple narrative: Earth’s shield weakened, gases escaped, oxygen levels increased, and life took a bold new turn.

By tracing that chain of events, scientists gain insight into how planets and biospheres interact, a lesson that will guide the search for living worlds far beyond our own.

The full study was published in the journal Communications, Earth and Environment.

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