What set the stage for the emergence of life on early Earth?

There will never be a single answer to that question, but there are many partial answers related to the global forces at play during that period.  Two of those globe-shaping dynamics are the rise of the magnetic fields that protected Earth from hazardous radiation and winds from the Sun and other suns,  and plate tectonics that moved continents and in the process cycled and recycled the compounds needed for life.

A new paper published in the Proceedings of the National Academies of Science (PNAS)  reports from some of the world’s oldest rocks in Western Australia evidence that the Earth’s crust was pushing and pulling in a manner similar to modern plate tectonics at least 3.25 billion years ago.

Additionally, the study provides the earliest proof so far of the planet’s magnetic north and sound poles swapping places — as they have innumerable times since.  What the switching of the poles tells researchers is that there was an active, evolved magnetic field around the Earth from quite early days,.

Together, the authors say, the two findings offer clues into how geological  and electromagnetic changes may have produced an environment more conducive to the emergence of life on Earth.

 

The early Earth was a hellish place with meteor impact galore and a choking atmosphere.  Yet fairly early in its existence, the Earth developed some of the key geodynamics needed to allow life to emerge.  The earliest evidence that microbial life was presented is dated at 3.7 billion years ago, not that long after the formation of the planet 4.5 billion years ago. (Simone Marchi/SwRI)

According to author Alec Brenner, a doctoral student at Harvard’s Paleomagnetics Lab,  the new research “paints this picture of an early Earth that was already really geodynamically mature. It had a lot of the same sorts of dynamic processes that result in an Earth that has essentially more stable environmental and surface conditions, making it more feasible for life to evolve and develop.”

And speaking specifically of the novel readings of continental movement 3.25 billion years ago, fellow author and Harvard professor Roger Fu said that “finally being able to reliably read these very ancient rocks opens up so many possibilities for observing a time period that often is known more through theory than solid data.”

“Ultimately, we have a good shot at reconstructing not just when tectonic plates started moving, but also how their motions — and therefore the deep-seated Earth interior processes that drive them — have changed through time.”

Today, the Earth’s outer shell consists of about 15 shifting blocks of crust, or plates, which hold the planet’s continents and oceans. Over eons the plates drifted into each other and apart, forming new continents and mountains and exposing new rocks to the atmosphere, which led to chemical reactions that stabilized Earth’s surface temperature over billions of years.

Earth has eight major plates and seven minor tectonic plates. Scientists have shown that Earth’s atmosphere owes its longevity, its components, and its incredibly stable Goldilocks-like temperature — not too hot, but not too cold — to the recycling of its crust made possible by plate tectonics. Earth’s oceans might not exist if water were not periodically subsumed by the planet’s mantle and then released. (National Park Service.

In 2018, members of the project revisited the Pilbara Craton, which stretches about 300 miles across Western Australia and is considered one of the oldest and most stable portions of the Earth’s crust. The team drilled into the primordial crust there to collect samples that were later analyzed for their magnetic history.

Evidence of when plate tectonics started is hard to come by because the oldest pieces of crust are thrust into the interior mantle, never to resurface. Only 5 percent of all rocks on Earth’s crust  are older than 2.5 billion years old — including many in Western Australia — and no rock is older than about 4 billion years.

Using cutting-edge techniques and equipment, the scientists concluded that some of the Western Australia surface was moving at a rate of 2.4 inches per year. That speed is more than double the rate the ancient crust was shown to be moving in a previous study by the same researchers.

Several other theories non-tectonic have been put forward to explain movements on the early Earth’s crust,  but the authors contend that both the speed and direction of this latitudinal drift leaves plate tectonics as the most logical and strongest explanation for it.

Overall, the study adds to growing research that shows that tectonic movement occurred relatively early in Earth’s 4.5-billion-year history and that early forms of life came about in a more moderate environment than earlier predicted.

Geologists Alec Brenner and Roger Fu, focused on a portion of the Pilbara Craton in Western Australia, one of the oldest and most stable pieces of the Earth’s crust. (Roger Fu)

In the paper, the scientists also describe what’s believed to be the oldest evidence of when Earth reversed its geomagnetic fields, meaning the magnetic North and South Pole flipped locations.

Since the forces that generate our magnetic field are constantly changing, the field itself is also in continual flux, its strength waxing and waning over time. This causes the location of Earth’s magnetic north and south poles to gradually shift, and to even completely flip.

These magnetic pole reversals are a relatively common occurrence in Earth’s geologic history, with the poles reversing 183 times in the last 83 million years and perhaps several hundred times in the past 160 million years, according to NASA.

The reversals themselves do not change much — unless you’re a bird or sea turtle or fish using magnetic lines to navigate — though for a period they do let in a somewhat greater amounts of cosmic radiation, solar wind and material from solar flares and coronal mass ejections.  But scientists say the effects have been found to be limited.

The reversal tells a great deal about the planet’s magnetic field 3.2 billion years ago. Overall, the message is that Earth’s magnetic field was likely stable and strong enough to keep solar winds from eroding the atmosphere — as some think happened to the early Martian atmosphere, which had less magnetic field protection.

 


Schematic illustration of the invisible magnetic field lines generated by the Earth, represented as a dipole magnet field. In actuality, our magnetic shield is squeezed in closer to Earth on the Sun-facing side and extremely elongated on the night-side due to the solar wind. Geophysicists are pretty sure that the reason Earth has a magnetic field is because its solid iron core is surrounded by a fluid ocean of hot, liquid metal. The flow of liquid iron in Earth’s core creates electric currents, which in turn creates the magnetic field. (Peter Reid, The University of Edinburg)

The authors used  magnetometers, demagnetizing equipment, and the Quantum Diamond Microscope — which images the magnetic fields of a sample and precisely identifies the nature of the magnetized particles — to create new techniques for determining the age and manner in which the samples became magnetized. This allowed the researchers to determine how, when, and in which direction the crust shifted billions of years ago, as well as the magnetic influence coming from Earth’s geomagnetic poles.

Their insights about the Earth’s magnetic field, combined with the results on plate tectonics, offers clues to the conditions under which the earliest forms of life developed in Earth.

But it  does not follow that if a planet has these conditions that life will emerge — there are way too many other conditions that need to be met.  But it may be the case that a strong magnetic field and some form of crustal movement and separation need to be present in some form for life as we know it to rise on other planets and moon.