How This Planet became Our Planet
A history for Earth Day 2022
Modern Earth as seen from orbit. Originally taken Nov 15, 1969 by Apollo 12 astronauts. Restored by Toby Ord.
📸 NASA AS17-148-22727 AKA The Blue Marble, an image taken on December 7th, 1972 by the crew of Apollo 17
Today is the 52nd celebration of "Earth Day" since the holiday's conception in 1970. Though it's a relatively new observance, the meaning behind the holiday is incredibly important. It's an observance of the one thing every human from our early ancestors to the modern day has in common: the shared experience of our incredible planet.
That isn't hyperbole either; Earth truly is incredible as far as planets go. With liquid water, plate tectonics, a breathable atmosphere, and habitable living conditions, there's a lot that sets our blue dot apart from the pack. Of course, it's also worth mentioning that Earth is the only planet known to harbor the fragile yet persistent cycle of matter and energy we call life. 🌱
To celebrate Earth Day this year, we want to take you on a journey through Earth's history and discover how this planet came to become our planet!
📸 An artist's depiction of the formation of a star with visible accretion disk
📸 A closer perspective of the presolar accretion disk. Asteroids and rocky planetoids collide over a period of millions of years
4,568,200,00 years ago...
our solar system was a dust cloud of gasses and debris floating in outer space. There were no planets or stars here yet, but there was still plenty going on. Gravity pulled dust particles closer and closer together, slow at first but gaining in speed and power over time. This cloud condensed into a flat, rotating disk with a burning hot center. This was how our sun came to be formed.
Particles outside the sun spun rapidly and collided with one another, eventually accumulating enough material to become rocky masses known as "planetoids". These planetoids were made of the oldest known material in our solar system and formed the building blocks of the planets. Those which didn't grow into planets stuck around too, joining the inner and outer asteroid belts. Sometimes, they even rain down on Earth, bringing clues to our solar system's formation.
Over a period of 100 million years, the slow accretion of material caused one of these objects to gain so much mass that a gravitational force strong enough to curve and heat the planet into a spheroid, making our planet: Earth.
📸 Early Earth was a ball of molten rock. Any solid masses were quickly melted by bursts of lava and asteroid impacts.
Without a strong atmosphere, it was still easy for other objects to crash into our planet's surface. Most of these impacts only left behind craters on the new planet, but one exceptionally massive asteroid known as Theia blasted a large chunk of Earth's mass off the planet. Over time, the debris of this collision would also accumulate into a celestial body: Earth's unusually large moon.
This is where Earth's geologic story begins, but we still have a long way to go before the blue and green landscapes we know today. At this point, due to massive volcanoes, asteroid impacts, and heat generated from gravitational stress, the Earth was essentially a molten ball, meaning there's little geologic evidence from this time period. The oldest terrestrial material we know of, zircon crystals from Australia, date back 4.4 billion years (4.4 Ga), around the time of the Hadean Eon.
By the time of the Archean Eon (4-2.5 Ga) though, the Earth's surface began to cool. Due to the previously liquid state of the planet, elements would sink or float depending on their weight which created distinct layers known as the crust, the mantle, and the outer and inner cores.
📸 Modern stromatolites in Western Australia. Portions of the Earth would have looked quite similar at the dawn of life.
This layering caused many distinct features to appear on Earth. Despite the crust, molten rock could still push its way to the surface, causing portions of the crust to shift around the planet to compensate for the new mass. This process is known as plate tectonics and is responsible for our planet's volcanoes, earthquakes, and continental drift.
Another addition to our planet at this time was its magnetic field, which is believed to have been generated by the dynamo interaction of the metallic liquid outer core and solid inner core. Such a field is incredibly important and not just for your compass; a strong field prevented our atmosphere from being torn away by charged solar particles. Mars, which was similar to Earth at this time in some respects but lacked such a field, was not so lucky.
Finally, thanks to cooler temperatures it was possible for liquid water to exist on the Earth surface. It was in these pools that the earliest known forms of life appear in the fossil record. Microscopic bacteria lived in the shallow water and produced miles wide excretions of sediments that became the world's first fossils, stromatolites. The oldest of these fossils comes from the Dresser Formation in Australia and date back over 3.4 billion years.
📸 Banded iron from MM3 shows the effect of the Great Oxygenation Event on the geologic record. A sudden increase in oxygen created these beautiful red layers.
The course of evolution
With a stable atmosphere, plenty of water, and cooled temperatures, the most tumultous period of Earth's history had ended. This would begin the long development and diversification of life. The spread of cyanobacteria led to mass photosythesis across the planet, setting forth a chain of events that would bring about an enormous rise of oxygen in the atmosphere.
Over the course of nearly 2 billion years, oxygen levels would rise, lowering the greenhouse effect and allowed for development of the first multicellular life. Today, we can see evidence of this period of history in the red streaks of banded iron formations, where iron oxides formed between the two elements.
📸 A mass of hungry Trilobites scour the Cambrian seafloor for food
An Explosion of Life
Suddenly, 538 million years ago (538 Ma), a rapid diversification and evolution of new life forms began. The Cambrian Explosion, as it has come to be known, changed everything. Though some multicellular animals existed before this point, they were simple life forms which just floated through the ocean or sat on the sea floor. In the Cambrian, creatures were fast, strange, and hungry.
Arthropods, molluscs, and early fish all appeared within a few million years of each other, carving out a pecking order on the food chain. That may seem like a long time, but it is orders of magnitude faster than the slow accretion of solar particles and surface cooling at our planet's inception. Things were changing faster than ever before.
The game of predator and prey had begun between the creatures of the sea. Anomalocaris reigned supreme, but trilobites and nautiloids developed their own tactics of survival with hard calcite shells that survive today as amazing fossils.
📸 A sudden series of eruptions wreaked havoc on Earth's ecosystems.
As the oceans filled with new species, Earth's land masses began to see development of life as well. By the Silurian Period (443-419 Ma), plants, fungi, and arthropods explored the terrestrial surface. In the Carboniferous Period (358-298 Ma), Amniotes roamed the land as well. These early ancestors of mammals and reptiles produced offspring in sturdy eggs which allowed development of embryos on land.
Soon, green forests crossed the supercontinent Pangea, massive insects buzzed through the trees, and diverse groups of animals battled it out both above and below the sea line. Then, in a geologic instant, everything changed.
Over 90% of life on Earth perished over the course of only a million or so years. A series of massive volcanic eruptions covered 7 million square kilometers in lava, releasing immense amounts of carbon dioxide, methane, and nickel. Sudden global warming devastated life on Earth to an extent never before seen. The Permian-Triassic extinction event, or Great Dying, was not the first mass extinction in Earth's history, but it was the largest. Life did survive though in one form or another and out of the ashes came the age of dinosaurs.
📸 The iconic T. Rex is a symbol of the scale the dinosaurs could reach.
The Age of Dinosaurs
The Mesozoic Era (252-66 Ma) saw yet another boom of life after major change. Archosaurs that had survived the Great Dying now found themselves as the most prolific group on land and the iconic dinosaurs would flourish on Earth for almost 200 million years without challenge.
If you were to visit this era, it might not look too much different from the modern day if you ignored the dinosaurs. Conifers dominated the landscape and flowering plants appeared in the Cretaceous. It was even possible to find a few early mammals.
The end of the dinosaurs came just as sudden as their dominance. The origin of their demise is debated among scientists, with massive volcanic activity and meteorite impact being top contenders. Either way, the dinosaurs faced an unfortunate and fiery end.
As an aside, if you'd like to dig a little deeper into the Mesozoic, we'd recommend checking out the Age of Dinosaurs collection page!
📸 Though not as ferocious as a T. Rex, the Woolly Mammoth was just as impressive in statue, albeit millions of years later. Despite its size, it would meet its match from a much smaller predator...
The Cenozoic (66 Ma-Modern Day)
The last 66 million years is one of the shortest periods of time we've talked about so far, but includes everything from the extinction of the dinosaurs to this very year!
After the end-Cretaceous event, our planet was ruled by the small. It was only tiny creatures that were able to survive and reproduce following the extinction of large food sources. That wouldn't stay the case forever though, as with the absence of great lizards, mammals and birds were able to grow quickly in size.
The climate cooled and tectonic plates shifted into recognizable continents. Cycles of new species came and went with some leaving behind descendants and others disappearing entirely. Then, about 2.5 million years ago a primate learned to walk on two legs...
📸 Rock art from the Tadrart Akakus in Libya, which date back to 12,000 BCE.
📸 Another shot from Toby Ord's restorations. This photo was originally taken on July 24th, 1969 by the Apollo 11 crew on their way back to our home.
When we describe Earth as our planet, we cannot mean it in a possessive sense. Between the massive timescale it has experienced, the many feet that have traveled its surface, and the incredible changes it's overgone in its 4 billion year life span, Earth is a thing far beyond our small grasp. It is a wonder to be able to experience even a small fraction of it.
By "our" planet we mean that we are of Earth — it is an essential part of our species' story, not the other way around.
This Earth Day, remember that while Earth may not have been made for us, we were most certainly made for Earth.
If you'd like to learn more about our planet's history in depth and explore the rise of human cultures as well, check out our book, Relics!