The Precambrian includes about 3.5 billion years of Earth’s history and is so named because it lasted until the Cambrian explosion, when almost all multicellular life on Earth evolved. The Precambrian is by far the longest era in the Geologic timescale and it saw the early evolution of the Earth itself. Despite this, relatively little is known about this period of time because even rocks rarely survive such a long interval. The oldest rock ever discovered is 4 billion years old and is one of the only remnants of the early Precambrian, while other Precambrian rocks are scattered throughout mountain ranges across the United States and other continents.

 

The Precambrian is further divided into the Archean (4-2.5 Ga) and Proterozoic (2.5-0.5 Ga) eons. (Read Ga as “billion years ago”.) During the Archean, the earliest continents and glaciers formed, plate tectonics may have started, and the earliest forms of life began. By the Proterozoic, the Earth had experienced its first ice ages and cyanobacteria and cellular nuclei had evolved. Especially important to us, the Proterozoic was the time of the Great Oxidation Event. Around 2.4-2.2 Ga newly evolved cyanobacteria began producing huge amounts of oxygen, eventually changing the Earth’s early atmosphere from one dominated by carbon dioxide, methane, nitrogen, and water vapor, to almost entirely nitrogen and oxygen.

 

While there are any number of intricate details I could list about the Permian, let’s break this down into my favorite summary tool ever: the timeline. About 4.4 billion years ago, the first crustal rocks formed as Earth, originally molten from a constant bombardment of meteorites and debris, began to cool. A few million years later, the Precambrian began (4 Ga), and early life may have originated around 3.5 Ga. All early life was microscopic, and one easily recognizable organism, stromatolites, are still alive today. Plate tectonics, the movement of the plates that make up Earth’s crust, may have started around 2.7 Ga.

 

Around 2.4-2.2 Ga widespread glaciers formed, starting the Earth’s first ice age. Around the same time, the Great Oxidation Event pumped oxygen into the Earth’s early atmosphere. Cell nuclei evolved around 2 billion years ago, and multicellular algae evolved several million years later (1.2 Ga). Between 750 – 580 million years ago (Ma) a second ice age covered much of Earth, informally called “Snowball Earth”. The melting of these enormous ice sheets may have allowed large animals with only soft parts unlike anything on Earth today to evolve at the very end of the Precambrian, about 570 million years ago. (One example is the Dickinsonia, described in this short PBS Eons video.) By 540 Ma, the Precambrian had ended.

While the history of Precambrian rocks is certainly interesting to future geoscientists like me, I’m well aware we’re a bit of a niche audience. Which is why many scientists researching the Precambrian choose to study not the Earth, but the life that evolved in a hugely inhospitable environment. Scientists now think that life evolved on Earth as early as 3.5 billion years ago, possibly in warm springs on the ocean floor. Or life may have ridden to Earth on a meteorite. No one really knows, but everyone wants to find out. Knowing where life first originated on Earth could be the key to finding life on another planet. While NASA now has the technology to send probes to other planets, the chances of finding evidence of life without knowing whether to search at the bottoms of oceans or somewhere on land are incredibly small. Once they know where to look, NASA scientists can use sensitive new tests to search for organic materials. We might find alien life not because we explore distant solar systems but because we learn more about our own planet.

 

The Earth of the Precambrian would likely be unrecognizable compared to the pictures we see today. The continents wouldn’t be covered in any greenery at all and wouldn’t even be in the same place or the same shape as we know today. So, whenever you next find yourself with a few spare minutes, try to picture your own Precambrian Earth. Would the continents be flat and grouped together or cratered and spread apart? Then compare it to the Earth we know today. Any differences prove that the Earth is a wonderfully dynamic planet which shapes the organisms that call it home and is shaped by them in return. This Earth is our Earth. It has never been this way before and never will be again. So, take care and remember: your actions have an impact, and will shape the Earth of the future.

 

Thanks for visiting the Precambrian with me!

 

All the data in this post was gathered from a wonderfully informative, yet simple USGS pdf created by D.A. Lindsey. Check it out if you want any more in-depth explanations about the Precambrian!