Precambrian facts for kids
Pre-cambrian (or Precambrian) is a term used to describe the eons before the Cambrian. It lasted from about 4.567 billion years BC to about 542 million years BC. It starts with the beginning of the Earth, about 4.567 billion years BC. When it ends, there were many hard-shelled animals around. It is named after the Roman name for Cymru / Wales, because rocks thought to be from this time were first studied there.
The Precambrian is sometimes shortened to pC or Cryptozoic. It makes up about 88% of all the time Earth has been around! That's a really, really big chunk of time!
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What is it?
The Precambrian isn't a specific period like "Jurassic" or "Triassic." Instead, it's a general term for all the time from when the Earth formed (about 4.6 billion years ago) until the start of the Cambrian Period (about 538.8 million years ago). It's divided into three big chunks of time called eons: the Hadean, the Archean, and the Proterozoic.
- Hadean Eon (4.567 billion to 4.031 billion years ago): This is the Earth's babyhood! It was a time of volcanoes, hot temperatures, and lots of crashing rocks from space.
- Archean Eon (4.031 billion to 2.5 billion years ago): Things started to cool down a bit. The first continents might have started to form, and the very first life appeared!
- Proterozoic Eon (2.5 billion to 538.8 million years ago): This is the longest part of the Precambrian. During this time, the atmosphere started to get more oxygen, and more complex life forms began to evolve.
Why Don't We Know More About It?
Imagine trying to read a book that's been buried in the ground for millions of years. That's kind of like studying the Precambrian! Many of the rocks from this time have been changed by heat and pressure (metamorphosed), or they've been worn away by wind and water (erosion), or they're buried deep underground. Also, there weren't many creatures with hard parts like bones or shells back then, so there aren't many fossils to find. The fossils we do find, like stromatolites (more on those later), can be tricky to understand.
Most of what we know about the Precambrian has been discovered since the 1960s. Scientists are still working hard to piece together the story of this ancient time.
How the Earth Formed
Scientists think the Earth formed about 4.543 billion years ago from dust and rocks orbiting the Sun. A little later, a Mars-sized object might have crashed into the Earth, and the debris from that crash formed the Moon! By about 4.433 billion years ago, the Earth had a stable crust. We know this because scientists have found tiny crystals called zircons in Western Australia that are about 4.404 billion years old.
Important Note: The term "Precambrian" is handy for talking about this long stretch of time in general. However, it's not an official term on the geologic time scale. The official time scale uses the names of the eons (Hadean, Archean, and Proterozoic) instead.
Life in the Precambrian
The biggest mystery of the Precambrian is when life first appeared. It's hard to say for sure, but scientists have found clues that suggest life might have been around as early as 3.8 billion years ago! They found carbon in rocks from Greenland that might have come from living things.
In Western Australia, scientists have found tiny fossils of bacteria that are over 3.46 billion years old! They've even found possible fossils that are 100 million years older in the same area. Some evidence suggests life might have evolved even earlier, over 4.280 billion years ago.
For most of the Precambrian, the only life on Earth was bacteria and other tiny single-celled organisms. These organisms lived in the oceans and formed big mats called stromatolites. Stromatolites are layered rocks made of sediment and the remains of these tiny organisms. They're some of the oldest fossils we have!
Around 2.1 billion years ago, more complex life forms might have appeared. These were multicellular organisms, meaning they were made up of more than one cell. However, it's hard to be sure if these ancient fossils are really multicellular organisms. Some possible examples include a red alga from Russia (2.45 billion years old), carbon traces from China (1.65 billion years old), Rafatazmia (1.6 billion years old), and a red alga from Canada (1.047 billion years old).
The oldest fossils that scientists widely agree are complex multicellular organisms come from the Ediacaran Period (635 to 542 million years ago). These fossils are found all over the world and include a variety of soft-bodied creatures. They're called the Ediacaran biota or Vendian biota.
At the very end of the Precambrian, creatures with hard shells started to appear. This marked the beginning of the Cambrian Period and the start of the Phanerozoic Eon, which is the eon we're living in today! During the Cambrian Period, there was a big increase in the variety of life forms. This is called the Cambrian explosion.
Even though there weren't any plants or animals on land during the Precambrian, bacteria and other microbes formed mats that covered the land. Scientists have even found tracks from an animal with leg-like things in mud that's 551 million years old!
How Life Might Have Started
One idea about how life started is called the RNA world hypothesis. RNA is a molecule similar to DNA. The RNA world hypothesis says that RNA came before DNA and proteins. During the Hadean Eon, there were lots of hot, watery places on Earth where RNA could have formed and replicated. Scientists have shown that RNA can replicate itself in these kinds of environments. This could have led to the evolution of the first life forms.
The Earth's Environment
It's hard to know exactly what the Earth was like during the Precambrian. Scientists think that the continents moved around a lot, forming supercontinents (big landmasses made up of all the continents joined together) and then breaking apart again.
One supercontinent called Rodinia formed around 1.13 billion years ago and broke up around 750 million years ago. There were also several ice ages during the Precambrian. One of the most famous is the Sturtian-Varangian glaciation (850 to 635 million years ago). Some scientists think this ice age was so severe that the entire Earth was covered in ice! This is called the "Snowball Earth" theory.
The atmosphere of the early Earth was probably very different from today. Most scientists think it was made up mostly of nitrogen, carbon dioxide, and other gases, with very little oxygen. However, some evidence suggests that there might have been some oxygen in the atmosphere as early as the Archean Eon.
Eventually, photosynthetic life forms (organisms that make their own food using sunlight) evolved and started producing oxygen as a byproduct. This caused a big change in the Earth's atmosphere, called the oxygen catastrophe. At first, the oxygen combined with other elements in the Earth's crust, like iron. This formed banded iron formations, which are rocks with layers of iron oxides. After all the iron was used up, oxygen started to build up in the atmosphere, leading to the high-oxygen atmosphere we have today.
How the Precambrian is Divided Up
As we mentioned earlier, the Precambrian is divided into three eons:
- Hadean Eon (4.567 billion to 4.031 billion years ago)
- Archean Eon (4.031 billion to 2.5 billion years ago)
- Proterozoic Eon (2.5 billion to 538.8 million years ago)
The Proterozoic Eon is further divided into three eras:
- Paleoproterozoic Era (2.5 billion to 1.6 billion years ago)
- Mesoproterozoic Era (1.6 billion to 1 billion years ago)
- Neoproterozoic Era (1 billion to 538.8 million years ago)
The Neoproterozoic Era includes the Ediacaran Period (635 to 538.8 million years ago), which is when the first complex multicellular organisms appeared.
Some scientists have suggested dividing the Precambrian into eons and eras based on major events in Earth's history, rather than just using numbers. For example, they could divide it into eons based on the formation of the Earth, the end of the Late Heavy Bombardment (a period of intense asteroid impacts), the appearance of banded iron formations, the first continental red beds (rocks with iron oxides), and the first animals.
Precambrian Supercontinents
The Earth's plates are always moving, and this movement has caused continents to collide and form supercontinents. The earliest known supercontinent was Vaalbara, which formed about 3.636 billion years ago and broke up about 2.845 to 2.803 billion years ago. Other supercontinents that formed during the Precambrian include Kenorland (formed about 2.72 billion years ago), Columbia or Nuna (formed about 2.1 to 1.8 billion years ago), and Rodinia (formed about 1.3 to 0.9 billion years ago).
See also
In Spanish: Precámbrico para niños
