History of Life and the Formation of the Earth Study Guide

🗻 Big Picture: The first microbes that ever existed on earth were around 3.7 billion years ago. Then came photosynthesis and multicellular organisms with complex cells. How old is life on earth? Let's dig deep into it!


Do you ever wonder how everything - the world and our lives today - started? How did the earth come into existence?

These questions remain a mystery to us, and while we’ve made some huge advances in our understanding, they continue to be unsolved mysteries in some ways! In this guide, we will discuss the origin of life on earth and the process of earth formation to help us crack the code.

How can we track the history of life on earth?

Fossil records

Well, the major source to know the history of life on earth is fossil records. Fossils are basically the remains of organisms that existed centuries ago. They form when an organism dies and is encased in dirt/rocks, and is solidified by minerals over the years. Fossils are nature’s own photographs! Fossil records refer to a group of fossils that have been preserved and analyzed in chronological order.

Molecular clocks

Another major source of evidence of life on earth are molecular clocks. Molecular clocks are used for estimation of the time period when species existed in geologic history with the help of DNA. Molecular clocks trace the genetic mutations in species over time, to understand their origin and evolution.

Formation of the Earth

So far, we know that our planet earth is the only planet in the solar system that supports living organisms. Let’s dive into a short story:

  • Earth formed when the Solar system settled into the current look around 4.5 billion years ago.

  • Earlier it was a part of cloud dust from the sun that then changed into molten magma.

  • These changes took place over a few hundred years, and those years were responsible for cooling the earth's atmosphere, giving the earth its recognizable modern form.

  • The process mentioned above is also known as the accretion of Earth.

  • After the formation of Earth, many asteroids fell onto the earth's surface.

  • This led to the destruction of many landforms and the origin of water bodies. Scientists also say that many living species vanished during such asteroid bombardments.

But how did life begin on earth before asteroids started randomly attacking the planet?

When and how did life begin on earth?

history of earth in a day


The history of life can only be predicted with the help of traces of living organisms and fossil records or molecular clocks. According to fossil evidence, how far back in time did life on earth exist?

  • As we discussed above, Earth formed around 4.5 billion years ago, and as per many pieces of evidence, life came into existence around 3.7 billion years ago.

  • If we talk about the earliest life on earth, evidence shows that it started from microscopic organisms (microbes) around 3.77 billion years ago in Western Australia.

  • After that, we found microscopic organisms, stromatolites, preserved in hard rocks around 3.5 billion years ago. Stromatolites are considered the first forms of life on earth.

  • Fossils aided in identifying organisms in the preserved remains.

  • Cyanobacteria were the first organisms to synthesize their food in the presence of sunlight. These organisms were also the earliest to release oxygen into the environment.

  • These microbes then started connecting and living with each other, and scientists traced the presence of multicellular organisms at that time.

  • Multicellular organisms then changed into complex bodies and complex cell structures like eukaryotes.

Through molecular clocks, the DNA of mutated organisms could be traced back to their origin. After years of evolution, these animals evolved into different species - humans, fishes, birds, and more - all from a common ancestor.

✅ Conclusion

  • Life on earth came into existence long after the formation of the earth. Before that, the earth was full of toxic gases.

  • The first life on earth was a microbe that survived on chemicals and not oxygen.

  • Multicellular organisms came long after the first-ever life on earth and then evolved into more complex structures and organisms.

  • By now, we have found out several species that are currently present on the Earth or that ever existed on it. However, there are several life forms that we still do not know about.


1. What is the history of the formation of the earth?

Researchers say that the earth formed around 4.5 Billions year ago. Earth formed after the amalgamation of multiple solar nebulae present in space at that time.

2. What is the origin of life on the earth?

Earth used to be a lifeless planet with several elements on it. Scientists suggest that a nuclear geyser is likely to be the actual origin of life on earth.

3. When was life first formed on earth?

Traces show that the first life formed around 3.7 billion years ago. It was a fossil of a stone with a microscopic organism that can be evidence of it.

4. When and how did life begin?

Life first on Earth began around 3.7 billion years ago. A layer of stromatolites was traced on a stone, which shows the evidence of life in that time. These organisms were not dependent on oxygen for life; instead, they utilized other earth's atmosphere chemicals.

5. What were the first life forms that existed on Earth?

Microbes, mainly stromatolites, were the first life form that existed on Earth.

6. Where did the first life on earth come from?

Direct evidence of the first life on earth comes from Australian Apex chert rocks. Evidence shows that these rocks first had microbes on them.

7. Who came first on Earth?

The first thing that ever existed on earth were anaerobic organisms. Those microbes lived in the absence of oxygen under tough conditions. According to the fossil evidence, these microbes existed around 4.5 billion years ago.

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  1. History of Life. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/5.1/primary/lesson/history-of-life-bio/ Accessed 13 Dec, 2021.

  2. Formation of Earth. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/5.2/primary/lesson/how-earth-formed-bio/ Accessed 13 Dec, 2021.

  3. Evolution and the history of life on Earth. https://www.britannica.com/science/life/Evolution-and-the-history-of-life-on-Earth Accessed 13 Dec, 2021.