The Beginning of Life On Earth

Abiogenesis- The Origin of life

The origin of life is formally called abiogenesis. It refers to the process, when non-organic material transformed into the living forms. The details of this process remain misty, though it probably began about 3.5 to 3.9 billion years ago. Moreover, despite knowing when it happened, the beginning of life is one of the most hypothetical concepts in the theory of evolution. As a pretty advanced form of live we might think that this final step to living matter was a huge, revolutionary change. In fact, studies show that it was more like the rest of evolution processes- increasing slowly, and gradually. The oldest known organisms are fossilized single-celled forms that were found in hydrothermal vent and may appeared over 4 billion years ago.

beginning of life
Oldest known life-forms

The Oparin-Haldane hypothesis- gradual rising of life

In 1920s both scientists independently formed a hypothesis, that organic molecules could be formed from inorganic material in certain conditions. First of all, there had to be a source of energy (for example ultraviolet radiation). Secondly, atmosphere with low oxygen level, but containing ammonia, water vapor. Oparin-Haldane hypothesis also suggest that first organisms appeared in a warm ocean, and were heterotrophic (which means they had to get nutrition from organic substances, just like you). There are a lot of similarities in a way Oparin and Haldane thought, however there are also differences. Due to Oparin, life developed from spontaneously formed precursor of cells- spherical aggregate of lipids. He also confirmed that membrane-bound spheres were a key to functioning enzymes, and therefore- efficient metabolism. On the other hand, Haldane believed that simple organic molecules formed due to the energy from UV radiation, and then became more complex, therefore becoming cells.

beginning of life
That's how Earth might look like 3.5 billion years ago

Miller-Urey experiment- inorganic beginning confirmed

Three decades after the Oparin-Haldane hypothesis scientists took the next big step. Miller and Urey produced organic molecules from inorganic compounds, which proved the previous hypothesis. Scientist reconstructed conditions on prebiotic Earth. They combined warm water with a mix of gases- water vapor, methane, ammonia, and molecular hydrogen (that imitated the ocean). Afterwards they pulsed the atmosphere electrical discharges, which imitated heat in the form of lightning. That also proved that at the beginning of life climate was more intense than today, however there is no proof how first organisms adapted to it.

beginning of life
The Miller-Urey hypothesis

Modern concepts of the beginning of life

Recent studies of the beginning of life base on the previous two. The metabolism first hypothesis suggests that the first life-forms were metabolic networks. However, the RNA world hypothesis says that first life was self-replicating RNA, and metabolic networks showed up way later. Additionally, some theories suggest that abiogenesis never happened because life appeared on Earth via collision with a space object. That object carried some living forms, that migrated from outer space to our planet. Scientists call that hypothesis panspermia. Some studies show that inorganic material came to Earth on meteorites. Moreover, some scientists believe that organic compounds also came to our planet from space. In both theories, we are literally made of stardust.

beginning of life
Inorganic compounds came to Earth on meteorites

Oldest known life forms

The oldest known forms of life are dated to be even older than we expect. We do not know the exact date, place or form of life that was for sure the first one. The earliest direct evidences we know are micro-fossils of single-celled organisms, found in 3.465-billion-year-old Australian Apex chert rocks. In January 2018 scientists described a 4.5 billion year-old meteorite, found on Earth. It contained liquid water and organic substances, that may be ingredients for life. Finally, there is archaea. This group of organisms was thought to be bacteria-like, but in fact it is not closer to bacteria than it is to eucaryotes. The archeal lineage may be the oldest organism, due to sediments found in Greenland. The relation between archaea, eucaryotes and procaryotes remains unknown. The three domains of organisms evolved in different directions, though knowing their common or uncommon origin might be solution to the problem.

First organisms were single-celled

First eucaryotes

Eucaryotes are a domain of organisms with nucleus. The oldest evidences of their presence are 2.7 billion years old. The theory suggest that organelles (components of cells that have membranes) formed when prokaryotic organisms "swallowed" another one. It became endosymbiotic with the host and over the evolution process, they became one. Later, eucaryotes evolved forming multi-celled organisms, classified now in four kingdoms: Protista, Fungi, Animalia and Plantae. The last one's origin is a bit different though. The theory says that in case of plants, the "swallowed" prokaryotic organisms photosynthesized.

Multicellular life

Multicellularity evolved within different domains, first 3 billion years ago. Some studies suggest it started off as symbiosis or unsuccessful reproduction, other that a group of function-specific cells aggregated into a mass. Whatever it was, evolution from simpler, unicellular microbes was a turning point in the history of biology on Earth. Complex forms evolved in six eukaryotic groups, however, the very first multicellular organism was probably bacteria-like form. Some groups lost multicellularity, for many it evolved repeatedly. First multicellular organisms were soft mixes of cells, so there are no fossilized forms of them. Then, due to evolution process, cells in multicellular organisms became very diverse, forming more and more complex living forms. It changed the ecology of the planet, giving life incredible options of development. Those possibilities were used in a spectacular event called Cambrian Explosion, that will be the topic of the next article of this series.

Multicellularity completely changed life on Earth


Despite all studies, the origin of life on Earth remains quite misty, though very important. Knowing how life was born on our planet, we can try to understand how it might been born on other planets. Moreover, it would lead us closer to understanding the concept of life itself. But study of origin of life can tell us about way more than just life. It is a key in theory of evolution, for its mechanisms, and it might help us understand geological history of Earth. Exploring other fields like geology or astrophysics may also help us understand how life appeared on Earth, and if it was even born here. Another unanswered question is how common life is in the universe, and how different is it for the one we know.

Also Read: Basics of Astrophysics Series

Author's message

I hope this article helped you understand the concept of the beginning of life. For further studies feel free to contact me, also check out the links. In the fourth article we will discuss a misty revolution, that was a key to our existence. If you enjoyed the article, please share it with your family and friends.

Previous in series: How do organisms adapt to change?


  1. Abiogenesis article i enjoyed. Eventhough i knew about Haldane and stanley miller experiment, i was not knowing the word Abiogenesis. Thanks for the article. I request another elaborate article on this subject matter.

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