What Was It Like When Life Began On Earth?

Life has existed on our planet in some form for nearly its entire history.

If you were to visit our Solar System shortly after its formation, the view would have been unlike anything we know today. The Sun would have had the same mass but was only about 80% as bright, as stars gradually increase in luminosity over time. The four inner rocky planets remained, with Venus, Earth, and Mars all possessing thin atmospheres, surface liquid water, and the essential organic compounds that could foster life.

While it remains uncertain if life ever emerged on Venus or Mars, we know that by the time Earth reached the age of 100 million years, it was already home to living organisms. After billions of years of cosmic evolution that created the necessary elements and conditions for life, Earth became a thriving habitat. Here’s an overview of those formative steps.

Life on Earth shares certain fundamental characteristics. It is primarily based on carbon-based chemistry, which requires elements like carbon, oxygen, nitrogen, and hydrogen, along with other essential elements. The four key traits that all forms of life share are:

1. Metabolism: Life extracts energy and resources from the environment for its own use.
2. Responsiveness: Organisms react to external environmental stimuli and adjust their behaviors.
3. Growth and Adaptation: Life can grow, adapt, and evolve over time.
4. Reproduction: Life can create viable offspring through its internal processes.

All these properties must be present for a group of organisms to be classified as living. Although crystals can grow and reproduce, their lack of a metabolism excludes them from being considered alive. Proteins can metabolize and reproduce but do not respond to environmental changes. Even viruses, which occupy a gray area between life and non-life, depend on living cells for reproduction, raising questions about their classification.

Numerous organic compounds, including sugars and amino acids, exist in interstellar space and were plentiful on early Earth. However, there is no evidence to suggest that life originated before the planet’s formation.

The prevailing theory suggests that Earth formed with these essential building blocks and possibly even more complex molecules. It’s conceivable that nucleotides were common, or that proteins formed spontaneously, setting the stage for life. However, the right environmental conditions were crucial for transforming these precursors into living organisms.

Venus, Earth, and Mars all likely had suitable levels of surface gravity, thin atmospheres, and liquid water. The key difference for Earth was its Moon, which may have provided unique opportunities for life that the other planets lacked.

The water available on these planets was probably sufficient to form oceans, lakes, and rivers without completely submerging the land. This created environments like tide pools, where water meets land and can sustain various energy gradients.

Sunlight, shadows, evaporation, and mineral interactions could create conditions for molecules to bond in new ways. While tides on Earth were influenced by the Moon, all three planets experienced tidal movements due to the Sun. Additionally, Earth's thermal activity likely played a significant role in the origins of life, particularly around hydrothermal vents.

Hydrothermal vents serve as hotspots of geological activity, providing ideal conditions for life. These environments host extremophiles—organisms capable of surviving extreme conditions that would typically destroy life processes.

These vents create significant energy and chemical gradients, where alkaline vent waters mix with acidic ocean waters, facilitating unique chemical reactions. Such environments suggest potential habitats for life beyond Earth, such as on moons like Europa or Enceladus.

The most promising sites for life’s origins on Earth are hydrothermal fields. Volcanic activity occurs both underwater and on land, providing heat and energy necessary for stabilizing temperatures and facilitating evaporation and concentration cycles. This allows for the accumulation of essential ingredients in a confined space while also providing cycles of sunlight exposure.

On our planet, tide pools, hydrothermal vents, and fields were likely common, setting the stage for the transformation from non-life to life.

Earth has undergone significant transformations over time, as have its living organisms. It remains unclear whether life originated once or multiple times in different locations. However, all evidence points to a shared ancestor among all extant organisms on Earth today.

Through genomic studies, scientists can trace back to LUCA: the Last Universal Common Ancestor. By the time Earth was less than a billion years old, organisms had already developed the ability to transcribe and translate genetic information, a process that persists in all life forms today. While it is uncertain if life arose more than once, it is widely accepted that all current life descends from a single lineage.

Despite geological processes obscuring the fossil record, we have managed to trace the origin of life back remarkably far. Microbial fossils dating back 3.5 billion years have been discovered, alongside biogenic graphite deposits traced to 3.8 billion years ago.

Even earlier deposits suggest that biological processes may have been at work 4.3 to 4.4 billion years ago, shortly after Earth and the Moon formed. Life on Earth may have existed almost as long as the planet itself.

At some point, under the right conditions, the abundant molecules that serve as precursors to life began to exhibit the characteristics of living organisms: metabolizing energy, responding to environmental changes, growing, adapting, evolving, and reproducing. This marked the dawn of life—a continuous thread of biological success that has persisted ever since.

Although Venus and Mars had similar beginnings, drastic changes in Venus’ atmosphere turned it into an inhospitable environment after a few hundred million years, while Mars lost its magnetic field and became a frozen wasteland. While life may have been transported off Earth through asteroid impacts, evidence suggests that our planet is where it all began.

By 9.4 billion years after the Big Bang, Earth was alive with diverse life forms, and humanity has continued to thrive ever since.

Further reading on the Universe's history: - What was it like when the Universe was inflating? - What was it like when the Big Bang first began? - What was it like when the Universe was at its hottest? - What was it like when the Universe first created more matter than antimatter? - What was it like when the Higgs gave mass to the Universe? - What was it like when we first made protons and neutrons? - What was it like when we lost the last of our antimatter? - What was it like when the Universe made its first elements? - What was it like when the Universe first made atoms? - What was it like when there were no stars in the Universe? - What was it like when the first stars began illuminating the Universe? - What was it like when the first stars died? - What was it like when the Universe made its second generation of stars? - What was it like when the Universe made the very first galaxies? - What was it like when starlight first broke through the Universe’s neutral atoms? - What was it like when the first supermassive black holes formed? - What was it like when life in the Universe first became possible? - What was it like when galaxies formed the greatest numbers of stars? - What was it like when the first habitable planets formed? - What was it like when the cosmic web took shape? - What was it like when the Milky Way took shape? - What was it like when dark energy first took over the Universe? - What was it like when our Solar System first formed? - What was it like when planet Earth took shape?

Starts With A Bang is now on Forbes and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.