Chapter 1: The Beginnings of Life

What was it like when life first emerged on our planet? Earth has hosted life in various forms for nearly its entire existence.

If you had arrived in our Solar System shortly after its formation, the scene would have been utterly alien. The Sun would have had a mass similar to what it is today, but its brightness would only have been about 80% of its current luminosity, as stars gradually become more luminous with age. The four inner rocky planets would still be present, but Venus, Earth, and Mars would have shared strikingly similar characteristics: thin atmospheres, liquid water on their surfaces, and essential organic compounds that could lead to life.

While it remains unclear if life ever developed on Venus or Mars, evidence suggests that by the time Earth was just 100 million years old, it was already inhabited by simple organisms. After billions of years of cosmic evolution that created the necessary elements, molecules, and conditions, Earth became a vibrant cradle for life. Here’s a look at those initial steps.

The question of whether life began on Earth or existed prior to its formation is still debated, but the evidence leans towards life originating on our planet. (ERIC ERBE, DIGITAL COLORIZATION BY CHRISTOPHER POOLEY, BOTH OF USDA, ARS, EMU)

Life on Earth has several universally acknowledged characteristics. While it primarily relies on carbon-based chemistry involving elements like carbon, oxygen, nitrogen, hydrogen, and others such as phosphorus and sulfur, it’s conceivable that alternative elemental combinations could also support life. Four fundamental traits define all known life forms:

1. Life possesses a metabolism, deriving energy from external sources.
2. Life responds to environmental stimuli, adapting its behavior accordingly.
3. Life has the capacity to grow, evolve, or adapt to its surroundings.
4. Life can reproduce, producing viable offspring through internal processes.

All four characteristics must coexist for an organism to be deemed alive. Although snowflakes and crystals can grow and replicate, their lack of metabolism disqualifies them from being considered living entities. Proteins may have metabolic functions and the ability to reproduce, yet they do not respond to external stimuli. Even viruses, which straddle the line between life and non-life, can only reproduce by infecting living cells, raising questions about their classification.

Numerous organic compounds—such as sugars, amino acids, and more complex structures like polycyclic aromatic hydrocarbons—are abundant in interstellar space, in asteroids, and were prevalent on early Earth. However, there’s no evidence to suggest that life originated before the planet formed.

During the early Solar System, comets and asteroids bombarded nearly every celestial body. This era, known as the late-heavy bombardment, likely delivered many of the building blocks for life, but not living organisms themselves. (NASA)

The prevailing theory is that Earth was born with these essential raw materials and possibly more. Nucleotides may have been common; proteins could have been pre-assembled; and lipid bilayers might have spontaneously formed in aqueous environments. However, transitioning from precursors to living organisms required a conducive environment.

These three promising planets—Venus, Earth, and Mars—all had adequate gravity, thin atmospheres, and liquid water, along with biochemical precursors. Yet, Earth’s Moon provided an additional benefit that likely helped create conditions more favorable for life than those on its counterparts.

The early Earth had sufficient water to form oceans, lakes, and rivers, but not enough to envelop the planet entirely. This allowed for the development of continents and oceans, where tide pools—areas where water meets land—could exist, presenting opportunities for unique energy exchanges.

Sunlight, shadow, evaporation cycles, porous fluid movement, and varying water activity all contributed to the potential for molecules to bond in novel ways. While tides would have existed on all three planets due to the Sun's gravitational influence, Earth’s Moon enhanced these effects. Additionally, thermal activity from the planet's interior played a crucial role.

Hydrothermal vents, located at the ocean floor, are hotspots that may have been prime locations for life to emerge. Even today, these vents support extremophiles—organisms that thrive in extreme temperatures that typically disrupt life processes.

These vents generate significant energy and chemical gradients, where alkaline vent water interacts with acidic ocean water. They also contain ions like sodium and potassium and structures like calcium carbonate, which could have served as templates for the earliest cells. The existence of life in such extreme environments suggests that celestial bodies like Europa or Enceladus may also harbor life.

Deep beneath the ocean’s surface, around hydrothermal vents devoid of sunlight, life continues to flourish. The question of how life originates from non-life remains one of science's great mysteries. If life can exist in these depths, there is hope for life in the oceanic subsurface of Europa or Enceladus as well. (NOAA/PMEL VENTS PROGRAM)

The most plausible sites for life’s emergence on Earth are hydrothermal fields. Volcanic activity occurs not just underwater but also on land, providing additional heat and energy sources that stabilize temperatures, create energy gradients, and allow for the accumulation of essential ingredients.

On Earth, tide pools, hydrothermal vents, and fields were likely prevalent. While precursor molecules originated beyond Earth, it is probable that the transformation from non-life to life occurred here.

This aerial view of Grand Prismatic Spring in Yellowstone National Park exemplifies one of the most iconic hydrothermal features on land. The vibrant colors result from various organisms thriving under extreme conditions, influenced by the sunlight that reaches different parts of the springs. Hydrothermal fields like this are prime candidates for where life may have first appeared on our planet. (JIM PEACO, NATIONAL PARKS SERVICE)

As time passed, both Earth and its life forms underwent tremendous changes. It remains unclear if life began once, multiple times, or in various locations. What we do know is that all living organisms today share a common ancestor.

By examining the genomes of contemporary organisms, scientists can trace the timeline of LUCA: the Last Universal Common Ancestor of life on Earth. By the time Earth was less than 1 billion years old, life had already developed mechanisms for transcribing and translating genetic information, which persist in all organisms today. While whether life arose multiple times is uncertain, it is widely accepted that today’s life forms descended from a singular lineage.

Despite geological processes often obscuring the fossil record, we have been able to trace the origin of life back remarkably far. Microbial fossils found in sandstone date back to 3.5 billion years. Graphite found in metamorphosed sedimentary rocks suggests biogenic origins dating back to 3.8 billion years ago.

At even earlier times, certain crystal deposits in rocks indicate biological processes, implying that Earth was full of life as early as 4.3 to 4.4 billion years ago—just 100 to 200 million years after the formation of Earth and the Moon. To our knowledge, life on Earth has existed nearly as long as the planet itself.

At some point during Earth’s early history, abundant life precursor molecules, under the right energy and chemical conditions, began to metabolize energy, respond to their environment, grow, adapt, evolve, and reproduce. Although these early forms of life may have been unrecognizable to us, this marks the dawn of life on Earth. In an unbroken lineage of biological success, our planet has been alive ever since.

Hadean diamonds found in zircon/quartz, with some deposits dating back to 4.26 billion years, illustrate the planet's ancient history. (M. MENNEKEN, A. A. NEMCHIN, T. GEISLER, R. T. PIDGEON & S. A. WILDE, NATURE 448 7156 (2007))

While Venus and Mars may have had similar beginnings, drastic changes to Venus’ atmosphere turned it into a scorching world within a few hundred million years, while Mars’ magnetic field failure stripped away its atmosphere, leaving it cold and desolate. Although asteroid impacts may transport Earth-based life elsewhere in the universe, all evidence suggests that this is where life originated.

By 9.4 billion years after the Big Bang, Earth was bustling with life, and we have never looked back.