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2.3. Where could life have gotten started on Earth?

A core learning question from the Astrobiology Learning Progressions

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3.1. What are the characteristics of life?

Grades K-2 or Adult Naive Learner

As the early Earth began to “grow up,” it changed in many ways. Places like oceans and lakes and mountains formed on Earth. Each of these different places could have been a home for living things, as long as there was some water and a source of energy, like the Sun. Life needs energy to live – just like we eat breakfast to get us started for the day, early life on early Earth needed energy to get started.

Portrait photo of Comic-style illustration showing grey volcanoes spewing grey gaseous material, on a dark terrain and with blue-grey terrain behind.
Hydrothermal vents are one possible environment on Earth where life is thought to have started.
NASA/Aaron Gronstal

Grades 3-5 or Adult Emerging Learner

As Earth continued to change and develop, all kinds of environments formed. There was the land (the geosphere), the ocean (the hydrosphere) and the air (atmosphere); but also places like beaches, rivers, and even icebergs floating in the ocean. Each environment was unique and could have helped life get started in different ways. We know that life needs water and life needs energy. Some places might have gotten energy from lightning or from sunlight. There were also small volcanoes on the ocean floor that were shaped kind of like chimneys. Many of the building blocks needed for life could have come up from below and passed through the chimney where they interacted closely. The hot water that also came through the chimneys brought energy. On the early Earth, there were many places where life could have gotten off to a good start.

Portrait photo of Comic-style illustration showing grey volcanoes spewing grey gaseous material, on a dark terrain and with blue-grey terrain behind.
Hydrothermal vents are one possible environment on Earth where life is thought to have started.
NASA/Aaron Gronstal

Grades 6-8 or Adult Building Learner

The early Earth continued to change and evolve into a place where life could have gotten started. Oceans, beaches, rock surfaces, volcanoes, and other unique environments took shape as the young planet began to mature. Life arose through the interaction of life’s building blocks, or raw materials (small compounds like carbon dioxide, methane, and other carbon-containing molecules), in specialized environments on the early Earth where energy was available. These interactions led to the building of larger chemical compounds such as amino acids and the precursors to DNA . Where did these interactions take place?

One answer that some scientists think is likely is that these chemical reactions leading to life started in or around the early ocean, which was very different than today’s ocean. Just like salt from ocean water can form a crust on rocks near tide pools we see today, water from the early oceans that contained these raw materials could have evaporated, leaving them behind and concentrating them on the surfaces of nearby rocks where their chances of interacting are much higher than if they were swirling around in the vast ocean. Similarly, in hydrothermal vents (hydro=water; thermal=heat) on the ocean floor, the raw materials and hot water shot up through chimney-like vents and spewed into the cold water surrounding them. This environment may have provided a secluded place for the raw materials to interact as well as energy to facilitate the interactions. But wherever it happened, the key components to life’s initial emergence were the presence of raw materials, bringing them together so they can interact, and ensuring an energy source.

Portrait photo of Comic-style illustration showing grey volcanoes spewing grey gaseous material, on a dark terrain and with blue-grey terrain behind.
Hydrothermal vents are one possible environment on Earth where life is thought to have started.
NASA/Aaron Gronstal

Grades 9-12 or Adult Sophisticated Learner

As the early Earth settled down from its formation and continued the evolution of its crust, oceans, and atmosphere, many unique environments were taking shape – places like beaches, icy areas, rocky surfaces, hot springs, lakes and lagoons, seas, volcanoes, and even small hot water volcanoes on the ocean’s floor called hydrothermal vents (hydro=water; thermal=heat). Many of the raw materials of life (small, organic [aka carbon-containing] molecules such as carbon dioxide and methane) were available in these environments, and in order to for life to get started, they needed to interact with each other in the presence of energy. In today’s biology, enzymes help lower the activation energy needed for biochemical reactions to take place. But on the prebiotic Earth (pre=before; biotic=life), none of those enzymes existed yet. In order to get small molecules to interact with each other, they had to be brought into close proximity so chemical reactions could occur. This may have happened in a variety of ways on the early Earth.

In hydrothermal vent systems on the ocean floor, organic molecules in superheated water from below the crust could gush up through chimney-like structures (the vents) where there would be immediate interaction with the cold ocean water. Groups of molecules could have gotten caught in tiny pore spaces in the rock of the chimney (like a sponge made of rock) – spaces that may have emulated an early cell, in which prebiotic chemical reactions that gave rise to life could have taken place. Also, the interaction of the hot and cold water could have produced a temperature gradient in which the molecules could have sorted themselves and interacted in new ways.

On the ocean’s surface, hydrophobic (“water-fearing”) molecules could have formed in pools like small oil slicks floating on the water. Raw materials in ocean water that splashed up onto rock surfaces in early tide pool environments would have been condensed and concentrated as the ocean water evaporated, forcing interactions. As sea ice froze, small pockets of salty water containing organic molecules would have gotten progressively smaller, bringing the molecules closer together. Even the surfaces of some minerals like calcite can provide a platform for prebiotic chemical reactions, especially reactions which bind monomers of the same kind into longer chains (like a string of pearls).

There were many different kinds of environments on the early Earth that may have provided the conditions and energy needed for prebiotic chemistry to occur and to potentially evolve into biochemistry.

Portrait photo of Comic-style illustration showing grey volcanoes spewing grey gaseous material, on a dark terrain and with blue-grey terrain behind.
Hydrothermal vents are one possible environment on Earth where life is thought to have started.
NASA/Aaron Gronstal