Life on Earth Emerged from Extraterrestrial Impacts

September 10, 2015 | Sarah Tse

A meteor falls above the treeline
Photo credit: Navicore/Flickr/CC BY 3.0. Photo has been cropped.

Catastrophe, or genesis? Popular culture associates comets and meteors with ravage and ruin, but new studies show that without them, life as we know it might not exist.

Mankind has searched for the origins of life on Earth since we first began to question the world and our existence. Over the last few millennia we’ve proposed countless explanations, but only recently have we amassed enough knowledge and technological advances to let us hazard a guess at what caused that first spark of life. Recent findings bolster one theory in particular: the first organic molecules—the ingredients of life—formed amidst the impact of a celestial object.

Before we had cells, we had complex organic molecules like proteins and carbohydrates. Proteins are especially critical for studying the inception of life (or "abiogenesis") because they can spontaneously catalyze or drive chemical reactions that form other organic structures. A groundbreaking experiment conducted by Stanley Miller and Harold Urey in 1952 tested abiogenesis by replicating what they thought to be Earth’s original atmosphere, and simulating lightning using electric shocks.

After a week, the resulting solution was found to contain amino acids, the smaller molecules that link together to form proteins. This experiment established the theory that life began in a primordial soup of inorganic molecules that, once stimulated by heat and electricity, came together into the first organic molecules.

In the last few decades, scientists have found that comets and meteors can also carry precursors to organic molecules like amino acids. This has led to hypotheses that extraterrestrial impacts may have played a role in forming the building blocks of life.

A team of Japanese researchers duplicated the impact of a meteorite on a primordial soup designed to resemble ancient oceans. They combined forsterite, a compound often found in comets, with bicarbonate and nitrogen, the main components of the oceans and atmosphere of prebiotic Earth, and finally shocked the mixture with a propellant gun.

The result? A medley of organic molecules, from various amino acids to nucleobases—the components of DNA and the blueprints for all life on Earth.

A second study carried the model a step further by analyzing how amino acids might react to a subsequent impact. Dr. Haruna Sugahara and Dr. Koichi Mimura used a different mixture—let’s say, a primordial goulash this time—containing amino acids, water ice, and forsterite at a cryogenic temperature of 77 K (about -200 °C or -321 °F).

After galvanizing the concoction with the propellant gun, they found that the amino acids had bonded together into short peptides, or amino acid chains, up to 3 units long. These types of peptides, formed under icy comet-like conditions, are more likely to evolve into longer peptides, which can eventually fold into a three-dimensional, functional protein. Once a comet provides this initial boost of energy, longer chain peptides, and even the enzymes that catalyze reactions, can form with less work.

An artist’s rendering of the Late Heavy Bombardment. (NASA)
Artist's conception of a comet smashing into an icy planet



Scientists currently estimate that life first appeared between 3.8 and 4 billion years ago—coincidentally, the time period called the Late Heavy Bombardment, when an abundance of asteroids collided with all the planets of the inner solar system. These findings piece together the story of how a series of collisions first created amino acids and nucleobases, then formed the amino acids into the very proteins that would assemble the nucleobases into DNA.

As if that weren’t exciting enough, an extraterrestrial basis for abiogenesis opens up the possibility for life to form on other celestial bodies. The moons of Jupiter and Saturn likely experienced a similar bombardment by asteroids and comets, and their icy conditions could also provide the perfect setting for chemical evolution, and eventually biological evolution, to begin.

This discovery reinforces the dualism that seems to underlie many phenomena in the universe: comet impacts have a reputation for causing cataclysmic destruction and mass extinction, but they also have the potential to sow the seeds of life.

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