How Did Life Begin?

November 4, 2015 | Sarah Tse

The Creation Michelangelo Vatican Museums Italy
Photo credit: gnuckx/Flickr (CC BY 2.0)

After centuries of research, we’re getting closer to an answer. Here are the leading theories on where that first spark of life came from.

Abiogenesis is the process of life arising from nonliving substances. Based on all we know about evolution and biodiversity, scientists are pretty sure that abiogenesis created the first, most simple organisms some time between 3.8 and 4 billion years ago. But there are a few very different theories floating around as to how abiogenesis happened.

The general consensus is that the first organic molecules formed from a “primordial soup” enriched with the chemicals that make up organic molecules: carbon, hydrogen, oxygen, and nitrogen. The famous Miller-Urey experiment in 1953 proved that such a mixture can spontaneously spawn complex organic molecules like amino acids and nucleotides, the basic components of proteins and DNA respectively. This initial chemical evolution must have been followed by biological evolution, as these organic building blocks assembled into highly structured cells.

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The original Miller-Urey experiment hypothesized that lightning may have provided that first spark of energy necessary to activate the organic molecules. Recent re-examination of the results of that experiment, using modern analysis techniques, indicated that an even greater variety of amino acids could be produced by adding hydrogen sulfide to the mix. Hydrogen sulfide is a prime component of volcanic gas, implying that the high volcanic activity of the early Earth, combined with lightning, could have created the perfect starter kit for life.

In addition to the peaks of volcanoes, the Earth’s molten interior escapes from chinks in the seafloor called hydrothermal vents. Our article on the discovery of ancient life-forms in these abyssal cracks describes how life may owe its first stirrings to this aquatic variety of primordial soup. Once the most fundamental organic molecules took shape, the minerals could act as catalysts to drive further reactions and form more complex structures. This theory is particularly completing because the ocean would have protected the fragile compounds from ultraviolet radiation and falling meteors.

Other recent experiments have turned to the skies as the origin of life on our planet. The Science Explorer has already covered recent experiments supporting a theory that the impacts of comets and asteroids delivered the ingredients necessary for this primal spark of life. Based on previous discoveries that these extraterrestrial bodies can contain amino acids, scientists recreated what might happen if a comet or asteroid impacted the ancient Earth. These experiments showed that collisions with Earth’s surface release enough energy that the mixtures of extraterrestrial and terrestrial chemicals are stimulated into forming complex organic compounds. A series of such impacts could have caused these compounds to rearrange into the first complete cells.

If comets and asteroids can plant the seeds of life, they can also sow those seeds amongst other planets. According to the theory of panspermia, life can spread from planet to planet via these celestial bodies, infecting new worlds like an epidemic. It’s possible that the very first organisms awoke on a distant planet and hitched a ride to Earth via a comet. This theory also implies that Earth is merely one of many habitable planets that has contracted life, a statistic in a galaxy full of living worlds.

This huge variety of possible methods by which the first organic molecules took form leads some scientists to believe that life could have independently arisen multiple times throughout the history of Earth. Life isn’t picky; no matter where the first organic molecules formed, they likely would have exploited any source of energy and minerals they could find. As we learn more about abiogenesis here on Earth, we also come closer to figuring out how it can happen on other planets.

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