This blog post examines scientific hypotheses about whether seeds of life from space became the origin of Earth’s life or were the cause of extinction.
In 1908, Svante Arrhenius first proposed the bold hypothesis known as the spore theory, suggesting that seeds of life from beyond Earth flew in and became the origin of life on our planet. This hypothesis, based on the concept that microorganisms could travel through space, was highly innovative scientifically at the time. However, it was not accepted as a scientific theory because there was no way to verify it. Nevertheless, the spore hypothesis was intermittently mentioned among scientists as one possible explanation for the origin of life. While it failed to gain support due to the scientific limitations and technological shortcomings of the time, the questions it raised about the relationship between life and the universe continued to capture scientists’ interest.
Later, Francis Crick, who won the Nobel Prize for discovering the double helix structure of DNA, revived the spore hypothesis in his 1981 book ‘Life Itself’. Crick seriously explored the possibility that Earth’s life originated in space and proposed the idea that life might not have arisen only on Earth but could be widespread throughout the universe. However, this claim sparked considerable controversy even then. Even his wife remarked that he seemed a bit off after receiving the award. This serves as an example showing that scientific achievements and personal fame do not necessarily translate into trust in scientific hypotheses.
There is still no clear evidence to believe life exists beyond Earth. Nevertheless, some scientists are inclined to accept the existence of extraterrestrial life as fact, stemming from human curiosity about our origins and the spirit of cosmic exploration. They argued that when astronomers detected simple organic molecules like methanol in nebulae between stars via spectral analysis, this constituted evidence of extraterrestrial life. However, interstellar space is nearly a vacuum, making life unlikely to exist there. Therefore, while such discoveries do not directly prove life’s existence, they are significant in suggesting that the universe can provide the fundamental building blocks necessary for life.
Another piece of evidence supporting the possibility of extraterrestrial life is the extraction of organic molecules from meteorites. As demonstrated by the detection of 74 different amino acids in a fragment of the meteorite that fell in Murchison, Australia, in 1969, it is clear that organic molecules arrive on Earth from space, carried by meteorites. This discovery raises the possibility that life’s origin could have begun beyond Earth, prompting scientists to explore how organic molecules formed in space and how they were transmitted through the cosmos to Earth. Such research bridges the boundaries of space chemistry and biology, contributing to a deeper understanding of life’s origins.
Conversely, another hypothesis suggests that meteorites may have actually caused the extinction of life on Earth. Dinosaurs, once the dominant life on Earth, suddenly went extinct at the end of the Mesozoic Era. In 1980, Luis Walter Alvarez proposed a meteorite impact as the cause. This hypothesis was not mere conjecture but was proposed based on scientific evidence. He noted that the K-T boundary layer, a sedimentary layer marking the transition between the Mesozoic and Cenozoic eras, was found in numerous locations worldwide. This K-T layer contained high levels of iridium. Iridium is a rare element scarcely found on Earth’s surface but is relatively abundant in meteorites. Based on this, he hypothesized that a meteorite approximately 10 km in diameter struck Earth at the end of the Mesozoic Era. He argued that this event generated an enormous amount of dust, blocking sunlight for decades and causing a rapid drop in global temperatures, leading to the extinction of most life, including the dinosaurs.
Through fossil research, scientists have discovered that large-scale extinctions occurred multiple times throughout Earth’s history. For example, at the end of the Paleozoic era, trilobites and fusulinids suddenly vanished. Several hypotheses exist regarding the cause of these mass extinctions, and the possibility of an asteroid impact cannot be ruled out. Beyond asteroid impacts, factors like massive volcanic eruptions or climate change have also been suggested as potential contributors to extinction events. These factors may have acted in combination, causing abrupt changes to Earth’s ecosystems.
Today, many scholars readily accept the possibility that organic matter, the source of life, arrived from outer space via meteorites, and that meteorites could have caused the extinction of life on Earth. However, there is no consensus on the process by which organic matter formed on Earth. Even if organic matter arrived from outer space, the specific process by which it evolved into life on Earth remains unclear. Therefore, Carl Edward Sagan proposed a compromise view that both extraterrestrial and Earth-formed organic matter likely contributed to the birth of life. This perspective attempts to integrate various theories on life’s origin, emphasizing that its emergence was not a single event but a complex, multidimensional process.
Until decisive evidence is found, diverse hypotheses explaining life’s origin will continue to be proposed. Scientists are constantly searching for new evidence, refining existing theories, and striving to unravel the mystery of life’s origin. Through this process, our understanding of life and the universe deepens, and someday, the ultimate answer to life’s origin may be revealed. Until then, however, diverse hypotheses will coexist, and the journey of exploring the mysteries of life and the universe will continue.
