A study raises doubts about one of the theories of the origin of life on planet Earth

A new computer simulation, which contradicts decades of scientific ideas, suggests that the popular theory that comets played a crucial role in starting life on Earth billions of years ago may not be true. These results indicate that the fragile organic molecules that these icy bodies were supposed to carry with them would have been destroyed at the moment of impact. This undermines an established theory about the origin of life that dates back to at least 1961.

The broader idea - known as the panspermia hypothesis, which posits that the components of life came from space - is even older, as it was first scientifically proposed in 1908. Life on Earth requires complex organic molecules, but it is unclear whether our planet in its early stages contained sufficient quantities of the raw materials needed for the chemistry that preceded the emergence of life; That is, the chemical reactions that produce the basic building blocks of life.

Comets are composed of dust and ice, and are also known to contain organic materials. This structure - which caused scientists to call them "dirty snowballs" - led them to hypothesize that comets may have brought water and organic molecules to the Earth's surface.

A new study, published on Wednesday in the journal Scientific Reports, indicates that planet Earth may have witnessed, at an early stage in its history - more than 4 billion years ago - a period in which it was subjected to an intense bombardment from more than 100,000 comets, each of which exceeded 100 meters (328 feet) in size. However, most of these collisions were too violent for the molecules that make up life to survive.

The research team used the latest models related to comet formation and evolution to simulate that early era in the planet's history 100,000 times. “Given what we know about environmental conditions on early Earth, our simulations showed, on average, that no collisions would have occurred that would have allowed life to arise,” says Richard Anslow, lead author of the study, who conducted this research while doing his doctoral thesis at the Institute of Astronomy at the University of Cambridge in the United Kingdom. The comets contained high concentrations of basic molecules necessary for life, but simulations showed that these objects were arriving too quickly to successfully deposit those molecules.

“Given the high speeds at which these comets were colliding with Earth, it is unlikely, if not impossible, that they would have delivered intact organic molecules,” Anslow says. “So it appears that comets are not as important to pre-life chemistry, contrary to what was previously thought.” Anslow adds that 54% of the simulations showed no collision at all, while the remaining 46% saw at least one collision, while a "small minority" recorded up to three collisions.

He continued: "We cannot be certain that a successful collision is impossible; one of those extremely rare collisions may have occurred." However, for such exceptional cases to occur, an “ideal site” must be present that meets critical conditions, including the presence of liquid water and other chemicals necessary to support life.

He points out that the combination of these circumstances makes the entire process unlikely. Alternative Explanations The study focuses on a natural chemical compound known as hydrogen cyanide (HCN), an important precursor to the chemistry that preceded life, and has been observed in dozens of comets, including the famous Halley and Hale-Bopp comets.

Hydrogen cyanide (HCN) is composed of hydrogen, carbon and nitrogen, and when combined with water can form amino acids, the molecules from which proteins are built. Other studies have previously indicated that this compound may be a possible essential element for the origin of life on Earth.

“When comets collide with Earth, they are exposed to enormous temperatures and pressures,” Anslow says. “Recent studies have revealed that many of these elementary molecules can be damaged by these high pressures and temperatures, as well as by the effects of the collision itself.”

The study's authors believe that even if a successful collision occurred, it is unlikely to have provided this chemical compound in a useful concentration; Also, if the impact had occurred in the ocean, the compound would likely have melted very quickly. Anslow explains that to create the conditions for chemistry prior to the emergence of life (prebiotic chemistry), other elementary molecules are needed; Such as compounds containing phosphorus and sulfur, which are elements not usually found in comets.

“Comets are an excellent means of transporting hydrogen cyanide to Earth, but because comet collisions with Earth occur randomly, there is no guarantee that the rest of the components necessary for these chemical processes will be present at the same impact site,” he adds. Anslow points out that there are other promising paths that could be explored if comets were not the source that brought the building blocks of life to Earth. Another theory that has been proposed for a long time suggests that the planet Earth may have been able to create the molecules necessary for life on its own.

Asteroids can also be considered an alternative cosmic source of organic materials. They were hitting the ground at a slower pace and in greater numbers. In this context, Anslow refers to recent results reached by studying samples brought from the asteroids Bennu and Ryugu, which were found to contain most of the amino acids from which proteins are built.

The latest study also raises another interesting possibility: while conditions on Earth may not have been ideal for the formation of hydrogen cyanide (HCN) from comet impacts, ancient Mars may have been more suitable for it. “Because Mars is much smaller than Earth and farther from the sun, the speed of comets hitting it is much slower,” Anslow says. “So while on Earth these impacts would have destroyed most of the hydrogen cyanide, that was not the case on Mars.”

This idea may add a new dimension to the old question of whether the Red Planet harbored life at some point in its history. “It's just another angle to consider as we try to understand the early evolution of Mars in more detail,” Anslow adds.

The comet theory still stands. Queenie Chan, an associate professor of earth sciences at Royal Holloway University in London - who was not involved in this study - points out that while researchers have carefully analyzed how hydrogen cyanide can survive comet impacts, it is likely that this chemical underwent reactions before the icy bodies entered the Earth's atmosphere. This process may have led to the formation of biologically important compounds - such as the building blocks of proteins - that are less volatile and more durable.

“Therefore, the inefficient arrival of hydrogen cyanide (HCN) in its intact state does not completely negate the possibility that comets may have played a role in providing other important organic compounds that may have contributed to the organic stock that preceded the emergence of life on Earth,” Chan wrote in an email. For her part, Nicole Zellner, a physics professor at Albion College in Michigan, said in an email that she was not surprised by these results. Although she, too, does not rule out comets as a possible factor in the emergence of life, she acknowledges that “researchers have already established precise and reasonable limits to assess how likely this is.”