Scientists question a decades-old theory about the origin of life

A new computer simulation casts doubt on a decades-old theory that assumes that comets contributed to the origin of life on Earth, as it indicates that the organic molecules they carried would have been destroyed by violent collisions. This theory dates back to the year 1961, while the idea of ​​the arrival of the components of life from space, known as “mass scavenging,” dates back to the year 1908.

Comets, made of dust and ice, contain organic materials, which has led scientists to believe that they may have transported the water and molecules needed to start life to Earth, which is not known whether it had enough raw materials in its early days to form the basic building blocks of life. According to a study published in the journal "Scientific Reports" on Wednesday, more than 4 billion years ago, the Earth was bombarded by more than 100,000 comets, each of which was more than 100 meters in diameter, but most of the collisions were so violent that they did not allow these particles to survive.

To simulate this stage, the researchers conducted 100,000 simulations using the latest models of comet formation and evolution. “Our simulations showed that, on average, no successful collision occurred, based on what we know about early Earth surface conditions,” said Richard Anslow, the study's lead researcher from the University of Cambridge.

Although comets contain molecules essential for life, their speed upon collision with Earth prevented them from delivering these molecules intact. “Comets were not as useful to prebiotic chemistry as previously thought, as very few, if any, of them were able to transport intact organic molecules,” Anslow said.

The simulation showed that 54% of the collisions did not occur at all, while 46% of them witnessed a minimum of one collision, and only a very small percentage recorded three collisions. He added: "We cannot rule out a successful collision, as one of the exceptional collisions may have achieved this," but explained that the matter would have required an ideal location in which liquid water and chemicals necessary for life were available, which makes this possibility unlikely.

Alternative Explanations The study focuses on hydrogen cyanide (HCN), an essential chemical compound in the early stages of life that has been observed in dozens of comets, including Halley and Hale-Bopp's comets. Hydrogen cyanide (HCN) is composed of hydrogen, carbon, and nitrogen, and can produce amino acids, the molecules that make up proteins, when it reacts with water.

Previous studies considered this compound as one of the possible components of the origin of life on Earth. "When comets collide with Earth, they are exposed to high temperatures and pressures, and recent studies have shown that these conditions and subsequent collisions destroy many elementary molecules," Anslow said.

The researchers believe that even if a successful collision occurred, the comet is unlikely to have transported a sufficient concentration of this compound, and it would have dissolved quickly if it had fallen into the ocean. Anslow explained that initiating the chemical reactions that preceded the emergence of life requires other elementary molecules, including phosphorus and sulfur compounds, which are not usually found in comets.

He said: “Comets are a good source of hydrogen cyanide, but their random collision with the Earth does not guarantee that all the components necessary for these reactions are present in the same location.” He added that there are other possible explanations, including an old theory that assumes that the primitive chemical mixture on Earth itself produced the molecules necessary for life, or that its source was asteroids that collided with Earth at lower speeds and in greater numbers.

He pointed out that samples from the asteroids Bennu and Ryugu revealed the presence of most of the amino acids involved in the formation of proteins. The study raises another possibility, which is that ancient Mars may have been more suitable for hydrogen cyanide to arrive via comets.

Anslow said that the small size of Mars and its distance from the sun make the speed of comets colliding with it much less, which reduces the collapse of this compound, unlike what happens on Earth. This could add a new dimension to questions about the possibility of Mars harboring life in the past.

"It's another angle that should be taken into consideration to understand early Mars evolution in more detail," Anslow said. The comet theory still stands. On the other hand, Queenie Chan, assistant professor of earth sciences at Royal Holloway University in London, who was not involved in the study, said that hydrogen cyanide may have undergone chemical reactions before comets entered the Earth's atmosphere, although researchers are focusing on studying its ability to withstand collisions.

She added that these reactions may have produced compounds important for the origin of life, such as amino acids, that are less volatile and more resilient. Chan continued: “The inefficiency of comets in transporting hydrogen cyanide intact does not negate their potential role in delivering other organic compounds that contributed to providing the materials necessary for the emergence of life on Earth.”

For her part, Nicole Zellner, a professor of physics at Albion College in Michigan, said she was not surprised by the results, but she does not rule out the contribution of comets to the origin of life, noting that researchers have set clear criteria to estimate the extent of this possibility.