Updated: Apr 18
As more discoveries are made, the probability of life coming from outer space grows, the panspermia theory has grown in plausibility over the years, could humans simply be ancestors from Mars or derived from asteroids or comets.
World renowned astronomer Carl Sagan once famously said that the question of the nature of life on Earth and the question of whether life can be found beyond Earth are two sides of the same question. With the case of panspermia, both questions are answered.
The panspermia idea is that life does exist beyond Earth and life as we know it on our planet came from outside of our atmosphere. Advocates for the panspermia theory have suggested several celestial objects are the source of life on Earth, including other star systems, asteroids, comets, and even Mars.
Early Suggestions About Panspermia
Panspermia is an ancient theory, dating back to the ancient Greeks. Some of the most well known astronomers who have backed this theory in modern times include, Chandra Wickramasinghe, and Fred Hoyle.
To-date, no hard evidence has been brought forward confirming this hypothesis to be factual, however, discoveries have occurred in the past century that certainly makes panspermia more plausible. In some of the more recent discoveries, some of the most intriguing evidence points to the fact that life may have once existed on Mars.
Another line of evidence that has given panspermia more interest and credibility is the fact that the earliest indisputable evidence of life in the geologic record occurs around 3.8 billion years ago, which was close to the end of the Late Heavy Bombardment (LHB), this was a period of time where the rate of collision between planetary bodies in the solar system was much greater than it is today.
Many argue that any life that existed on Earth prior to the LHB would have been wiped out, meaning that we would have required an extraterrestrial origin for present life as we know it today. This is just speculation, but when looking at the timeline of the emergence of life on Earth, it certainly makes the theory more plausible.
Did Life Come From Mars?
In 1984, in the Allan Hill region of Antarctica a meteorite was discovered. It was simply named, ALH84001, and it went almost unspoken about and generated little interest for the next ten years. In 1994, following geochemical and microscopic analysis it was proven that the meteorite was of Martian origin, it was also a very unusual Martian meteorite as that.
Scientists uncovered that the meteorite contained complex organic molecules known as polycyclic aromatic hydrocarbons (PAHs), which are normally associated with microbial activity.
They also found iron sulphide mixed with magnetite, these two minerals do not normally occur together in the presence of carbonates, unless produced by life.
The scientists investigating the meteorite also found strange looking nanostructures, shaped like rods, which had a strong resemblance to certain types of bacteria. Given these three unusual discoveries within the meteorite, this led many scientists to view this as probable evidence of life on Mars.
These discoveries made surrounding the meteorite ALH84001 forced many scientists to take another look and express further interest in it. Over the years, the evidence that was seemingly pushing towards the belief of life on Mars has now become considered doubtful my many who have favoured a non-biological origin.
It is now known that PAHs can form readily in non-biological settings when organic molecules are subject to heating and the rod-like structures that resemble bacteria are likely too small to have ever been living things.
However, the magnetite and iron sulphide are still believed to be evidence for the presence of biology in ALH84001, many of the grains of magnetite are approximately the same shape and size as the grains produced by bacteria.
To this day, there is currently no known formation mechanism in which magnetite grains of that nature form in association with iron sulphide without the presence of biology.
The answer to whether life existed, or indeed does exist on Mars will likely not be answered until a sample return mission is completed, which is planned for some time in 2030s. More evidence is currently mounting to support the case that Mars was once a wetter and more habitable planet that life could have thrived on in the distant past, however, this is far from conclusive as yet.
Even if it is proven that life existed on Mars, the idea that life from Mars could have seeded life on Earth is met with even more implications.
Scientists say there are two main issues with the possibility of life on Mars seeding life on Earth. The first one is could any type of life have survived the journey from Mars to Earth. The second issue, is there a reason to believe that life would have been more likely to have formed on Mars first rather than on Earth. Surprisingly, the answer to both questions, might just be yes.
Even if microbes could potentially survive the impact event and the extremely hostile environment in outer space, would they be able to live long enough to make the journey from Earth to Mars, which in theory could last for millions of years?
Interestingly, bacterial spores recovered from extinct bees that had been trapped in amber and brine inclusions in ancient salt crystals suggest that bacterial spores can last up to 250 million years and remain viable. However, it must be noted that a brine or amber inclusion in a terrestrial salt crystal are still mild environments compared to that of the interior of a collisional fragment from Mars drifting through space.
Scientists have also explained that they have a reason to believe that it could potentially have been easier for life to form on Mars first rather than Earth. While life as we know it requires water, the formation of the building blocks of life are inhibited by the presence of water. In the very early history of Earth, after it cooled, it was likely covered in a global ocean. Mars however, went through periodic wet and dry periods during its very early history.
A recent hypothesis for the origin of life is that life first formed in shallow bodies of water, or pools, that would frequently be exposed to sunlight that periodically dried out. This would have gradually resulted in the build up of organic compounds with each dry cycle, leading to layers of complex organics being formed that could become the ingredients for life. The fact that Mars was unlikely to have been covered in a global ocean as well as having periodic wet and dry periods may have made early Mars an easier place for life to form than Earth. There is now a lot of scientific support which claims that this is a very plausible theory. For all we know, the human race could in fact all be Martians.
Could Life Have Come From Comets or Asteroids?
For those who believe in panspermia, another popular theory is that of comets and asteroids. Astronomer and mathematician, Chandra Wickramasinghe believes that life is likely to have been formed in the interiors of comets. He was consistently argued that the trillions of cometary objects that are likely to exist in the solar system provide much more opportunity for life to be formed than that of the early environment of Earth.
Another alternative that has been discussed is that it is possible that a comet could have been infected with life whilst passing through the upper atmosphere of a life-rich planet, allowing life to implant itself on a comet without having to actually have been formed on the comet. This theory doesn't help to explain how that life could have formed, it pushes the question further back in time and to a different planetary body that we may know little or nothing about.
If comets did contribute living matter to Earth, they would also likely have contributed water. It has long been suggested that water on Earth was provided by comets, however, isotopic studies of comets in the past have discovered that many of them have different deuterium-hydrogen ratio than water that is found in the oceans on Earth. Recent studies in comets show that some of them have consistent deuterium-hydrogen ratios that are consistent with those in the oceans on Earth, suggesting that comets could potentially be the source of some of Earth's water, life and organics.
Another potential for the origin of life could be asteroids, rocky bodies that are mainly found between the orbits of Jupiter and Mars. Most asteroids are believed to be the fragments of protoplanetary bodies that could have been the size of Vesta, approximately 530 kilometers (321 miles) in diameter. These primordial parent bodies of the asteroids would have been destroyed in huge collisions during the early days of the solar system. Most of the current knowledge we have of the composition of asteroids comes from the study of meteorites, most of which have come from asteroids. Many studies of meteorites show that the asteroid parent bodies would have been large enough and had enough radioactive isotopes in their interiors to go through either full differentiation or at least metamorphism. Many meteorites have shown evidence of aqueous alteration in the interiors of their parent bodies, showing that they had been heated enough for a liquid water to be present.
Many meteorites have been found to be rich in organics, including amino acids. Many asteroid and meteorite parent bodies could potentially have had conditions in their interiors not very chemically different from the environment in which life is believed to have formed on Earth, and even Mars.
The undeniable evidence of aqueous alteration, including the presence of organics and clays in asteroidal meteorites, provides more evidence to impacting asteroids being a probable source of life on Earth as far as the panspermia hypothesis is concerned.
We must stress that this is currently just speculation, so far, no evidence has been proven to conclude that life did indeed come from asteroids or comets, but recent discoveries in the study of comets and asteroid science makes the argument for the panspermia hypothesis much stronger.
Did Life Come From Other Star Systems?
To start with, most scientists were doubtful that microbes within dirt clods or small rocks could survive the huge distances between the stars, especially since a trip could potentially last hundreds of millions or even billions of years. It was also believed to be highly unlikely that any objects from other star systems would ever come close enough to a planet in an alien star system to collide with it.
In recent years this opinion has changed quite considerably, mainly because of the discovery of two interstellar bodies that have passed through our solar system in the last few years. The first one being Oumuamua, the name which means "first messenger from afar" in Hawaiian. Astronomers using a telescope operated by the University of Hawaii discovered Oumuamua in 2017. In 2019, the second object was discovered, named 2I/Borisov, this was a more typical object, being indistinguishable from local solar system comets in all but its hyperbolic orbit.
Scientists are still debating over the nature of Oumuamua, it has been described as a collisional fragment of an exo-Pluto to being an alien light sail, the planetary scientific community as expected, favoured the first description.
The significance surrounding both 2I/Borisov and Oumuamua is that they were both interstellar objects that were discovered within just two years of each other. This suggests that interstellar objects passing by is much more common that we previously thought. This has led to many scientists being more open to the possibility that life could travel not only between planets in the same solar system, but potentially between solar systems themselves.
If life is not restricted to one solar system until a technological species evolves capable of interstellar travel, then it is plausible that interstellar space, rather than being dead as first presumed, could be full of life.
The Panspermia Theory Today
Ever since panspermia was first taken seriously as a potential scientific answer, it has been controversial to say the least. Recent discoveries and the advance in research technology have redeemed the possibility of the panspermia hypothesis, giving credence to the fact that it could be a strong possibility.
Although there has been no evidence released to show that life did form on other planets, with every new discovery it is becoming more likely that life will soon be discovered beyond Earth.
The one question that still remains is that if we do find life outside of our planet, will it be completely different to any life we have experienced, or could it potentially turn out to be just like ours?
Now you have read the panspermia hypothesis, make sure you check out the story on the Wandjina sky beings show in Aboriginal rock art.