Panspermia huh? Sounds a bit unsavoury doesn’t it? Suspiciously like something you’d find on the internet; not on a science blog.
It’s actually the idea that life on Earth could have come from elsewhere in our galaxy, that life may have travelled through space originating from a different planet, and when it reached ours it flourished and evolved into all the different forms we see today. So we may actually all be aliens. Great idea huh? Pretty awful name though.
Sperm actually means ‘seed’ and pan means ‘all’ so panspermia means all-seed, OK that doesn’t really help, I’m not sure who first coined the term but they certainly didn’t have a flair for marketing. Anyway, how likely actually is it?
In this first post I’ll examine the ‘traditional view’ of panspermia, including some evidence, and in the next post I’ll look at some more extreme versions of panspermia, such as the idea that intelligent aliens deliberately seeded the galaxy with life.
The inspiration for these posts came from commenter hartm242 who gave me the idea of writing about panspermia in the requests section of this blog. You should definitely check out his excellent blog too, it’s about exploring alternative forms of life on planets unlike our own, and he’s created a hypothetical alien planet with its own biosphere.
So all life on Earth could be alien? But how could life get to Earth, in what form and where from?
The traditional view of panspermia is that simple single-celled life, like bacteria, may be able to survive in space for long periods of time, lying dormant, until the bacteria land on a planet that can support life, at which point they awaken and begin to metabolise, grow and reproduce.
Its similar to the way that many plants release seeds that can be carried by the wind. Such plants, living on one island in an archipelago, could eventually colonize other islands if their seeds were blown to and landed on those islands. Sure, most of the seeds may end up falling into the sea and dying, or may even reach the islands but land in inhospitable areas, but over time enough seeds may reach the other islands to create viable colonies. A similar process could happen in space, with bacteria travelling between planets seeding life.
However, for bacteria to be able to do this, they would have to overcome some big problems, as space is not the most hospitable of places. They would have to be able to survive harsh conditions such as no oxygen, extremely low temperatures (≈ -270°C), desiccation and sterilizing radiation. The cool news is that many bacteria can do some of these things, they’re called extremophiles. Extremophile bacteria can survive and even thrive in tough conditions, there are species that live in 120°C water, or can survive freezing to -196°C, live in acid or even survive strong radiation. It has even been suggested that some bacteria could be effectively immortal, that they could only be killed if eaten by another organism. Genetic evidence also suggests that all life on may Earth have evolved from a particular kind of extremophile microorganisms called Archaea.
With the discovery of extremophiles, scientists have realised that life can live under some pretty awful conditions. There is even evidence that bacteria from Earth survived a stay on the Moon’s surface. Surveyor 3 was an American lander sent to the Moon in April 1967. It’s still on the Moon today, but in 1969 its camera was brought back to Earth as part of the Apollo 12 mission. When analysed by NASA, the camera was found to have a common bacterium called Streptococcus mitis living on it, which was thought to have contaminated the camera prior to the Surveyor 3 launch, and thus survived exposed on the surface of the Moon for around 31 months. This is pretty amazing, proof that bacteria can survive in space? Maybe not unfortunately, as I should point out that a NASA scientist involved in the recovery of the camera has claimed that a breach in a sterile procedure may have contaminated the camera after it was brought back. Damn! As far as I’m aware, whether or not this bacteria got on the camera before or after it was launched into space has not yet been determined.
Even if Streptococcus mitis did survive on the surface of the Moon for over two and a half years, critics of panspermia have pointed-out that this is nowhere near as challenging as surviving for the long periods in space required to travel between planets. Even the toughest extremophiles would find it difficult to survive such a journey. But what if the bacteria had protection?
One variant of panspermia is that the bacteria could travel as spores. A spore is a protective structure created by an organism to house a seed; many mosses, fungi and plants, such as ferns, reproduce using spores carried in the wind, which germinate once they land in a favourable location. Spores can survive harsh conditions such as heat, cold and drying, and can even survive fire; after forest fires ferns are often amongst the first plants to grow back as their spores survive the burning.
Spores could travel through space via a number of mechanisms. If they are carried high into their planet’s atmosphere by strong winds they may be propelled into space by the stellar wind (ionised particles thrown off the surface of a star) or could be ejected from their planet’s surface by explosions such as those caused by energetic meteorite impacts. Once in space the spores could be propelled between planets by radiation pressure (photons emitted from a star push the spores to further out planets in a solar system) or could even be propelled by sections of a planet’s magnetic field ejected into space and carried along an interplanetary magnetic field line; such a mechanism could accelerate spores to speeds capable of carrying them to different solar systems. Life on Earth could have originated from a planet orbiting a different star!
Another idea, called lithopanspermia, is that bacteria could travel through space protected in rock. Many rocks contain small empty spaces called vesicles that can house bacteria colonies (solidified lava is often quickly colonized by bacteria living in vesicles). Bacteria could thus travel between planets within rock inside asteroids and comets. These asteroids or comets would thus provide both a means of protection and a means of transport.
So maybe extremophile bacteria could survive in space and may have been protected and transported inside asteroids or comets, but where could it have come from?
One candidate is our nearest neighbour, Mars. Geological evidence suggests that the Red Planet may have had large bodies of water on its surface in the past. This suggests that when the planet’s internal thermal engine was still firing Mars may have had a warmer and wetter surface and an atmosphere, and may therefore have had life of its own. Rocks are periodically ejected from Mars’ surface due to impacts of asteroids and some of these rocks fall to Earth. If life once existed on Mars then maybe some of the Martian rocks that fell to Earth could have seeded life here. Of course, it’s also possible that the reverse happened, that life from Earth may have travelled to Mars and may have survived on its surface for a time, but as Mars became cold and dry it would have probably died out. Other candidates in our Solar System are the Jovian moons Europa and Callisto, which may have oceans under their surface in which life could be found. However, it would be much more difficult for life to travel from these moons to the Earth.
Life could even have come from other solar systems. The Universe has been around for about 13.7 billion years. At first it was mostly composed of hydrogen and helium, but as the first stars formed they began to create more complex atoms as they fused protons and neutrons into larger atomic nuclei (this is called stellar nucleosynthesis), when some of these stars went supernova and exploded, they showered the Universe with atoms like carbon and oxygen. Cosmologists think that after around 2 billion years solar systems could have formed that had planets and the right atoms to support life. Life could therefore have emerged in our Galaxy around 11.2 billion years ago. Our Solar System is roughly 4.56 billion years old, so life could have emerged much earlier in our Galaxy and could have spread to new Solar Systems as they formed. Life appears to have emerged very early in Earth’s history, maybe after less than half a billion years. Some scientists find such an early emergence unlikely, but under panspermia life could have emerged very slowly in a different solar system and then seeded the Earth soon after it formed.
So what about the evidence? All this sounds like nice theory, but maybe a bit unlikely. Well, there’s no conclusive evidence but there are some cool findings, the best is from 2001, when a geologist called Bruno D’Argenio and a molecular biologist called Giuseppe Geraci (both from Naples University) claimed to have found bacteria living inside a meteorite, inside the crystal structures of its minerals. They even managed to grow some of the bacteria in a medium in their laboratory. They claimed the bacteria had a different DNA structure to life on Earth, and that they were extremely resistant, surviving both heating and washing in alcohol. They estimated the bacteria were 4.5 billion years old, similar to the age of the Earth. (As far as I know, no further studies have been conducted on these bacteria, let me know if I’m wrong!)
Other scientists have made slightly wilder claims for proof of panspermia, including Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe suggesting that alien microorganisms entering Earth were responsible for epidemic disease outbreaks and even supplying new genetic material to stimulate evolution.
Personally, I think panspermia is a great idea, but I think that life probably did emerge on Earth rather than travelling to us through space. Biologists have found that simple molecules, in the right combination and in the right conditions can form more complex biological molecules like amino acids and DNA and I believe that life probably did emerge through such processes (called chemical evolution). I don’t think we have enough evidence for panspermia, and that extraordinary claims require extraordinary evidence.
However, the idea can’t be truly ruled-out, not until we have a better idea of how life emerged on Earth, and how common life is in our Galaxy. In the next post I’ll examine some of the more extreme aspects of panspermia, from cosmic amino acids to complex life forms travelling through space.
Live long and prosper.