Roughly five billion years ago, a giant molecular cloud some 65 light years in diameter collapsed inward, probably as the result of a nearby supernova. The shockwave from a dying star passed through the nebula, compressing it, changing its density and causing it to collapse. As a the molecular cloud, composed almost entirely of hydrogen and helium, fell in on itself, the conservation of angular momentum caused the gas to swirl and spin, slowly forming a disc, the heart of which would become our sun.
Distinct planets formed 4.6 billion years ago, after the slow, steady, clumping of material accumulated into gravitationally significant objects. But Earth was a hellish, seething mass of volcanic activity, constantly being bombarded by gigantic asteroids. Current thinking is that the Moon was formed within a hundred million years of Earth, as the result of a collision with another planet roughly the size of Mars.
For the next five hundred million years, the Earth cooled as oceans formed and the atmosphere increased in density as the result of volcanic out-gassing.
Life arose on Earth almost as soon as the planet had cooled, before Earth’s protective magnetic field had formed at around 3.5 billion years. That life arose while the planet was subject to a late-heavy bombardment of asteroids from space is surprising, especially as scientists are now considering evidence that suggests this bombardment lasted long than they initially thought, until around 3.8 billions years ago. Given that the oldest fossil records, dating from 3.5 to 3.8 billion years ago, are already quite advanced, scientists speculate that the simplest self-replicating molecules arose around the 4 billion year mark. This is astoundingly quick when you consider how hostile the environment was at that time. The subject of abiogenesis, or how life arose from inorganic molecules is one of the last, great frontiers of modern science.
Abiogenesis is remarkable given the Late Heavy Bombardment of asteroids pummelled the Earth with what should have been killer blows that sterilized the planet time and again. During the bombardment, it is estimated that there were:
- 22,000 or more impact craters with diameters up to 20 km (12 miles)
- about 40 impact basins with diameters about 1,000 km (620 miles)
- several impact basins with diameter about 5,000 km (3,100 miles)
The Chicxulub impact, that is thought to have contributed to the demise of the vast majority of dinosaurs some 65 million years ago, left a crater 180 km (110 miles) in diameter, making it comparable to a moderate/modest impact during the Late Heavy Bombardment.
Given the sheer number of impacts during the bombardment, the question arises, how did life survive in its infancy? And this brings up the contentious and somewhat speculative notion of panspermia. Rather than decimating the fragile beginnings of life, the bombardment may have delivered life to Earth in the first place.
Panspermia is the theory that life need not arise on planets like Earth, but that it may be spread, either naturally, deliberately or accidentally through out the universe. Although it should be noted that panspermia (the spread of seeds) doesn’t answer the question of abiogenesis, it simply pushes it out to be resolved at some other time/place.
There’s a number of good objections to the idea of panspermia, or life on Earth arising from space, like the vast distances between planets, let alone between stars, and the sheer amount of energy required to propel matter from one star to another, escaping gravity wells, etc. And yet we find meteorites on Earth that originated on Mars. The reverse journey is not quite as likely, but it is possible.
Don’t underestimate microbes or the importance of microbial life. Without them, there would be no life on Earth.
Space may be a harsh and unforgiving environment for Homo sapiens, bringing death within seconds, causing our blood to boil as the pressure drop allows gases to come out of suspension, but this isn’t the case for microscopic life. In fact, the opposite is true. Exposure to space preserves microscopic life!
In 1967, NASA landed Surveyor 3 on the Moon. Two and a half years later, Apollo 12 touched down next to Surveyor to retrieve the camera so scientists could examine the effects of prolonged exposure to the harsh temperature extremes and radiation in space. To their surprise, scientists found life! Not extraterrestrial life, but microbes that had originated on Earth. Although there’s some conjecture that the analysis of the camera may have inadvertently introduced the contamination after the camera had been returned to Earth, NASA still stands by its original finding, that the contamination was present before launch, enjoyed a holiday on the lunar surface, and was subsequently revived when returned to Earth.
Even if the Surveyor contamination is discredited, there is a certified experiment using tardigrades that demonstrates that complex microbes can survive in space. Well, survive may be too strong a term. A better description would be that microbes can be freeze-dried and revived.
Tardigrades, or water bears, are remarkable examples of the resilience of microscopic life. Barely a millimetre in length, they’re prolific on Earth, being found everywhere from the frigid Arctic oceans, to the tropics and even the Himalayas.
Tardigrades been found living in environments as cold as -80C and can survive an almost complete loss of body water, entering a state called cryptobiosis where their metabolism drops to 0.01% of its normal functioning level, with its body fluids dropping 1% of normal. Although the animal is effectively dead through extreme dehydration, it’s a case of “just add water,” and within a few hours the water bear is alive again. It’s the microscopic equivalent of reviving an Egyptian mummy.
While in a cryptobiotic state, tardigrades can withstand temperatures of -270C and heat of 150C, that’s a viable span of 420 degree Celsius, from almost absolute zero right up to temperatures that would scorch most living cells.
It’s no wonder, then, that NASA sent Tardigrades into space for 10 days in 2007, unprotected from the harsh effects of vacuum and solar radiation, and the water bears took it in their stride, coming back to life once they’d returned to Earth.
Although there’s a huge difference between surviving for a week in space and being blasted off the face of the planet by a meteorite, floating through space for tens of thousands to millions of years before crashing into Mars or one of the moons of Jupiter, in theory, Tardigrades or their eggs could survive such a journey and find themselves revived on contact with liquid water. Whether they could thrive in those new environs is debatable, but that such a notion is in anyway possible lends credence to the idea of panspermia.
If life on Earth arose as the result of panspermia from that original molecular cloud some five billion years ago, arriving during the Late Heavy Bombardment, then we are confirmation of life on other planets!
Our solar system covers roughly a light-day in diameter, while there are approximately 1400 other star systems within 50 light years of Earth.
This image of the Orion nebula spans 24 light years. The molecular cloud from which our system was formed spanned 65 light years, meaning it encompassed such a vast volume of space that panspermia from any one of hundreds of other systems would be plausible.
Panspermia, though, wouldn’t mean aliens would look like us. We share various elements of DNA/RNA with every other living organism on the planet, but the vast majority of them look nothing like us. We may, however, be the descendants of freeze-dried bugs.
UPDATE: Although panspermia is entirely speculative, comets and objects in the Kuiper Belt give us the opportunity to study the kind of asteroids/comets that made up the Late Heavy Bombardment that “might” have seeded Earth. So far, spectroscopy has detected methane in the belt, while amino acids have been found in comets. Perhaps one day we’ll find a primitive, overly simplistic RNA molecule that’s capable of self-replication, and that may be the precursor to life on Earth.