New research into how life might have evolved elsewhere in the Universe has thrown up a rather depressing possibility - we haven’t found any aliens yet because they’re probably already dead.
The hypothesis is based on what we know about the first billion years or so after a new planet forms - its environments are extremely unstable, fluctuating in temperature and atmospheric composition so wildly that the chances of a new life form evolving quick enough to cope are very slim (which means give yourself a pat on the back, fellow human, we made it.)
"The Universe is probably filled with habitable planets, so many scientists think it should be teeming with aliens," said lead researcher, Aditya Chopra from the Australian National University. "Early life is fragile, so we believe it rarely evolves quickly enough to survive."
Chopra and his team reference the early environments on Venus and Mars - some 4 billion years ago when they first formed, they were likely fairly habitable, and there is a chance that life forms did in fact appear on their rocky surfaces.
But we have evidence to suggest that around a billion or so years after these planets had formed, Venus was well and truly on course to become the smouldering hothouse of death we know it as today, and Mars did the opposite - its temperatures plummeted, and any early life that might have formed on its surface was likely frozen out of existence.
So what makes Earth so special? Turns out, it’s us. Well, our earliest ancestors, at least. The researchers suggest that the success of life on Earth is down the fact that the earliest life forms actually had a stabilising effect on the rapidly fluctuating environment. "Most early planetary environments are unstable," says Chopra. "To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable."
The researchers simulated this scenario using a model they call the Gaian bottleneck, which basically means that if life can’t evolve fast enough to stablise its environment, it dies out. If we’re looking at examples such as Venus and Mars, the Gaian bottleneck states that if you don’t make it through that brief period of rough but possible habitability, you’ve probably missed your chance.
We know from previous research that life on Earth managed to evolve so fast, it ended up regulating greenhouse gas emissions on a planetary scale, and this appeared to have a favourable effect on what’s known as our planet’s albedo - the ratio of reflected radiation to absorbed radiation.
That's important, because when Earth was just forming, the Sun was up to 25 percent less luminous than it is now, but all the evidence points to the oceans being liquid, or at least not completely frozen. This seemingly impossible scenario is known as the faint young Sun paradox.
Back in 2012, researchers in the US came up with a possible solution - the high concentrations of warming greenhouse gases, such as carbon dioxide (CO2), that were being regulated by early life likely kept the planet sufficiently warm before the Sun could. This ensured that Earth escaped the 'snowball state' that we see in the majority of rocky planets we know about today.
The interaction between these large patches of frozen and liquid water on the surface of Earth interacted with the radiation, and established the planet’s albedo, which ultimately determined its surface temperature.
"In extremely rare cases - like on Earth - the relatively rapid evolution from single- to multicellular organisms to complex life forms did not produce enough greenhouses gases to cause runaway negative feedback and heat the planet enough to evaporate all its liquid water," explains Campbell Simpson at Gizmodo. "It’s that particular and so far unique quirk that has kept us alive, if the Gaian bottleneck explanation is accurate."
The hypothesis, if we can somehow prove it, answers the problem posed by the Fermi paradox: if the Universe is a colossal space, filled with trillions upon trillions of potentially life-sustaining stars and habitable planets, why haven’t we found any aliens?
"The mystery of why we haven't yet found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces," says one of the team, astronomer Charles Lineweaver.
The research has been published in the journal Astrobiology, and the team has also posted it here with free access, to encourage other scientists to come forth and scrutinise it. If what they're proposing ends up matching what really happened all those billions of years ago, it's a huge bummer that alien life probably died out before we ever had a chance to meet them. But it's also incredibly humbling to think about how incredibly lucky we all are to exist. High fives all 'round.