Does alien life exist?

Editor’s note:

This is third and final edition in a trilogy of posts on Nick Lane’s theories of life and how they relate to the Fermi Paradox — or why we haven’t detected alien life yet. The first looked at how life first began, while the second looked at why complex life took so long to evolve. Now we’re going to assess what this all means for the prospect of there being alien life out there in the wider universe.

The Voyage of Life, Manhood — Thomas Cole (1842)

Sometime during the summer of 1950, physicists Enrico Fermi, Edward Teller, Emil Konopinski, and Herbert York were walking to lunch at the Los Alamos National Laboratory. They were discussing the possibility of flying saucers and faster-than-light interstellar travel, when Fermi asked the famous question: “Where is everybody?”

Fermi’s contention was that given the time elapsed since the beginning of the universe thus far (13.8 billion years), the size of the Milky Way Galaxy (100,000 light-years in diameter), and the probability of there being a huge number of Earth-like planets scattered among the stars, there would have been plenty of time for civilisations to arise, invent interstellar travel, and colonise their way across the galaxy. So why can’t we see them?

The current estimate is that there are somewhere between 100-400 billion stars within the Milky Way, with at least 17 billion Earth-like planets orbiting them. Given the overwhelming number of possible locations from which an interstellar civilisation could have emerged, Robin Hanson proposed the ‘Great Filter’ concept in 1996.¹ He argued that there must be some sort of natural barrier somewhere along the chain of events from life emerging to interstellar travel that’s preventing an alien civilisation from arising in our vicinity.² But what was it?

The most widely accepted theory today is that the hard part is abiogenesis — life emerging at all. Nick Lane disagrees.

Life seems to have arisen within alkaline hydrothermal vents some 4 billion years ago. As the Earth is only 4.5 billion years old, this was very early in our planet’s history. Does this mean it was easy for life to emerge here? It seems so! Does this mean it will be easy for life to form elsewhere? Maybe! But not definitely. It’s hard to know if life can form easily in general, or if it can form easily only if the conditions are right. Given we have seen zero evidence of life anywhere else in the universe, it’s probably better for us to assume the latter. That being the case, to assess the possibility of life out there in the wider universe, we need to assess how often Earth-like conditions crop up.

What were the ingredients that first permitted life to emerge here? Surprisingly, the shopping list is remarkably short.

The first ingredient is obvious: water. All life on Earth is predominantly made up of water — we are about 60% water, fish about 80%, and plants between 80% and 90%. It’s necessary for all the metabolic chemical reactions that sustain living cells, including the simple prokaryotes which emerged first. How common is water out there in the wider universe? Really common! In fact, it’s the most common molecule in the entire universe! Hydrogen and oxygen, the two elements from which water is made, are especially abundant, and they love reacting with one another. So water is definitely not a problem.

The second ingredient for life is actually rock. Recall that the formation of the alkaline hydrothermal vents on the early Earth was caused by seawater seeping deep into the ocean floor where it could react with a mineral found in the upper mantle called olivine. This bubbled up the extremely reactive raw hydrogen which powered the early organic syntheses necessary for organic molecule formations — the building blocks of cells. How common is rock? Extremely common of course! Half of the planets in our solar system are made from rock, asteroids are made of rock, as are most moons. Rock is everywhere in the universe, so that won’t be a limiting factor either.

The third ingredient is carbon dioxide. All life on Earth is carbon-based, and it was the carbon dioxide dissolved in the early ocean that reacted with hydrogen to form organics in the vent systems. How common is carbon dioxide? Also extremely common! It’s the third most common molecule in the universe.

This is really exciting! The three ingredients that first allowed life to emerge here on Earth — water, rock, and carbon dioxide — are extremely common across the universe. Does that mean life will be extremely common? Nick Lane thinks so!

All life on Earth is chemiosmotic, meaning proton gradients across membranes drive carbon and energy metabolism, thus powering cells and sustaining life. We’ve seen how this first arose naturally and spontaneously in the micropores of the alkaline hydrothermal vents, synthesising the organic molecules that eventually gave rise to protocells, and then living cells. Since then, all life has continued to use proton gradients to sustain itself. The line between geochemistry and biochemistry is perhaps not as sharp and distinct as we’re accustomed to drawing it. There is no reason to think that on wet, rocky planets elsewhere in the universe chemiosmosis wouldn’t arise in the same way. In fact, it may even have arisen elsewhere in our solar system.³

If we hold this to be true, then the Great Filter is not life’s emergence in the first place. It must be something else. But what?

As I’ve already mentioned, life emerged very soon after the formation of the Earth. However, complex life did not. Bacteria and archaea, the first two of the three domains of life, spread across the entire world, adapting to every conceivable environment. To do so they evolved a dizzying array of different genes and metabolic processes. But they remained structurally the same for 4 billion years.

As we’ve seen, bacteria and archaea were locked in their simplicity by an energy constraint. It was only after an extremely rare occurrence of successful endosymbiosis — one cell crawling inside another — that this constraint was overcome. The eukaryotic cells that formed from this merger were thus able to grow larger and more complex, before eventually giving rise to multicellularity (on possibly as many as 30 different occasions) and the baroque complexity of life we see today.

But successful endosymbiosis doesn’t happen very often. In fact, in the entire history of life on Earth, this jump from simple prokaryotes to complex eukaryotes has happened once and only once. Or has it?

Off the coast of Japan, some 1200m below the surface, there is in the inky depths a submarine volcano called Myojin Knoll. A team of Japanese biologists spent over a decade trawling its waters looking for interesting lifeforms. They didn’t find anything noteworthy for a long time, until, in 2010, they collected some polychaete worms clinging to a hydrothermal vent. Upon the back of one of these ugly creatures they found a single specimen of a fascinating little microbe.

Parakaryon myojinensis, as it came to be called, has some confounding features. A passing glance at its morphology and one can see a nucleus, a cell wall, and some endosymbionts that look like hydrogenosomes, which are ordinarily derived from mitochondria. The cell is quite big, about 100x larger than a bacterium like E. coli. From these details we might deduce that Parakaryon is clearly a eukaryote.

And yet, a closer look gives us pause. Its nuclear membrane is a single layer punctuated by a few gaps. Every other nucleus we know of has a doubled membrane with complex nuclear pores. Meanwhile, Parakaryon’s DNA is formed of fine fibres like bacteria, not the thick chromosomes of other eukaryotes. Even weirder, there are ribosomes in the nucleus. By the time we realise the cell holds nothing resembling an endoplasmic reticulum, a Golgi apparatus, or a cytoskeleton, our classification of Parakaryon as a eukaryote looks very shaky indeed. So what’s going on?

All eukaryotic life that we know of emerged from a single successful case of endosymbiosis — that’s why all their cells look so similar. Parakaryon looks halfway between a prokaryote and a eukaryote; perhaps because it might well be exactly that. Lane suspects that the enigmatic cell is a prokaryote that has acquired endosymbionts, and is in the process of changing into something like a eukaryote. Perhaps complex life is emerging a second time, right before our eyes!

Alas, we might be getting overexcited. The Japanese biologists who dredged up Parakaryon have never found another trace of the microbe since they found that sole specimen, despite 15 years of searching. A population density that low is most likely bound for extinction. Perhaps evolutionary history is littered with attempts to escape the prokaryotic energy constraint, all but one of which failed. Parakaryon is thus the exception that proves the rule; the evolution of complex life is extremely rare.

So what does this all mean for the Fermi Paradox? What is the possibility for alien life, somewhere out there among the stars?

Lane argues that while genes seem capable of endless diversity, energy does not. As such, he expects the basic metabolic processes that power life on earth (chemiosmosis) to be universal to all life, no matter where it is in the universe. This means that wherever we find the three ingredients that gave rise to life here — water, rock, and carbon dioxide — we should find life. Amazingly, these ingredients are incredibly common, and as such the universe might be teeming with life.

But — and it’s a big but — there appears to be a stringent bottleneck at the jump from simple bacteria-like organisms to more complex lifeforms. It’s a depressing thought: we might well be surrounded by extra-terrestrial life on all sides, but none of it has evolved past simple bacteria.

To compound things further, there’s no evolutionary principle that says intelligence must arise even in complex lifeforms. It took 2 billion years after the emergence of the first eukaryotic life for natural selection to push in favour of the large and more calorie-intensive brains found in hominids. Even in cases where life overcomes the eukaryotic bottleneck, it might never give rise to the intelligence necessary to build an interstellar civilisation. For all we know, intelligence is a second Great Filter.

There are 17 billion Earth-like planets scattered across our galaxy. If intelligent life had evolved among them, we should have seen it by now.

In the Milky Way at least, it seems we are alone.

1. FYI Robin Hanson is on substack.

2. He also came up with another really interesting concept he calls ‘Grabby Aliens’, but we’ll save that for another time.

3. I think I might do a bonus post on the candidates in our solar system where simple life may also have emerged.

Notes

Lane, Nick (2010). Life Ascending: The Ten Great Inventions of Evolution

Lane, Nick (2015). The Vital Question: Why is Life the Way it is?

Lane, Nick (2023). Transformer: The Deep Chemistry of Life and Death