by Ben Miller
What’s more, in the years since what is now known as the Miller–Urey experiment, we have found amino acids and nucleobases in some far-flung places: on comets, meteorites, and even in interstellar space.4 That’s significant, because for its first billion or so years the Earth was pummeled by comets and meteorites, with a particular flurry of blows taking place roughly 3.9 billion years ago during what is known as the Late Heavy Bombardment. The primordial seas could very well have been a soup of amino acids and nucleobases, some earthly and some alien. Could a lightning strike have kick-started life as we know it?
In the minds of many contemporary evolutionary biologists—and this sketch comedian—there are two reasons why the answer to this question is a polite “not really.” The first is what is called the concentration problem. For a series of chance encounters to produce something as complex as a self-replicating molecule you need a lot of the right kind of molecules in the right place, which is why chemists tend to do their experiments in test tubes rather than oceans. A broiling primordial sea is simply not the place you can expect to find delicate organic chemistry.
As a workaround, some evolutionary biologists have suggested that Darwin had it right, and the vital reactions took place in a pond, where—thanks to evaporation—the “soup” was much thicker. Which sounds promising, until you consider the fact that, as already mentioned, the early Earth was being bombarded by comets. The exact time when life began is subject to intense debate, but, to be brief, we have fossil evidence of so-called “microbial mats” at around 3.5 billion years ago,5 and chemical evidence of cellular life from 3.7 billion years ago.6 As a result, most pundits would be happy to set a date for “prototype” life at around four billion years ago. The really peculiar thing about that is it predates the Late Heavy Bombardment. Whatever this “prototype” life was, it appears to have survived a cataclysm.
But for me all that fades into insignificance when it comes to the second problem: energy. As we learned in the last chapter, to sustain life we need a ready supply of information-rich energy. The “wham, bam, thank you, ma’am” of lightning just won’t do the trick; life needs a lover with a slow hand. And when I say slow, I mean slow; for natural selection to do its thing, we need an energy supply that lasts for tens, hundreds, maybe even thousands of years. Thankfully, in a flash of inspiration over two decades ago, the aforementioned Mike Russell pictured what he considered to be just the right place. It’s time to meet your maker: an alkaline hydrothermal vent.
LIFE SPRINGS ETERNAL
“And what,” you may ask, “is one of those?” The simplest answer would be a hot spring on the seabed, caused by a chemical reaction between sea water and a common mineral, olivine. Olivine—a greenish crystal made of iron, magnesium, silicon, and oxygen—reacts with sea water to form serpentinite. And the striking thing about the booking hall here at Euston is that it is paved with serpentinite marble, a green stone with white serpent-like marbling, a fact that the fatalist in me can’t help feel is significant.7
Anyway, back to the plot. When sea water reacts with olivine to form serpentinite, it releases a great deal of heat, and this hot, mineral-rich fluid then rises as alkaline springs on the seabed. When it meets the cold sea water at the bottom of the ocean, the minerals precipitate out, like the limescale in a kettle, producing vents of porous white limestone. One famous example is the so-called Lost City in the middle of the North Atlantic, a ghostly hoard of some thirty gnarled chimney stacks, the tallest of which is some twenty stories high.8
It’s in the microscopic pores of this limestone, Mike Russell realized, that life may have gotten its start. The clue came from the way that all single-celled organisms store energy. Essentially, they act like tiny electric batteries, pumping protons across their membranes so that their insides become less positively charged than their outsides.9 We call this a proton gradient. If energy is required for a chemical reaction somewhere within the cell, a proton is allowed to fire back through the membrane. The energy of this proton is then harnessed to create a molecule of energy-carrying ATP,10 which then carries it to wherever it is needed.
Which begs the question, “Why bother?” Why go to all the trouble of storing energy in a proton gradient, instead of just making ATP straight away? Maybe, thought Mike Russell, it’s a hangover from an earlier stage of life. McDonald’s sells hamburgers in 119 countries, but banks its profits in dollars because its first store was in San Bernardino, California. Maybe single-celled organisms bank in protons because that’s the way it was done back in the ’hood. And one place you are sure to find a proton gradient is in an alkaline hydrothermal vent.
Why? Because “acidity” and “alkalinity” are just “proton concentration” by another name. Acidic fluids have a lot of protons; alkaline fluids have few. The present-day oceans are slightly alkaline,11 but thanks to a much higher concentration of atmospheric carbon dioxide, back in the day they would have been much more acidic.12
EXAMINE YOUR PORES
With that in mind, let’s zoom in on one of the pores in the limestone chimney of a primordial alkaline hydrothermal vent and see what’s going on. Amazingly, it’s full of microscopic bubbles. Each one acts like a tiny battery. Inside, we’ve got warm alkaline fluid. Immediately outside, we’ve got acidic sea water. And betwixt the two we have a thin gel-like membrane made of iron sulfide, with a proton gradient across it.
Could such a bubble be the place that some prototype of life set up shop, using a natural proton gradient to drive chemical reactions? As we saw in the last chapter, to organize matter we need to be able to do work. Thanks to the proton gradient across its membrane, a bubble in an alkaline hydrothermal vent has energy on tap. What’s more, its confined space is also a great place to concentrate chemicals. The concentration problem and the energy problem have been solved in one fell swoop.
And those aren’t the only things these kinds of vents have going for them, because they are also rich in another vital constituent of living cells: transition metals. Cast your mind back to the periodic table, and you will remember that the middle of it is made up of a block of colorful, dense, mildly reactive metals with dependable names like iron, nickel, copper, and zinc. Ever wondered why these are recommended as part of a balanced diet? Crucially, it’s because we find them embedded within a bewildering number of proteins.13
Why might that be? Well, the thing about transition metals is that they make great catalysts.14 Essentially they are wealthy philanthropists, with more electrons than they rightly know what to do with, and are happy to donate a few to the needy, safe in the knowledge that they will regain them somewhere further down the line. Alcohol dehydrogenase, for example, the enzyme in the liver that breaks down alcohol, contains a socking great zinc ion right in the middle of it, as do some three hundred other known enzymes. So far as we can make out, transition metals were the first catalysts, and were later enslaved by enzymes.
Membranes, proton gradients, and transition metals: All of them make a good case for alkaline hydrothermal vents as the cradle of life. Add to that the fact that they are tucked away on the seabed, out of reach of the 700 million year asteroid bombardment that rattled the newborn Earth, and I hope I’ve got your attention. So how was the trick done? How do you get from something non-living to something living?
MAKE MINE A SINGLE-CELLED ORGANISM
Knowing that life is a symphony written in long-chain carbon molecules, it’s clear that we need a source of carbon. What better place to get it than from the carbon dioxide dissolved in the primordial ocean? In long-chain molecules we most often find carbon bonded to hydrogen, so we are going to have to find a source of that as well. What about water? We’ve got plenty of that at hand.
No dice. It’s possible to get carbon dioxide to react with water to produce long-chain carbon molecules and free oxygen, but it’s far from easy. Plants do it, but they use sunlight, harnessed by a convoluted chemical pathway called oxygenic photosynthesis. That particular party trick didn’t emerg
e until something like 2.8 billion years ago (bya) at the very earliest.15 No, water won’t do. So where are we going to get our hydrogen from?
This is where another feature of alkaline hydrothermal vents comes to the fore; serpentinization produces lots of dissolved hydrogen. Carbon dioxide and hydrogen will react to make methane and other long-chain molecules, but you need both energy and a catalyst to get things going, much in the same way that you need a match and a fire lighter to get a decent fire to take in a grate. As we know, not only do our bubbles have energy on tap, but their membranes are made from iron sulfide, a catalyst which is perfect for the job.
One of my favorite Monty Python sketches is from The Life of Brian. “All right, all right,” says a revolutionary John Cleese, “but apart from better sanitation and medicine and education and irrigation and public health and roads and a freshwater system and paths and public order . . . what have the Romans ever done for us?” After my chat with Nick Lane, I feel the same way. Apart from the protection from meteorites, the membranes, the transition metals, the proton gradients, the iron sulfide catalysts, the dissolved hydrogen, and the carbon dioxide . . . what have alkaline hydrothermal vents ever done for us?
PUSHING CARBON UPHILL
Of course it’s a long way from a few smallish carbon molecules in an iron-sulfide bubble to a single-celled life-form. Unlike the primordial soup theory, however, which produces life like a rabbit out of a hat, our vent-based proto-life can make its way there in stages. One of Mike Russell’s triumphs has been to show that the iron sulfide membrane of an individual bubble is permeable. That means that small molecules can escape, but larger ones are trapped, ready to undergo further reactions.
In the broadest of terms, the reaction of carbon dioxide with hydrogen would first produce small carbon molecules like methane, formate, and acetate,16 all of which would be allowed in and out of the membrane. In the next stage, these small molecules would react together to form medium-sized molecules such as amino acids and nucleobases. These would be trapped by the membrane, the proverbial fish in a barrel for further reactions which would then produce larger and larger molecules.
A crucial step would have been the creation of the first long-chain carbon molecule that was capable of making copies of itself, or, as we say in the jargon, self-replicating. In present-day organisms, this role is played by DNA, but it’s unlikely to have been the molecule of choice for the very first life. For a start, DNA has a complex double-stranded structure, being a sort of “twisted ladder,” with “sides” made of ribose and phosphate groups, and “rungs” made of pairs of nucleobases.17 In fact you can think of a DNA molecule as more or less being two RNA molecules fused together,18 a simple fact that has led many to suppose that RNA came first and later evolved into DNA.
Supporting this is the fact that DNA is essentially passive. To decode it, translate it into a recipe for amino acids, then assemble those amino acids into proteins requires RNA. Add this to the fact that RNA is able to write code into DNA, and catalyze reactions, and you start to build a picture of a busy chef who has decided to write his favorite recipes in a cookbook. Meaning that RNA is the chef, and DNA is the cookbook. All of which hints at an earlier epoch of life, predating DNA, when RNA ruled the kitchen. Evolutionary biologists call this the “RNA World”; life, but not as we know it.
The grail of researchers like Lane is to create RNA from scratch in a model alkaline hydrothermal vent. At the climax of my visit, he leads me into a pristine lab, where a glass cylinder the size of a bricklayer’s thermos flask sits on a bench top, trailing wires and surrounded by electronic monitors. It looks like a lava lamp on life support. I peer through the glass, not really sure what I’m expecting to see. Frankenstein’s Molecule, perhaps? Could it be possible that, inside this sterile-looking experiment, new life is taking its first shuffling steps?
A JUMBO IN A JUNKYARD
One of the most famous critiques of the primordial soup theory was made by Fred Hoyle. In his 1981 book The Intelligent Universe, he professed bemusement that something as complex as a single-celled organism could come about by chance. With characteristic Yorkshire phlegm, Hoyle put it this way:
A junkyard contains all the bits and pieces of a Boeing 747, dismembered and in disarray. A whirlwind happens to blow through the yard. What is the chance that after its passage a fully assembled 747, ready to fly, will be found standing there? So small as to be negligible, even if a tornado were to blow through enough junkyards to fill the whole Universe.
Can a proton gradient across the membrane of an iron sulfide bubble trapped in an alkaline hydrothermal vent achieve what a tornado in a junkyard cannot? After my encounter with Nick Lane, I’m in the “yes” camp. Most importantly, unlike the tornado, the vent doesn’t have to make the entire cell in one go; it can do it in incremental steps. First, it just needs to do something simple, like take one molecule of carbon dioxide and react it with hydrogen to make methane. Next it synthesizes the building blocks: amino acids and nucleobases. What’s more, any large molecules that form can’t ever leave, trapped as they are within gel-like bubbles of iron sulfide, encouraging them to form ever longer chains: handy stuff like proteins, and nucleic acids.
And here’s the crunch. Far from being a shot in the dark, life is a slam dunk. We should expect to find it anywhere there’s a hydrothermal vent bubbling alkaline vent fluid into an acidic ocean, and such vents are a feature of all newborn, volcanic, wet, rocky planets. Far from being a statistical fluke, life is just the chemical pathway by which carbon dioxide reacts with hydrogen to form methane. Or, as Mike Russell succinctly puts it: “The meaning of life is to hydrogenate carbon dioxide.”
FIRST TO THE PARTY
Can that be true? Certainly life got started very quickly on Earth. To see just how quickly, let’s remind ourselves of how planetary systems form. First, a shockwave from a supernova creates pockets of high density in surrounding gas and dust. These pockets then collapse under gravity to form clusters of new stars. As each new solar system condenses, it spins faster and faster, flattening into a disk, much in the same way that a skater spins faster as she draws in her arms.
Out in the disk, planets begin to clump together under gravity. The temperature of the protostar determines what type of planet forms where. Close to the star is the rock line, where the temperature is cool enough to allow rock to solidify. Here’s where we find small rocky planets like Mercury, Venus, Earth, and Mars. Further out is the snowline, beyond which water, methane, and ammonia all freeze, and we find giant ice planets like Uranus and Neptune. In between are the giant gas planets, like Jupiter and Saturn.19
Eventually, the temperature and pressure of the protostar become so great that it “switches on,” and begins nuclear fusion. A blast of charged particles strafes the newborn solar system, blasting away the remaining gas and dust and leaving naked planets. For the first time, their home star lights the horizon. By this time the solar system is barely fifty million years old. Another fifty million years on, and the Earth is much like it is today, with a carbon-dioxide rich atmosphere, little land, and an acidic ocean.
And here’s the kicker. Fast-forward a mere 300 million years, and the organism that we call LUCA, the Last Universal Common Ancestor of all life on Earth, is eking out an existence in an alkaline hydrothermal vent. It uses the proton gradient between vent fluid and sea water to hydrogenate carbon dioxide—meaning to replace one or both of its oxygen atoms with hydrogen atoms—releasing energy.
As we know, LUCA wasn’t the first life. It was the product of millions of years of evolution, one tier of which had probably been RNA-based. Other kinds of worlds almost certainly predated these RNA worlds, but their self-replicating molecules are lost to us. However you dice it, 300 million years was not a great deal of time to produce something as complex as LUCA. Life isn’t rare, at least not in its proto-cellular form. It works straight out of the box.
Yet LUCA, as I’ve hinted, hadn’t yet left the safety of the vent. To
do that, it needed to develop a membrane capable of generating its own proton gradient. Was that a roadblock on the path to complex life? It would seem not. This may come as a surprise, but there’s growing evidence that LUCA left the vent not once, but twice.
POPPING THE EVOLUTIONARY HOOD
Ever since Darwin sketched his first “tree of life,” expressing his idea that all life on Earth has evolved from a common ancestor, biologists have been arguing endlessly over which species begat which. Formally known as taxonomy, the guiding principle of this somewhat fraught discipline was to try and group organisms according to their physical traits.
The name of the game was to divide creatures into groups that shared the same characteristics, and then to rank those groups in some sort of evolutionary order. On one level, of course, this makes complete sense. The grand sweep of evolution can be more or less summarized as a progression from the simple to the complex, so you’d think that you’d be able to sift and sort organisms into some kind of time line. At one end, you’d have the simple stuff, like single-celled bacteria, and at the other you’d have the complex, multicellular things such as woolly mammoths. Get into the fine detail, however, and it’s a different matter.
For a start, the more closely related two species are, the greater the similarity in their outward appearance and the harder it is to rank them. A difficult job isn’t made any easier by the fact that traits can just as easily evolve out as evolve in. While the grand sweep may be toward complexity, on a shorter timescale there is a great deal of ebb and flow. Humans are a classic example. Neanderthal man, who lived alongside us in northern Europe only 39,000 years ago, appears to have had a bigger brain than we have, and may have been more intelligent than we are. If an alien taxonomist arrives on a barren Earth some million years hence, and finds a human and a Neanderthal skull, he could be forgiven for assuming that the Neanderthal version was the more recently evolved of the two.
