Spheres of destiny

Bubbles are the key to understanding how life on this planet evolved. By Martha Henriques

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Tatiana Gulenkina, “Untitled #17” from Things Merging and Falling Apart, 2012. Courtesy the artist 

Life began with bubbles. In a primordial soup, billions of years ago, tiny bubbles of fatty molecules formed to make spheres only a minute fraction of the width of a human hair across. The bubbles came about spontaneously, with molecules of similar shapes and properties lining up, trying to pack together like-for-like as closely as possible. These molecules were fatty chains, which wouldn’t mix with the watery solution of the early planet’s oceans.  

These chemical oceans were thought to be harsh places. All the basic building blocks of life were there, the fragments of proteins, sugars and salts, dissolved in a lifeless sea. The climate of early Earth was intense and probably entirely unrecognisable: thunderstorms struck the seas, tides raged up on land that was nothing but bare rock. Chemical reactions happened quickly and readily, with molecules coming together driven by excess energy and falling apart when the new structure turned out to be fragile or untenable.  

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Micelles form in liquid solutions when tightly packed lipid molecules line up with their water-loving heads exposed and their water-hating tails faced inwards. Illustration by Robert Galt 

The shape that let the fatty molecules get as close together as possible was a sphere – water on the outside, fatty chains on the inside. There was a compromise at the barrier between water and fat; the ends of the molecules were “hydrophilic” or water-loving, while the fatty tails were “hydrophobic”. You can guess what that means. 

So with this compromise at their boundaries, the tiny spheres were stable, water-loving outside and water-hating inside. These spheres were called micelles, and had nothing in their centre but the ends of the long fatty molecules. 

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A bilayer sheet forms in a liquid solution when lipid molecules join up in a tightly packed formation with their hydrophilic heads exposed on the outside and their hydrophobic tails hidden on the inside. Illustration by Robert Galt 

This was the start, but the fatty molecules could make more complex shapes, too. Rather than just forming one layer of fatty molecules with their heads turned outwards, they could line up as a double layer. Heads faced outwards to water, tails faced inwards and backed onto another row of tails – and on the other side, the second row of molecules’ water-loving heads. 

This double layer meant that the fatty molecules no longer made a micelle – it wasn’t just a sphere with nothing but fat at its centre – but instead a bubble.

Why would a bubble be any better for making life than a sphere? Because unlike a micelle, a bubble can hold something. The tiny pocket of watery primordial soup inside the bubble can be slightly different from that outside it. A bubble lays down a boundary between inside and out and can start to control what’s taken in and made useful. If the bubble’s surface can act as a partial barrier, it can begin to build an identity distinct from the outside world. This gives rise to what biologists call “integrity”.

But this kind of bubble is dead. There is nothing in it but a watery soup and nothing much to keep it distinct from the outside world. While scientific definitions for exactly what makes up life are rickety and riddled with exceptions, reproduction is not a contentious requirement. Bubbles alone might break off into smaller bubbles, but they don’t have unique characteristics that they can pass on to other bubbles. A bubble might merge with another slightly different bubble, and there’s no way for it to retain its identity. What’s missing is a code. In order for a bubble to come to life – at least by ordinary scientific definitions – there must be heritable information that defines the living thing. The bubble needs genetic material.  

Cellular life, in its earliest days, needed only two ingredients: a bubble of membrane and something heritable to pass on to other cells through reproduction. The most commonly known heritable substance is DNA. Scientists have been using this principle, of a semi-sealed pocket and its contents, to find the most stripped-back version of a cell that can be considered “alive”. The idea is an old one, and there have been many efforts to recreate earliest life from its simplest bubble-like “protocells”.

It’s a tricky job, as scientists don’t know what the early primordial soup was made of. They can guess, by mixing together the gases and simple compounds that made up the early atmosphere and seas. Scientists can attempt to divine which of these building blocks were around on early Earth from records scarred chemically and physically into ancient rocks. Add a bolt of lightning or two and see what happens to the mixture. In conditions such as these, recreated in the laboratory, some of the components of life form spontaneously.

So a membrane bubble has some genetic information inside. But is it alive? There are two reasons it might not be considered so. First, the genetic code doesn’t at this stage contain any meaningful information; it will just be a string of building blocks arranged in no particular order. Their order doesn’t translate into any structure, function or biological trait. Second, there’s no mechanism for one string of genetic information to copy itself and make a new bubble cell. A cell in this state might be rather lonely. 

Only one molecule solves both problems for the primordial bubbles in a neat trick. RNA can be thought of as the non-identical twin of DNA. It has the same property of coding information that DNA can have, but it also has other properties that are at least as important. RNA molecules can curl up into a ball, a very particular shape, which can interact with other RNA molecules to encourage them to replicate. Each RNA strand acts as a template for another one to form alongside it. The first strand of RNA can encourage the second strand to bind together and then break away. This process goes on in a runaway chain, which can end up with potentially limitless copies of the same RNA strand. This is replication: RNA can do it to itself, but DNA can’t.  

This is a puzzle, as we code our genetic information in DNA, not RNA. Was there a transition? If so, how did it happen?

A world of living cells based on RNA is one hypothesis, but it may not have been that way. We don’t know that cells ever encoded genetic information in anything but DNA, the more boring molecule that lacks RNA’s tricks. DNA does not tend to curl up into interesting shapes that can coax chemical reactions. The idea of DNA stimulating the replication of more DNA just doesn’t add up for most scientists. The RNA world, as it’s nicknamed, is a very neat idea in which many have faith but few can have certainty.

Let’s say that life is somehow made of DNA now. Tiny fatty bubbles replicate and split into daughter cells again and again, doubling each time. Your family tree has germinated. But what of the first problem, the genetic information that codes nonsense? Are these bubbles alive yet? By this point, theoretically, these bubbles must now contain a DNA sequence that matches up with an RNA sequence that can, in turn, do something useful. (Yes, RNA sticks around, just not as a coding molecule; it’s a helper molecule that copies DNA in order to do its bidding.) This RNA is curled up so that it can coax along replication not of RNA, but DNA. More strands are made and the replication process can continue. 

Other than this first explanation, the code might be nothing but arbitrary strings of genetic letters arranged with no words, sentences or punctuation. That doesn’t do much for the bubble. But the DNA code gains meaning over evolutionary time. Sections turn into phrases that translate to molecules of RNA that can perform a useful function, or - more often - encode another molecule that can do something useful.  An extra kink is added, a straighter section here, a stickier section there. That means RNA can curl up into different shapes and float off into the bubble to spark different processes: certain shapes encourage the building of other molecules while other formations of RNA may expedite these processes.  

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Liposomes form in liquid solutions when lipid molecules line up to make a double layer with their hydrophobic tails facing inwards and their  hyprophilic heads facing outwards on both sides, forming a bubble shape. Liposomes can be used as carrier molecules, keeping substances in their central pockets. Illustration by Robert Galt 

The many and varied jobs that these molecules do come about because of the imperfect replication of DNA. A typo in the code can give rise to a new kind of chemistry in the cell, and change the nature of the bubble.The precious DNA molecules themselves don’t do any dirty work – that’s reserved for RNA. In more complex cells, DNA is held hostage – changes to the code are minimised by keeping the DNA safe and often compartmentalised away in a part of the cell, perhaps to protect it from chaotic interactions going on elsewhere.

The new forms of RNA could make huge molecules – proteins – out of the fragments that littered the early seas. Proteins are more efficient than RNA at coaxing particular chemical reactions and these disposable giants can be used to shore up the bubble’s membrane, also transporting valuable treasures from the outside and expelling waste from the inside. They can break down high-energy molecules to fuel the cell’s activity. They can mend and destroy, communicate and be silent, make mess and clean up.

Through natural selection, living cells have been fortified and stabilised. They have developed an array of mechanisms to keep them intact and active that has continued to grow for as long as scientists have wanted to study it. After all that evolution, all that change, living cells have become the most delicate and intricate bubbles in the world. §