In the previous article, we saw how the basic building blocks in terms of elements came together for the death and rebirth of stars. So given we now have all the basic elements of life, how did it go forward to build up the complex structures called cells we see today? How did we emerge from the shackles of interstellar debris to the promised land of life?
Our Sun is a very ordinary star. Stars range in size from about 0.08 the mass of our Sun – which can barely shine, up to around 150 times our Sun, which rip themselves apart. We’re positioned somewhere in the suburbs of the Milky Way, out on one of the arms, far away from the center – it’s nowhere special, but we like it. Even our galaxy isn’t particularly special, it’s one of the bigger ones in the local group, but not the biggest, and it’s currently part-way through colliding with a smaller galaxy – not that we’ve noticed until recently.
Our Sun formed like many others when a cloud of interstellar gas contracted to form a proto-star. As the gas comes together, it heats up, and if there is enough of it, it gets hot enough to fuse hydrogen into helium – around 14 million kelvin (or °C if you prefer). Not all of the material in the cloud went into forming the Sun, some got left behind and stayed in orbit, clumping together to form ever bigger bodies. These things would become the planets, moons, and other bodies in the solar system.
Close to the Sun it was too warm for water and hydrogen to stick around, and so the inner planets consist of what’s left over, and are called the rocky planets. The outer planets didn’t care and consumed larger quantities of volatile gasses to become the gas giants. Out beyond that is a layer of debris that makes up comets every so often. Of course the planets also grew incredibly hot as they condensed out of the swirling debris, and our own Earth still retains a lot of heat from that initial collapse. Early on it would have been much hotter of course and much more geologically active.
After the Earth formed, it went through a number of traumas. It was continually bombarded by spare bits of matter left over from the formation that hadn’t found a place to live yet. It was also hit by a lot of cometary debris from the outer solar system, which probably brought a large amount of the water present today to the Earth. It also had a vicious collision early on with a something large – probably as large as the planet Mars. This hit the early Earth one almighty thump, and created the Moon in the process. This is the reason the moons rocks are very similar to those on the Earth.
The bombardment from outer space continued on for many millions of years making vast craters still visible on the Moon, but mostly recycled out of view on the geologically active Earth.
As the Earth cooled, a crust began to form, and the water started to make oceans and the bombardment slowed down as things started to fall into regular orbits and detritus was absorbed by the planets and Sun, or kicked out of the solar system altogether.
Now it was possible for the building blocks of life to get together. We have all the basic elements from the death of stars, but they need to be fitted together in just the right ways. Broadly speaking, there are some basic types of compounds that we need that are relatively complicated. These are fats, nucleic acids, and proteins. Pretty much no life we know of currently can exist without these three things. Obviously other things are required too – like water and salts and so on, but they are easy to make. In fact, up until 1828, it was thought there were two types of compound. Those that could be made by man, such as salt, nitric acid, ammonia and so on, dubbed the inorganic chemicals; and those made by living things, such as sugar and fats that were beyond the means of man to make – the organic compounds. This vitalism as it was known, was overturned by Friedrich Wöhler, when he accidentally made urea – the first “living” chemical to be made from non-living things.
Anyway – fats are required for lots of things, but crucially in the form of phospholipids, they make up the cell membranes that enclose all cells. Without these, the living chemicals would just drift apart and not do anything. Some viruses can get by without having a cell membrane, but they can’t live without parasitising a cell membrane clad host to reproduce in. So cell membranes are fairly essential, and so are the lipids they are made from.
Proteins have a huge number of uses, but probably principle amongst those forming life is their enzymatic role. They can speed up reactions in a catalytic manner, that would otherwise hardly ever happen without their presence. Proteins are not absolutely essential, although all living things have them today. One of their other big uses is structural, hair, skin, internal cell structure – all done by proteins. You could even stretch a point and say that prions, the instruments behind BSE and nvCJD, “live” just as proteins – but again they require help from real living things to get anywhere. Proteins are built up from chains of amino acids, so you need a good handful of those to get started, and we use up to 20 different types.
Nucleic acids are a major, MAJOR part of life, because they carry the instructions to build and operate the living cell, including building offspring. Without these there would be no way to transmit designs down to the next generation, and so everything would grind to a stop with the death of the first cell. Prions again manage without nucleic acids, but pretty much everyone agrees they are not living, and they can’t reproduce without a reservoir of existing proteins to subvert.
So how did these things form? Well in some ways we have too many options, and in other ways we have none that we know for sure are how it happened. No one has ever taken a handful of basic chemicals, stirred them up and out has popped a working organism. The evidence for things that happened so long ago is pretty much erased. The Earth’s crust gets recycled on a continual basis, so really old rocks that might have useful clues in them are few and far between. Those that we can find have had to survive for 4 billion years and in that time something is going to have happened to disturb them. So the traces we can find are very weak. Not all evidence has to come from rocks though – some can come from our what we find today. If we find, for instance, that some mechanism occurs in pretty much every living creature, plant and bacteria, we can probably assume they haven’t been invented anew by each, but that they are probably echoes of something ancient.
Anyway, on with the story, what have we found?
Well to begin with there has to be mentioned the classic Miller-Urey experiment. In 1953, Stanley Miller put together in a flask in a chemistry lab a collection of things that might well have been around early on in the Earths history. He had water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2) in the mix, all nice simple inorganic compounds (but some of them mildly explosive!). Then, in a health and safety nightmare, he subjected the mixture to a stream of small electrical sparks. After a week of this, a load of gunk appeared, and about 15% of this gunk created was organic compounds, including lipids, and at least 2 different amino acids. This showed how easy it was to make organic molecules from not much – makes you wonder why it was so hard before 1828! There have been various criticisms about the experiment since it was published – with people disputing the original mixture may not have reflected the early earth. Since then other combinations have been tried and most come up with some useful products.
However that isn’t the only source. Meteorites – those lumps of rocks that descend on us from space like an echo of the early bombardment. Some of these, particularly the carbonaceous chondrites, have been studied and within them are found all manner of interesting organic chemicals, including those amino acids again. The Murchison meteorite in particular has been extensively studied and all manner of chemicals found in it, even bits of nucleic acids. Meteorites get hot as they come rushing through the atmosphere, but the heat tends to stay on the surface, as they are only cooked for a short time before being cooled again by the atmosphere. Anything more than a few centimetres inside is pretty much untouched.
Still not convinced? Well there is another weird setup at the bottom of deep oceans, where volcanic vents pour out superheated water chock full of minerals, known as black smokers. Here a strange ecosystem has evolved that has no dependence on the Sun at all. Under these high temperatures and pressures all manner of interesting reactions happen and yet again its possible to make organic chemicals. There are suggestions that this might be how life started, deep in the sea protected from hostile UV light and away from the bombardment, it could have been quietly getting on with things. Lots of other ideas around too – including clays, seashores, ice, … well you name it actually.
So how would it have formed? That is more tricky to work through. It is well known that lipid bi-layers, which are the basic cell membranes form spontaneously. This is just the way they are. They have two different ends, one that is attracted to water (hydrophillic) and one that hates water (hydrophobic). So if you get a bunch of them together, they tend to shuffle around until all the hydrophilic bits are on the outside and the hydrophobic on the inside. You can see this if you wash a greasy pan – without detergent the fat settles out as circular blobs.
Under some conditions they will absorb more molecules, and so grow, and even split into separate parts. So actually that part is really the easiest part to work through – it just happens.
What would go inside though is more tricky. You can envisage a couple of possibilities.
One is a metabolism first model, and in this one you have a set of reacting chemicals that make up an auto-catalytic cycle. This means that the reaction moves in a cycle, starting with say compound A, going to B, to C to D and back to A again, in the meantime perhaps producing something. We see cycles a lot in modern organisms, the TCA cycle, the Calvin cycle and many others. If it started this way, there had to have been a inclusion at some point of a hereditary molecule like RNA/DNA. This is a bit tricky to see how it came about, but not beyond the bounds of possibility.
The other tack, which probably has more support, is the genes first model. In this case we have genetic material present from the start and they are what drives the metabolism. There is a certain amount of evidence for this, that comes from a number of angles. Most avenues focus on RNA as the original genetic molecule. This is because RNA has other properties besides its basic hereditary role. RNA can perform some of the functions of a protein, in that some forms of RNA can catalyse reactions. These particular forms of RNA, called ribozymes, loop themselves into complicated knots, a little like proteins do, with similar effects. In some cases it is believed they could even catalyse their own replication and that would be an excellent starting point. If it could reproduce itself and other strands, you could quickly have a self replicating cycle. What’s more, RNA is found in many places today. Some viruses use it as their sole genetic material – HIV for one. RNA is also found in the ribosome, which is the key machine found in all cells that converts RNA into proteins. RNA also occurs in other cellular machines – so it clearly has a heritage.
However against this we have a number of issues. RNA is not terribly stable left on its own – it has a habit of attacking itself leading to fragmentation. It’s also more difficult to see how RNA can get constructed without some framework. There have been suggestions of even more primitive nucleic acids, including those not found in nature such as PNA, TNA, LNA and GNA. Another theory suggests that poly-aromatic hydrocarbons could build a suitable framework for RNA to form on.
Anyway – this view leads to a primitive form of life based on RNA, called, without much imagination, The RNA World.
RNA has other problems too – it’s not a good long term storage for information. Enzymes that copy RNA tend to make mistakes, often introducing errors at about 1 in 10,000 bases or so. The HIV virus for instance, which uses RNA as its genetic material even today, normally introduces at least one error every time it copies its 10,000 base genome. This is one reason it mutates rapidly and is so difficult to hit, even for our adaptive immune system.
So long term stability is not something RNA is good at. This is an asset when you are trying to evolve into something successful, but not good when you have evolved into something successful.
This is where DNA comes in – somehow. DNA, because it exists as a double strand, is much more stable anyway – it doesn’t attack itself. Also having the second strand allows for proofreading to check for mistakes in copying. This pushes the error rate well down for DNA, making only perhaps as much as 100 mistakes when copying the 3 billion bases of the human genome. This is why all living organisms (bar viruses) use DNA today.
So somehow, and there is no clear consensus on the exact mechanism, life got started and settled down to copying itself around. But these were at best single celled organisms and a long way from anything like you or I.
What way could life progress? Tune in next time…