THE SPECIAL MESSENGER

Mitochondria are tiny structures that exist within every cell. They are not in the cell nucleus, the tiny bag in the middle of the cell which contains the chromosomes, but outside it in what is called the cytoplasm. Their job is to help cells use oxygen to produce energy. The more vigorous the cell, the more energy it needs and so the more mitochondria it contains. Cells from active tissues like muscle, nerve and brain contain up to one thousand mitochondria each.

Each mitochondrion is enclosed within a membrane. Arranged in an elaborate structure within the membrane are all the enzymes required for the final stage of aerobic metabolism. This is the part where the fuel we take in as food is burnt in a sea of oxygen. There are no flames and all the oxygen is dissolved, but it is as much a piece of combustion as what happens in a gas fire or a car engine. Fuel and oxygen combine to produce energy. Fires and engines produce their energy as heat and light. Mitochondria do not give off light when they burn fuel but they do heat up – it is partly the heat given off by mitochondria that keeps us warm. However, the main output is a high-energy molecule called ATP, which is used by the body to run virtually everything, from the contraction of heart muscles, to the nerves in your retina that is reading this page, to the cells in your brain that are interpreting it.

Buried right in the middle of each mitochondrion is a tiny piece of DNA, a mini-chromosome only sixteen and a half thousand bases in length. This is minuscule compared to the total of three thousand million bases in the chromosomes of the nucleus. Finding DNA in mitochondria at all was a big surprise. And it is very peculiar stuff. For a start, the double helix of this DNA is formed into a circle. Bacteria and other micro-organisms have circular chromosomes, but not complex multi-cellular organisms and certainly not humans. The next surprise was that the genetic code in mitochondrial DNA is slightly different from the one that is used in the nuclear chromosomes. Mitochondrial genes hold the code for the oxygen-capturing enzymes that do the work in mitochondria. However, many genes that govern the workings of the mitochondria are firmly embedded within the chromosomes of the nucleus.

How did this all come about? The current explanation is stunning. It is thought that mitochondria were once free-living bacteria that, hundreds of millions of years ago, invaded more advanced cells and took up residence there. You could call them parasites, or you could call their relationship with the cells symbiotic, with both cells and mitochondria doing something for each other. Cells got a great boost from being able to use oxygen. A cell can create much more high-energy ATP from the same amount of fuel using oxygen than it can without it. For their part, the mitochondria evidently found life within the cell more comfortable than outside. Very slowly, over millions of years, some of the mitochondrial genes were transferred to the nucleus, where they remain. This means mitochondria are now trapped within cells and could not return to the outside world even if they wanted to. They have become genetically institutionalized. Even now you can see the evidence of gene transfers between mitochondria and nucleus that didn’t work out. The nuclear chromosomes are littered with broken fragments of mitochondrial genes that have moved across to the nucleus over the course of evolution. They can’t do anything because they are not intact. So they just sit there, as molecular fossils, a reminder of failed transfers in the past.

There is something else which is unique to mitochondria. Unlike the DNA in the chromosomes of the nucleus, which is inherited from both parents, everyone gets their mitochondria from only one parent – their mother. The cytoplasm of a human egg cell is stuffed with a quarter of a million mitochondria. In comparison, sperm have very few mitochondria, just enough to provide the energy for swimming up the uterus as they home in on the egg. After the successful sperm enters the egg to deliver its package of nuclear chromosomes it has no further use for the mitochondria, and they are jettisoned along with the tail. Only the sperm-head with its package of nuclear DNA enters the egg. The plump, fertilized egg now has nuclear DNA from both parents, but its only mitochondria are the ones that were in the cytoplasm all along – and they all come from the mother. For that simple reason, mitochondrial DNA is always maternally inherited.

The fertilized egg divides again and again, forming first an embryo, then a foetus, which in turn becomes a new-born baby and, eventually, an adult. Throughout this process, the only mitochondria to be found are copies of the originals from the mother’s egg. Though both males and females have mitochondria in all their cells, only women pass theirs on to their offspring because only women produce eggs. Fathers pass on nuclear DNA to the next generation, but their mitochondrial DNA gets no further.

Changes to DNA, both in the mitochondria and in the nucleus, arise spontaneously as simple mistakes during the copying that accompanies cell division. Cells have error-checking mechanisms which correct most mistakes, but a few escape this surveillance and get through. If these mutations occur in cells that go on to produce eggs or sperm, known as the germline cells, then they can be passed on to the next generation. Mutations that occur in the other body cells, called somatic cells – the ones that aren’t going to produce germline cells – will not be passed on. Most DNA mutations have no effect at all. Only very occasionally, when they strike and disable a particularly important gene, will mutations be noticed. In the worst cases, these mutations can produce serious genetic diseases, some of which we shall encounter in a later chapter, but most of the time they are harmless.

The rate at which mutations occur in nuclear DNA is extremely low – roughly, only one nucleotide base in one thousand million will mutate at every cell division. Mitochondria, on the other hand, are not quite so vigilant with their error-checking and allow through about twenty times as many mutations. This means that many more changes are to be found in mitochondrial DNA than in the equivalent stretch of nuclear DNA. In other words, the ‘molecular clock’ by which we can calculate the passage of time through DNA is ticking much faster in the mitochondria than in the nucleus. This makes mitochondria even more attractive as a tool in investigating human evolution. If the mutation rate were very low, then too many people would have exactly the same mitochondrial DNA and there wouldn’t be enough variety to tell us anything much about developments over time.

There is yet another bonus. Although mutations are found all round the mitochondrial DNA circle, and this whole range was used by Allan Wilson and his students in ‘Mitochondrial DNA and human evolution’, there is a short stretch of DNA where mutations are especially frequent. This section, about five hundred bases in length, is called the control region. It has managed to accumulate so many mutations because, unlike the rest of the mitochondrial DNA, it does not carry the codes for anything in particular. If it did, then many of the mutations would affect the performance of the mitochondrial enzymes. This does sometimes happen when mutations hit other parts of the mitochondrial DNA outside the control region; there are some rare neurological diseases which are caused by mutations in genes that disable essential parts of the mitochondrial machinery. Because they are so damaged, these mitochondria do not survive well and are only very rarely passed on to the next generation. So these mutations gradually die out. The control region mutations, on the other hand, are not eliminated, precisely because the control region has no specific function. They are neutral. It appears that this stretch of DNA has to be there in order for mitochondria to divide properly, but that its own precise sequence does not matter very much.

So here we have the perfect situation for our research: a short stretch of DNA that is crammed full of neutral mutations. It would be much quicker and cheaper to read the sequence of the control region, just five hundred bases, than the entire mitochondrial DNA sequence at over sixteen thousand bases. But was the control region going to be stable enough to be useful in examining human evolution? If the control region were mutating back and forth at a great rate at every generation, then it would be extremely difficult to make out any consistent patterns over the course of longer time spans. We knew already from the work of Allan Wilson that if we were going to dig down deep into the genetic history of our species, Homo sapiens, using mitochondrial DNA, we needed to cover at least 150,000 years of human evolution – say 6,000 generations at twenty-five years per generation. If mutation in the control region were too frantic or erratic, it would be very hard, if not impossible, to distinguish the important signals from all the incidental, irrelevant changes after a few generations. We needed a way of testing this before embarking on the time-consuming and expensive commitment of a large study of human populations. How could we best do this?

Ideally, I wanted to find a large number of living people that could be proved to be descended through the female line from a single woman. In the course of my medical genetics research on inherited bone disease, I had worked with several large families; so now I took out the charts on which I had recorded their pedigrees. Although these went back several generations, there were depressingly few continuous maternal lines connecting the living members of these families. I could ask for the families’ help to put me in touch with relatives who were not shown on the charts; but it would be a long business. Still, there seemed nothing else for it, and I began to dig out their names and addresses. On my way back home that night, while I was thinking about something else, I experienced one of those rare moments when an idea suddenly arrives from the recesses of the mind, goodness knows how, and you know within a millisecond that it is the answer to your problem, even though you haven’t had time to work out why. I suddenly remembered the golden hamster.

When I was a small boy, I read in a children’s encyclopaedia that all the pet golden hamsters in the world were the descendants of just one female. I can definitely say that I had not thought about this again over the intervening decades. And yet the idea surfaced now. I do remember thinking at the time that the story couldn’t possibly be true. But what if it were? This would be the ideal way to test out the stability of the control region. All the golden hamsters in the world would have a direct maternal line back to this ‘Mother of all Hamsters’. It follows that they would also have inherited their mitochondrial DNA from her, since it is passed down the female line in hamsters just as it is in humans. All I had to do was collect DNA from a sample of living hamsters and compare their control region sequences. I didn’t need to have an accurate pedigree, because if there really had been only one female to start with they all had to trace back to her anyway. If the control region was going to be stable enough to be any use to us, then its sequence should be the same, or very similar, in all living hamsters.

I asked Chris Tomkins, an undergraduate student who, in the summer of 1990, had just started his final year genetics project in my laboratory, to see what he could find out about the golden hamster. The first thing he discovered is that, properly speaking, they are not called golden hamsters at all but Syrian hamsters. Chris went straight down to the Oxford public library and came back with some good news: he had found out that there was a National Syrian Hamster Council of Great Britain. He called the secretary and next day we were on our way to an address in Ealing, west London. Here we were greeted, with no little suspicion, by the secretary of the Syrian Hamster Club of Great Britain – Roy Robinson (now sadly deceased).

The late Mr Robinson was the product of a vanished age, a self-taught amateur scientist of great distinction. His dimly lit study was full of books on animal genetics, many of them written by himself. He pulled out his book on the Syrian hamster. His eyesight was very poor, and even with the help of very thick spectacles he needed to hold the text right up close to his face. He confirmed the story I had read as a boy. Apparently, in 1930 a zoological expedition to the hills around Aleppo (now Halab) in north-west Syria had captured four unusual small golden-brown rodents, one female and three males, and taken them back to the Hebrew University in Jerusalem. They were kept together, and the female soon became pregnant and gave birth to a litter. There was clearly going to be no difficulty in breeding them in captivity. The university began to distribute them to medical research institutes around the world, where they became popular as an alternative to the more usual rats and mice – though they were tricky lab animals, active only at night, bad-tempered and prone to bite their handlers (good for them!). The first recipient was the Medical Research Council institute at Mill Hill in north London, which passed some on to London Zoo. By 1938 the first golden hamsters had reached the United States.

Sometimes, lab animals that are no longer required are taken home by staff and kept as pets rather than being killed. Over time, hamsters spread from one household to another and, as their popularity increased, commercial breeders added them to their catalogues and groups of hamster enthusiasts started up. In 1947 a piebald hamster appeared in one breeding colony – the first of many coat colour varieties, caused by spontaneous mutations in the coat colour genes, it showed itself because of the inbreeding within the colony. It wasn’t difficult to mate the mutants with each other and produce a pure-bred strain. Breeders became ever keener to find new coat colours, and over the next few years many different such mutants were discovered and pure-bred strains established – cream, cinnamon, satin, tortoiseshell and many more. Hamsters made good pets and the availability of strains with different coats only added to the interest. Thus began the population explosion: today there are over three million hamsters kept as pets all over the world.

Mr Robinson lived in an old horticultural nursery, which at the time we visited was quite run down. A long, rectangular plot enclosed by walls of beautiful old brick contained overgrown flower beds and a handful of greenhouses with cracked and broken panes. There were also two substantial sheds, and we made our way to the first one on the left, where Mr Robinson unlocked the door to let us in. We could not believe our eyes. Inside were rack upon rack of cages, all labelled and numbered, within each of which nestled a family of hamsters. Mr Robinson had collected an example of every single coat variety that had ever been produced, and was interbreeding them to unravel the genetics. There were pure-white hamsters, lilac hamsters, hamsters with short dark fur and hamsters with long fine coats like an angora goat. So eminent was Mr Robinson in the world of Syrian hamsters that each time a new coat mutant was discovered, a pair would be sent to Ealing. We were looking at the world reference collection. To cap it all, he opened an old ‘Quality Street’ sweet tin and there inside, neatly stacked, were the dried skins of the original animals that had been sent to him. Martin Richards, who had made the trip along with Chris and myself, was so taken that he bought two hamsters from a pet shop in Ealing on the way home. He kept them in his flat for two years until they passed away. Of more immediate significance, we took away from Mr Robinson’s collection a few hairs taken from each strain.

Mr Robinson had also given us the contact details of Syrian hamster breeders’ and owners’ clubs throughout the world, and Chris was about to write to them asking for hair samples when it occurred to us that this might not go down very well. We had already discovered that you needed quite a number of hairs to get out the DNA. Hamster hairs were very fine and tended to break off above the root. Although the animals didn’t mind a few hairs being plucked, they were likely to feel a little uncomfortable, and so were their owners, if we asked for substantial tufts. That’s when we realized we needed another source of DNA. We hit on what seemed at first a completely wild idea. We knew the DNA amplification reaction was exquisitely sensitive, which is why it had worked with the ancient DNA from the archaeological bones. Would there be enough hamster cells shed from the walls of the large intestine to survive in their droppings? Surely, not even the most devoted owner would begrudge parting with a few droppings for the cause of science. But would it work? There was only one way to find out – so next day Martin appeared with a fresh crop from his house guests. They were dried and shrivelled, rather like mouse droppings, and totally inoffensive. Even so, Chris used tweezers to pick them up and put them into a test tube. He boiled the droppings for a few minutes, spun down the sediment in a centrifuge and took a drop of the clear liquid into the DNA amplification reaction. It worked a treat.

For the rest of the summer small packets arrived from hamster enthusiasts all over the world. With their characteristic rattle, we knew immediately what they were. We eventually got DNA from thirty-five hamsters, and it wasn’t long before Chris had sequenced the mitochondrial control region in all of them. They were all absolutely identical. So the story was true after all. All the pet hamsters in the world really do come from a single female. But more importantly for us, the control region had remained completely stable. From that very first hamster captured in the Syrian desert to its millions of great-great-great…great-grandchildren from every corner of the world, the control region DNA had been copied absolutely faithfully with not even a single mistake.

It was an amazing thought. Going flat out, hamsters can manage four or five generations a year. At that rate there would have been time for at least two hundred and fifty hamster generations since 1930. Even though all thirty-five of our hamsters would not have traced independent maternal lines all the way back to 1930, the fact that there were absolutely no DNA sequence differences between any of them had to mean that the anxiety I had that mutations in the control region might be happening too quickly was unfounded. Quite the reverse, in fact: this was a very reliable region of DNA after all, not given to fickle fits of mutation that would make it impossible to trace over the hundreds of generations we wanted to explore in our own human ancestors. Of course, there was a chance that even though the control region was stable in hamsters, it might not be in humans. I didn’t think this was very likely, given the very fundamental nature of mitochondria, and I was prepared to take that risk.

I was not alone in my interest. Before very long it was plain that other scientists were thinking along the same lines and had realized the potential of this very special piece of DNA to illuminate not only the grand schemes of human evolution but much more recent mysteries as well.