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The Genetic Evidence for Human Evolution
From: The Natural History Museum
| By:
Chris Stringer |
EDITOR'S INTRODUCTION |
Analysis of the human genome has revealed a wealth of information about our evolution, about patterns of historical migration and population movement, and about the relationships between humans and other primate species. Recent technological developments have allowed the extraction of Neanderthal DNA from fossil bones for the first time, shedding more light on how Homo sapiens came into being. In this interview Chris Stringer, Head of Human Origins at The Natural History Museum, explains the range of genetic evidence for human evolution. |
Fathom: The genetic evidence for human evolution is based on the analysis of DNA. What exactly is DNA? |
Chris Stringer: DNA stands for deoxyribonucleic acid. It is the DNA that makes up the genetic code; the code that identifies us and builds our bodies. DNA is made up of strands of nucleic acids and each of us is the product of a unique code which we inherit, in about equal measures, from our parents. Half of our DNA is inherited from our mother, half from our father and that combination is what makes us what we are. |
But what's interesting about DNA is that it carries not only the history of our immediate parental ancestry, but also a history that can be traced back over a much longer time. Scientists are able to look at our deep history by studying our DNA. DNA is copied from one generation to the next and sometimes little copying mistakes, or mutations, creep in. If those are not harmful, they are kept in the DNA pattern and passed on to future generations. So by looking at those changes through time and observing how they have accumulated, we can build up relationships between people and relationships between populations. DNA gives us a key to the past which is independent of the fossil evidence. It's another way of reconstructing our history. |
Fathom: Are there different types of DNA that scientists analyse in order to learn more about human evolution? |
Stringer: Yes, there are. Within most of the cells in our body we have a full complement of DNA coding and that DNA is contained in three different ways. In the nucleus of our cells we have chromosomes which are inherited equally from each parent; the chromosomes contain DNA from our mothers and our fathers. There are 23 pairs of chromosomes in total and of those, 22 are 'autosomal' chromosomes. They carry 'autosomal' DNA which is acquired equally from both parents. The other set of chromosomes, the 'sex' chromosomes, have X and Y variants. Females have two X chromosomes and males have an X and a Y chromosome. So the Y chromosome is unique to males and if we look at the DNA in the Y chromosome it shows a line purely through males, through fathers, going back into the past. |
Furthermore, outside of the nucleus there are little DNA-containing organelles, mitochondria, often referred to as the 'powerhouses of the cells'. There are many in each cell and their DNA, called mitochondrial DNA, is inherited only through mothers. When an egg and a sperm unite to make a new individual, the mitochondrial DNA in the sperm dies off without contributing to the new individual's genetic makeup. Therefore that egg contains only the female's mitochondrial DNA which gives us a DNA signal entirely through the maternal line. So by investigating the Y chromosome we can chart a line of fathers stretching back into the past, whilst mitochondrial DNA gives us a line of mothers stretching back into the past. |
Fathom: Has DNA analysis revealed anything about our relationship with other primates? |
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| Chimpanzee. | |
Stringer: Yes, it has been discovered that we share a huge amount of DNA with our closest primate relatives. It has been calculated that we share 98 to 99 percent of our DNA with chimpanzees. The differences between humans and chimps may seem profound, but when you look at the genetic codes they are overwhelmingly similar. There are in fact two species of chimpanzees, common chimpanzees and bonobos, and we are equally related to both. DNA studies have shown that of the apes, chimpanzees are our closest relatives, followed by gorillas, the other African great ape, followed by orang utans, the Asian great ape, followed by gibbons, which are smaller apes. This reflects the order of divergence. We think that humans shared a common ancestor with the apes and that the different species split off from there; first the gibbons split off, then the orang utans, then the gorillas, then the chimpanzees, leaving humans with their own line of evolution. Researchers have analysed the amount of difference between human and chimpanzee DNA and estimated that in order to accumulate those differences, the two species would have split between 5 and 7 million years ago. |
Fathom: And what has it revealed about our relationship with each other? People around the world look very different but how different are we really, and is that level of difference typical? |
Stringer:
Despite all the different shapes and sizes and colours of humanity, the evidence from mitochondrial DNA tells us that we are actually very, very similar. Comparing the differences between human mitochondrial DNA around the world with the differences found within just one small troop of chimpanzees or gorillas in central Africa shows that a single ape troop contains greater diversity than all of humanity! In fact relative to many other species, we're almost clones of each other. This is consistent with a recent origin for our species and also with the view, held by some scientists, that there was a bottleneck some time in our recent evolutionary past. It seems likely that within the last 200,000 years, the human population went down to a very small number, to maybe only 10,000 or 20,000 people, reducing the genetic diversity that gave rise to today's population. This may well be why humans today show a relatively low diversity compared with many other species. Chimpanzees, in comparison, have had a long and quite complex evolutionary history; their diversity may have been developing for over 1 million years. |
Fathom: When did DNA research start having a major impact? |
Stringer: Genetic studies had already confirmed our close relationship to the apes, but for modern human origins I think it really took off in 1987 when seminal work on mitochondrial DNA was published in the journal Nature. The three authors of that paper caused quite a sensation because, for the first time, they had looked at the mitochondrial DNA types of about 150 people from all over the world, from Africa, Europe, New Guinea, etc.--members of the different so-called races. They used a computer to build up a tree of the relationships of all those different types. The tree assumed the minimum number of mutations needed to account for all the difference found among the samples and charted common ancestors going further and further back in time until it eventually arrived at a single hypothetical ancestor for all the people living today. Since this is mitochondrial DNA, that ancestor is female, so she was termed 'Mitochondrial Eve'. By making calculations based on the number of mutations, the authors were able to pinpoint that Eve lived approximately 200,000 years ago--relatively recently. And that wasn't all. From the structure of the tree that the computer constructed, they deduced two fundamental branches. One branch was African and was found only in Africa, the other included both Africans and non-Africans. |
The authors thus concluded that Mitochondrial Eve lived in Africa and that some of her descendants stayed behind in Africa whilst others went out and founded the populations in the rest of the world. This was revolutionary because it indicated that the ancestral stock lived in Africa quite recently and gave rise to all the variation we find today in really quite a short period of time. It also concurred with the 'Out of Africa' theory which had suggested just this pattern of events, based on the fossil evidence. |
Fathom: How much work has been done on DNA from pre-Homo sapiens fossils? |
Stringer: DNA is often absent or very badly preserved in fossils and the technology for working with it was only developed a few years ago. In most cases the fossils are too old, or were buried in the wrong conditions, and no recoverable DNA survives. But under certain circumstances DNA does survive, particularly if the bones are fossilised in cool, dry conditions. In 1997, scientists recovered mitochondrial DNA from the Neanderthal type fossil, the German specimen discovered in 1856 that gave its name to the whole Neanderthal group. A piece was taken from the humerus (arm) bone of that fossil, tiny bits of mitochondrial DNA were extracted and, using a special technique known as the Polymerase Chain Reaction, those samples were copied in large quantities and then analysed. |
When the mitochondrial DNA of the Neanderthal fossil was compared with that of living humans and chimpanzees, it was found that it was more similar to humans than to chimpanzees, but that the mitochondrial DNA was significantly different from that of anyone alive today. It was equally different when compared with Europeans, Africans, Australians and so on. This suggested that the Neanderthals formed a separate branch to the human lineage that survives today, contradicting the idea that they might be partly ancestral to modern Europeans. Mitochondrial DNA has been sampled from about 10,000 individuals from Europe, and from at least five Neanderthals, and no living person so far tested has anything resembling the DNA types found in Neanderthals. The Neanderthal samples show common features, and analysis suggests that they began to separate from our own genetic line about half a million years ago, developing their own distinct diversity. As is also suggested by the fossil evidence, the Neanderthals are apparently genetically extinct--the patterns seen in their mitochondrial DNA cannot be found anywhere in the world today.
Fathom: Has any non-mitochondrial fossil DNA been sequenced? |
Stringer: No, unfortunately mitochondrial DNA is all that has been recovered from Neanderthal fossils and there are several reasons why. Mitochondrial DNA is present in much greater quantities and so is easier to copy. Also, because the whole mitochondrial DNA molecule has only about 16,000 base pairs, it is well-studied and differences and similarities can be easily recognised. It is possible that under exceptional conditions, nuclear (X, Y or autosomal) DNA could be preserved, but until now such DNA has not definitely been identified from a fossil. |
Fathom: Has DNA been recovered from any of our direct ancestors? |
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| Reconstructed head of Cro-Magnon man. | |
Stringer: Not from fossils it hasn't, and because of the preservation problems it's unlikely that DNA survives in fossils that are older than 100,000 years. This means that scientists will not be able to extract and examine the DNA from the African fossils that may well be our direct ancestors. But considering the more recent past--the last 30, 40 or 50,000 years--attempts have been made to get DNA from Cro-Magnon fossils in Europe. Unfortunately most of these have failed, or the DNA cannot be shown to be original and authentic. The problem is that the DNA types revealed are similar, in some cases identical, with DNA types that are found in Europe today. Because Neanderthal DNA is so different from that of anyone alive today, it confirms that the DNA came from the fossil and has not been contaminated by a modern human. In the case of Cro-Magnons it's much more difficult to prove that. |
There have been some successes. Two early modern human fossils from Cheddar, in Britain, one dating to about 13,000 years and one to 9,000 years, have produced types of mitochondrial DNA that are similar to those commonly found in Europe today which, given the age of the material, is not surprising. There have also been some extremely controversial claims made for mitochondrial DNA recovery from a number of Australian fossils, including a specimen from Mungo which dates to about 60,000 years old. It was suggested that the DNA does not fall within the range that would be expected for modern humans of 60,000 years ago, i.e., that it is too different to indicate a recent African ancestry. This claim was used to support the theory of 'Multiregional Evolution' which proposes that Homo sapiens did not only arise in Africa, but evolved at more or less the same time in different parts of the world. |
The claims are controversial, firstly because the DNA may not be authentic. The fossil was found in sandy conditions, a substrate which generally does not preserve bone or DNA particularly well. Furthermore, some colleagues and I reanalysed the data using a larger sample of Africans and Australians to make the comparisons, and our results suggested that the claimed ancient DNA falls entirely within the expected range for modern humans. Of course this is not to say that there might not be some surprises in store for us 'Out of Africa' theorists. If we could get DNA from early modern human fossils from around the world, we might discover that there were older lineages surviving. But so far this has not proved to be the case and the Australian evidence does not present the challenge it was claimed to. |
Fathom: Speaking of the 'Out of Africa' theory, can you tell me what light DNA analysis might shed on what we know about early population migrations? |
Stringer: Studies of mitochondrial DNA indicated that we share a recent common ancestry in Africa, and that at some point, perhaps 100,000 years ago, people started to emerge from Africa and founded the rest of humanity. The evidence from many other gene systems has now been put into the equation. Overall, the evidence shows that present-day Africa has the greatest genetic diversity and this probably reflects the fact that in the past, Africa had the largest population size, and that people have lived and evolved in Africa longer than they have lived and evolved anywhere else. Non-Africans show less diversity and that is consistent with them being the product of a dispersal, of the migration from Africa of a relatively small group or groups of people. |
Looking at the genetic pattern as a whole, it is still a source of dispute as to whether there was just one main migration event or several. Some evidence suggests that there was only one small group that came out of Africa, perhaps 50,000 years ago, who founded all the populations outside of Africa, whilst other evidence suggests that there were earlier migrations. There may have been a dispersal event around 100,000 years ago which went across southern Asia and eventually reached Australia, and a later one that gave rise to the populations in Europe (the Cro-Magnons), East Asia and the Americas. So there may have been multiple dispersals. It is even claimed by some people that there are still faint genetic signals of a dispersal from 600,000 years ago which can be picked up in Asia. If that were correct it would make the Out of Africa model more complex because, although most of our DNA came from a recent African migration, it would be possible that there was inter-mixture with older populations in Asia and that some of those genes have persisted. But most experts don't think that is the case. |
Fathom: What has DNA analysis told us about the settlement of Europe? |
Stringer: Mitochondrial DNA and Y chromosome DNA studies show that Europe was settled more recently than Africa and that's consistent with European ancestry being traced back ultimately to Africa. The genetic evidence suggests that Europe was settled within the last 50,000 years and that most of the DNA found in Europe today dates back to the Cro-Magnons of the later part of the Upper Palaeolithic, or Old Stone Age. At least 50 percent of the mitochondrial types found in Europe can be traced back to between about 14,000 and 25,000 years ago--the founding period for much of the mitochondrial DNA diversity seen in Europe today. There is also a very small component that dates from the beginning of the Cro-Magnon population and another component, maybe 10 to 25 percent, that seems to have come in with Neolithic farming practices from the Near East and Middle East in the last 10,000 years. This relatively small component might indicate that farming spread more as an idea than as a mass migration of people- that the aboriginal Europeans took up farming practices, rather than being replaced by farming communities coming in from elsewhere. However, study of the male-inherited Y chromosome DNA shows more recent immigration, indicating that greater male migration was associated with the spread of farming. |
Fathom: What can we hope for future genetic research into human evolution? |
Stringer: I think we can hope that more ancient DNA will be found. Recently some more Neanderthals have been sequenced for their DNA and those results will be published soon, giving us an increasingly detailed knowledge of the diversity of Neanderthals and how they relate to us. We can also hope for the discovery of DNA from early modern humans in areas other than Europe. By looking in the cooler regions of the world, where DNA is potentially better preserved, such as higher latitude or high altitude sites in Asia and the Americas, we may be able to find further DNA in fossils. But, unfortunately, the signs from the hotter regions of the world are that DNA survivability rates there will be very poor. |
Where modern humans are concerned, we are already seeing the most fantastic growth of studies of DNA of populations from all over the world, giving us unprecedented detail in mapping migrations, for example in Polynesia and from Asia into the Americas. An increasingly accurate picture of population dispersals is emerging as our ability to both identify and date such events becomes more refined. Researchers are looking at language patterns and genetic patterns and finding clear correlations; the history of languages and history of DNA can be studied together to build up even more complex and detailed patterns of migration. |
The human genome has now been completely sequenced and the same needs to be done for the chimpanzees, which means we will be able to better understand both the similarities and differences between us and them. Scientists are starting to home in on certain areas of the DNA, which might well be the key to some of the things which separate us from the apes such as language and higher brain function. These studies will help us to build up a picture of what it is that makes a chimpanzee a chimpanzee and a human a human, and how the differences may have evolved. |
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