I can’t help but feel rather sorry for scientists whose work involves evolution, because there is surely no other scientific topic that has become so charged with emotion.§ Physicists might grumble about those who don’t share their interpretation of quantum mechanics, or colleagues who spend their careers working on theory that has no resemblance to the real universe – but they are unlikely to be dragged into debates with school boards and legal battles over the reality of their theories. The odd thing from the outsider’s viewpoint is how straightforward and obvious evolution is.
We only have to make two assumptions, which are nothing but common sense, and evolution is inevitable. The first is that we have the ability to pass on various characteristics to our offspring, who are not carbon copies of us, because a mix of those characteristics comes from both parents. The second is that organisms with characteristics that help them survive and thrive are more likely to have offspring to whom they can pass on those characteristics. Combine these and you’ve pretty much got evolution happening whether you like it or not. Darwin didn’t know how it worked – he didn’t know about the genetics we’ll meet later on in the book – but it’s hard to see how anything else could happen in the biological world.
Science tells us that evolution has led over the billions of years that there has been life on Earth to the proliferation of species. It’s quite common for people to say ‘I accept micro-evolution – that’s obviously going to happen. So, for example, if birds with bigger beaks are better at breaking up the nuts they eat, then over time more of the birds with bigger beaks will be more likely to breed and big beaks will dominate. But I don’t see how a mouse can turn into a chimpanzee, or a chimp into a human.’
There are two problems here. One is the failure to recognise that ‘species’ is an arbitrary concept. Every single organism is the same species as its parents.¶ Which seems to imply that you never will get a new species emerging. However, we’re not dealing with a single change, but rather an accumulation of tiny changes to genetic makeup that eventually result in an organism that is not the same species as its earlier ancestors. Exactly how we define a species is a little vague, but the traditional point at which a species divides off from another is when it’s no longer possible to interbreed and produce fertile offspring. Depending on the rapidity with which an organism reproduces, such a change could take millions of years or just decades.
A good parallel with the paradox of changing species over time despite an organism always being the same species as its parent is in the colours of the rainbow. We know that there are far more colours than Newton’s original, arbitrary seven. Zoom in to the detail of a rainbow and there are millions of subtly different colours – my computer can display a palette of over 16 million. Look at two adjacent colours and they will apparently be identical. (Try this in a paint program on your computer if you don’t believe it.) Every one of those 16 million-plus colours looks the same as its ‘parent’ colour next to it. Yet across the whole spectrum we go from red to orange to yellow to green and so on, with all the variations in between. This is what happens with species too.
The other problem with the argument ‘I don’t see how a mouse can turn into a chimpanzee, or a chimp into a human’ is that you don’t need to, because this does not happen. We are not descended from our cousins, the other great apes. Rather, go back far enough and you will reach a common ancestor of both us and our closest living relatives, the chimpanzees and bonobos. Go back further and you will reach a common ancestor that also takes in other great apes. Further still you will find a common ancestor we have with monkeys as well … and so on, eventually reaching a common ancestor with a mouse. And so on again. It’s like an upside-down, back-to-front version of a family tree.
To take in the whole of human genealogy we need to go back to the point in time our species evolved from its predecessor. You could, of course, carry on further and further into those common ancestors – but it’s hard enough getting our head around just our human family trees. So, we will sensibly make the break when Homo sapiens came into being, around 200,000 years ago. This rough date comes primarily from fossil evidence. We’ll come back to our more distant ancestors in Chapter 6.
UNCOVERING MITOCHONDRIAL EVE
So, how do we demonstrate everyone’s parallels with Dyer’s royal blood? We need a way to look into the distant past, discovering how far back we need to go before we see shared ancestors for large groups of people. One way to do this is to use DNA. We will come back to DNA in a lot more detail in Chapter 9, but for the moment the important thing is that the bulk of your DNA molecules, which make a very significant contribution biologically to what you are, come from both your parents. However, a small amount of your DNA – so-called mitochondrial DNA – comes only from your mother. This is the tiny relic of DNA still remaining in mitochondria, essential parts of your cells which developed from bacteria. Mitochondria are often called the power units of our cells, because they are responsible for producing the molecules that store tiny amounts of energy to be transported around the body.
As the distant ancestors of mitochondria were independent entities, they had their own DNA, distinct from the main DNA of the cells they reside in. Like our chromosomes, each of which comprises a single long molecule of DNA, the DNA in mitochondria contains genes. Over the many millions of years that mitochondria have been in action in humans (and almost all other organisms with complex cells), the genes from the mitochondria have largely been transferred out to our chromosomes. In the case of humans, just 37 genes have been left. But this tiny fragment of DNA is special as it is inherited only from our mothers.
By combining data on variants in the mitochondrial DNA of a range of individuals with the rate at which this DNA was assumed to have mutated,|| it was possible to work backwards to deduce when the most recent common ancestor of all current humans was alive. Someone from whom every living person is descended. This so-called ‘mitochondrial Eve’ is thought to have lived 150,000 years ago, give or take a few ten thousand years. It should be stressed that mitochondrial Eve was neither the only woman living at the time nor the first woman – this process merely identifies a likely distance in time to a female individual who according to mitochondrial DNA was the most recent woman to be in the family tree of everyone now alive.
SEARCHING FOR COMMON ANCESTORS
Finding a timescale for mitochondrial Eve was an interesting exercise, but it doesn’t give a good picture of ‘what makes you you’, both because it was based on crude assumptions (some of which are outlined below) and because it only tracked through the female line. To get a better picture of your ancestry, we need both male and female lines – and some impressive statistics. Back in the late 1990s, Joseph T. Chang, Professor of Statistics and Data Science at Yale University, wrote a paper entitled ‘Recent Common Ancestors of All Present-Day Individuals’. In it, Chang took a trip back in time using a mathematical model that gives a fascinating picture of our heritage as individuals.
Initially, Chang kept some of the extreme simplification of the earlier exercise. The mitochondrial Eve calculation had assumed each generation dies off producing the new one – there is no overlap between generations – and that each individual only has a single parent. It also assumed that an individual’s parent is randomly selected from the entire population of the previous generation, clearly a huge over-simplification. (Just think how your parents met – would they have had an equal chance of getting together with every other person of the opposite sex of any age alive at that time? I doubt it.) Chang still simplified reality, but made significant improvements.