What Do You Think You Are The Science of What Makes You You - Brian Clegg Page 0,6
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