9. Mirror, mirror

Take a look in your mirror again. Try to forget that what you see is ‘you’, a human being. Just see an animal looking back from the mirror. An animal that isn’t particularly different from an ape in appearance, even though your brain and the capabilities it gives you sets you apart. There was much talk in the early days of evolutionary theory of humans having descended (or ascended, depending on your point of view) from the apes – but that’s a misleading picture.

Building your ancestor tower

To get a true picture of the evolution that produced your body in action we need to look back over your ancestry, all the way back to the earliest life on Earth that’s related to you. Trying to imagine how you can get to a human being from something as simple as a bacterium, say, can be hard to visualise. Apart from basics like cell structure, containing water and DNA, it’s difficult to see that you have a lot in common. But this is your heritage. A great way to picture how you got to that image in your mirror from these early life forms is to imagine building your ancestor tower.

We’re going to represent you with a piece of Lego – specifically, a violet piece of Lego. You are on the top of a Lego tower. Below you is another violet piece of Lego. This is one of your parents (it doesn’t matter which one). One of their parents is the next block down, and so on. Let’s imagine we’ve constructed the whole tower, many kilometres high, that gives us a Lego block for each living thing all the way back to your earliest ancestor, the first life form in your ancestry.

How that first life form came into being is a different story, one we don’t know the answer to. But let’s stand well back and take a look at the tower you have built. It has a couple of cunning design features. The obvious one is the colour of the blocks. We have coloured them so that they show a rainbow. They run in colour from red down at the earliest days of your ancestry through to violet with you at the top. It’s a complete rainbow.

How many colours in the rainbow?

When you look at a rainbow caused by raindrops when the Sun’s out, it looks as if there are some distinct colours there. You can see a red block, an orange block and so on. But those divisions are totally arbitrary. The seven colours of the rainbow we talk about today were made up by Isaac Newton. Few people actually see seven colours in a rainbow, but Newton wanted there to be seven, probably to correspond to the seven notes in a musical scale. Even the divisions you can see are as a result of your brain fooling you, looking as usual for patterns.

In reality, the rainbow forms a continuity of colours, merging without any sudden change from red to orange, orange to yellow, yellow to green and so on. If you go down to the differences between the colours, based on wavelength of light, or energy of the photons involved, there are billions upon billions of colours. And that’s what we have in the ancestry tower that culminates in your body. A true rainbow of colour.

No sudden changes

Pick any two adjacent blocks in the tower and to all intents and purposes they are the same colour, whichever blocks you choose. You will never see two adjacent blocks where, say, one is blue and the other is green. You will never see a transition from one colour to a different one. Yet over its height, the tower manages to shift in colour from red through to violet via all the other colours. There are, of course, very subtle differences between each brick, but they are far too small for your eyes to detect.

Similarly, with the creatures those blocks represent, each generation is, to all intents and purposes, exactly the same kind of animal as the previous one – you will never see a transition between one species and another. Each individual is the same species as its parents. Although your body is different from your same-sex parent, the differences are largely cosmetic. You are the same species as your parents.

Look further back and there is no sudden break between human and prehuman, or, going further still, between a dinosaur/lizard-like creature and a mammal. Each time, the offspring is the same species as the parent, yet paradoxically we manage to get a shift from single-celled simplicity through to the ancestors of plants, fishes, dinosaurs, mammals and our fellow apes.

This is why the Victorian concept of a ‘missing link’ is so misleading. It suggests more of a change between generations than has ever happened. The term ‘missing link’ is, in fact, totally out of date. It refers to the idea that nature is composed of a great chain from the simplest forms of life (like bacteria) to the most complex (humans) and everything that has lived can be put into this structure – except that there are some missing links in the chain. The trouble with this picture is that it isn’t possible to decide a sensible order for the chain. Is a humming­bird higher up the chain than a mouse? Is an earthworm higher or lower than a ragworm? It makes no sense.

A failure to link up

There was another subtlety in our tower design. Ordinary Lego has exactly the same pimples and holes on every block, so any block can clip into any other. On our ancestral blocks, the shape, size and number of pimples gradually changes with position in the tower. The differences between the block that represents your body and your parents will be indistinguishable. And we should be able to link a block with one many generations earlier. But if you run down the tower, eventually you will get to a block where your modern block will simply not link any more.

This is where the species boundary is as far as you are concerned. The ancestor block that yours couldn’t click into, who we’ll call Fred (but could be either sex), is a different species from you. You are incompatible. Biologically you would be unable to breed with Fred.

The really important, but rather puzzling thing about this is that we can’t label Fred as the point at which a new species began. Hundreds of blocks either side of Fred are the same species as Fred; they can interbreed. It’s just that Fred is a different species from you. What this demonstrates is that the whole idea of ‘species’ is a totally artificial one, devised by biologists before they understood evolution. The idea is useful as a marker, but it has to be seen as a relative one, not an absolute.

The babel of towers

The ancestor tower that produced your body doesn’t stand alone. Every living thing has its own tower of ancestors. Some towers will be very similar to your own. A chimpanzee will have a tower that is identical until it gets to the point, near the top, that the human tower and the chimpanzee tower diverged.

That point where we had our last common ancestor with chimps really is closer than you might think. Your ancestor tower goes back over three billion years, but we and the chimpanzees split between seven and twenty million years ago. That means that only around 0.3 per cent of the Lego bricks in your tower differ from the chimp’s. This doesn’t mean human beings are in some sense descended from chimpanzees, or from any other existing ape. We both descended from a common ancestor that was neither chimp nor human.

To take another point of divergence, our common ancestor with mice lived around 75 million years ago. Go back down your ancestor tower to this point and you will find a small mammal that probably looks more like a mouse than a monkey, but it is neither. This time span, 75 million years, doesn’t seem very long when you think that life has been around for around three billion years. It might not seem long enough to get from a mouse-like creature to you. But bear in mind the average generation time over those 75 million years might be around five years or less, which would mean at least fifteen million generations in which small changes could accumulate to make something very different.

There will be some ancestor towers that never make it to the present day – many of them, in fact. Think of the dinosaurs, for example. Each has just as rich an early part of the tower as you do, but stops short around 65 million years ago. (Note, by the way, that this was a similar time span to our divergence from the common ancestor we share with mice.) Other towers stop billions of years ago. None of these truncated towers represents a now-living creature.

Equally it is possible that there are ancestor towers that did not start from the same first block as our own. We don’t know how life began on Earth, but if it happened once, it could have happened more than once, independently, in different locations. It seems likely, though, that every living thing yet discovered – animal or plant – does originate from the same first block. This is because every living thing we know so far has significant aspects in common. We are yet to discover a totally unique form of life that doesn’t make use of a carbon-based structure and a DNA (and/or its related chemical RNA) mechanism as a control structure.

Proud to be ‘just a theory’

The mechanism for moving up the tower is evolution. The body you see in the mirror is the product of a long evolutionary process. There is a lot of nonsense talked about evolution. It is sometimes attacked as being ‘just a theory’. This is a fundamental misunderstanding of the nature of science – all of science is composed of ‘just theories.’

If we take a fundamental bit of science, like Newton’s laws of motion, we have a very simple set of rules that go something like this:

1. A body will remain at the same velocity (including stopped) unless acted on by a force.
2. The amount of force applied to a body is equal to the body’s mass times the acceleration in the direction of the force.
3. Every action has an equal and opposite reaction.

Surely these rules aren’t ‘just a theory’? Well, yes, they are. The way science works is that a scientist or team puts together a hypothesis, which might be something like the laws above. They then test it against experiment: ‘Do we find indeed this happening? Yes we do, so that strengthens the hypothesis.’ The more evidence we have for it being true, the more likely it is that this is a useful theory. Once something goes from being just a hypothesis to being a theory, it has stood up well and is usable. But it could still be disproved somewhere down the line.

Newton gets it wrong

This has actually happened with Newton’s second law from that list. Einstein’s special relativity shows that if something is moving, the relationship between force and acceleration is more complex than Newton thought. And special relativity has never let us down yet; it’s a better theory than Newton’s. However, as it happens, the way Newton’s theory fails doesn’t make a noticeable difference most of the time. So we can still happily use Newton’s simpler form in most circumstances.

Any theory can be disproved – it just requires some new evidence. And that includes theories that are given the misleading label ‘laws’. No scientific theory can be absolutely proved, because a new piece of evidence could always come along that shows our assumptions to be wrong. But this doesn’t mean science is no better than totally made-up ideas like magic. Science gives us the best picture, given our current information – it’s just that it must always be a work in progress.

Evolution is a theory in the same sense that Newton’s laws are. It could at some point be disproved, and our current best understanding is more complex than Darwin’s original picture. Nonetheless it is at the moment by far our best theory, given the available evidence. In a way this is not surprising because it is such an obvious theory – in fact it’s amazing that it wasn’t discovered long before Darwin.

Evolving makes a lot of sense

The basis of evolution is very simple. You inherited various traits from your parents, making your body the way it is, as they did from their parents, and so on back down the ancestral tower. In Darwin’s day they didn’t know how this happened, but we now know it’s down to genetics (and epigenetics). Some traits are likely to help a particular species to survive in its current environment. Others are likely to make it more difficult for the species to survive. Individuals that have the traits that help them survive will be more likely to live long enough to reproduce. And so those traits are more likely to be passed on.

Over a long, long time, these gradual changes, coming from a combination of the different mixes of DNA as different individuals breed and random changes occurring in DNA as a result of mutation, will inevitably lead to changes in the species. This is really all that evolution is about – the way different generations are randomly different from previous generations, combined with the survival pressures of the environment.

Many of those who are unhappy with evolution and would prefer that living things were designed by an external force point out that this kind of change will only result in gradual shifts within a species. It surely couldn’t result in, say, a fish-like creature evolving into a human being. People who have this problem need to go and play with an ancestor tower – as already mentioned, there are no jumps from species to species. Each and every generation is the same species as its predecessor. That is the wonderful paradox of biology, brought about by this arbitrary ‘species’ label. There don’t need to be any species-to-species leaps.

What use is half an eye?

Another problem those who are unhappy with evolution have is that, bearing in mind that change happens very slowly, what would be the advantage of a partly formed change that didn’t deliver any benefits? This is an issue that plagued Darwin for some time. When you look at your body in the mirror, it has many complex structures. How, for example, could something as complex as an eye come into being? How could you get from primitive creatures with no sight to something with a fully-formed eye?

This does not prove as much of a problem as it first appears. It might be that there is an intermediate stage that has a different benefit – for all we know, creatures with half-formed eyes might have looked more attractive to potential mates. But in fact, with the eye we know that there are straightforward benefits, because there are creatures out there right now with pretty well every intermediate stage between nothing at all and a complex eye. Some have light-sensitive patches on the skin; others have pinhole camera eyes – no lens, just a cavity with a retina; some have very crude optics; others have different variants on seeing, like an insect’s compound eye, and so on.

Another example of a capability that seems to have little value if half-formed is having wings – either you can fly or you can’t. But again, the reality is more subtle. With small wings, for instance, you might not be able to fly, but you can use them to get along a little more quickly when escaping a predator. And you may have some alternative use for them, like cooling yourself. It’s entirely possible for a part-formed feature to have a different use that later gets discarded.

Part of the problem those who don’t like evolution have with these complex structures is that, try though they might, creationists and others can’t get the idea into their head that evolution is not being directed. So in asking ‘Why would you have a partially formed wing?’ there’s an implicit assumption that evolution has a purpose, that it is working towards a wing. But evolution isn’t like that – it is truly random, merely selecting along the way for things that are useful (or at least not a hindrance). Without the idea of some guiding principle being involved, there is less concern about these part-formed features.

Science can always be proved wrong

The problem with creationism and intelligent design, the alternative viewpoints usually put up against evolution to explain why your body is here – along with all the other facets of nature – is that they aren’t science. Remember that the way science works is to test a theory against the evidence. But those who believe in an external designer say that there is no testable evidence for the existence of the designer – it is something that has to be taken on faith.

Most scientists will tell you that for a theory to be science, it has to be ‘falsifiable’. That means that there has to be a mechanism for proving that the theory isn’t true. One of the early scientific theories was that everything with weight tried to get to the centre of the universe, which was thought to be the centre of the Earth. It was wrong, but it was science. As more and more data became available from observing the Solar System and the universe around us it became clear that the Earth wasn’t at the centre of everything. The theory was disproved. Similarly, evolution, quantum theory and relativity could all be disproved by the appropriate observations.

I’m not saying that scientists abandon their theories with good grace. Many cling on to them for a long time, until an overwhelming weight of evidence to the contrary forces them to admit their mistake. But a belief in a supernatural designer isn’t like this – it can’t be disproved. You can show that it isn’t necessary, but you can’t show it isn’t true. I have to emphasise that just because it can’t be disproved doesn’t make it false, but it stops it from being part of the realm of science. Intelligent design and creationism aren’t science and should not be taught as such.

Even some scientific theories suffer from this problem. Hundreds of scientists have dedicated their working life to string theory, a theory designed to explain the structure of all the different particles that make up the universe. But as yet no one has come up with a way of testing the theory (or any particular variant of it) and proving it false. Some argue that this means that as yet string theory is also not science. It is mathematics that may or may not have a link to the real world, but without that ability to test it and potentially disprove it, string theory must remain a second-class citizen as far as science goes.

The sense of wonder

With the paradoxical simplicity of evolution and its magical capability to change organisms from one species to another without ever seeing such a change from generation to generation, we have come to the end of our exploration of science using your body as our laboratory.

I hope that you will never again look in a mirror and just think ‘I really need a bit more exercise.’ Take a moment every time you see that remarkable structure to enjoy a sense of wonder. There’s all of science coming together to make what you see work.

Your body is a window on the universe.