It is often referred to as the ‘great chain of being’, which is the very old idea – much older than evolution – that there is a scale of complexity. On this depiction of the great chain of being you can see that plants are higher than inorganic things, animals are higher than plants, humans are better than animals, angels are above humans and so on. You might say, ‘Oh, we don’t believe in that any more.’ Yet, if you pick up any evolution textbook or even a popular science evolution book, you will often find something that looks very similar to this.
Many biology books start with single cells; then go on to simple multicellular algae; then plants; then animals and fungi. Animals with backbones are presumed more advanced than those without backbones; and furry animals with backbones rate higher than those with scales. The last chapter in the book will usually be on humans as if we were the pinnacle of evolution.
You might say, ‘Oh, well, this is just a way to present things; it doesn’t really matter,’ but it does actually influence our thinking. For example, there is an ad on ABC Radio at the moment for a program on hearing, on which there is a sound bite from an expert who says, ‘Insects have hairs on their body that detect air movement and this evolved into hearing in humans.’ We know that humans did not evolve from insects and yet that is the kind of error of thinking we find ourselves making, when we arrange this orthogenetic series. So this is one kind of objection to presenting the beginning-to-end model of evolution.
But there is a more practical reason: beginning-to-end is not how evolutionary biologists usually work. Most evolutionary biologists like me start with what we have here today, the evidence we have in the present, and we use that to work backwards: to go back through the history of life, to reconstruct the past and the processes that we cannot directly witness.