For me as a biologist, evolutionary and developmental biology – evo devo for short – is one of the most wonderful, illuminating, useful areas of study. In the last few decades we have gone from guessing at how things might have evolved, to having some actual mechanisms of how organs, and even whole organisms, can change. As a Christian, I am interested in this subject first of all because it’s fascinating. Having been freed by Biblical Scholars from feeling that I need to read the Bible as a science book, I can now go and explore God’s world using the tools of science, thanking him for all the incredible things I find. Secondly, this knowledge is incredibly useful. The more we learn about how our bodies develop and grow, the more we can treat disease.
Sean Carroll was heavily involved in the evo devo revolution, and his 2005 book Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom, is still one of the iconic popular texts about the subject. To give you a flavour of this bestseller, I’ll summarise some of the introduction.
“Simply put, development is the process that transforms an egg into a growing embryo and eventually into an adult form. The evolution of form occurs through changes in development. Both processes are breathtaking”, writes Carroll. Darwin invited his readers to think about how small changes in development – perhaps the length of a forelimb or beak – that affect a particular type of animal at different stages in its life, and in different parts of the body, could accumulate over countless generations to cause massive differences in form and function.
We accept the process of development from egg to adult without blinking an eye. We might marvel at the process, but it happens right in front of our eyes so we accept it as one of the facts of nature. For Carrol, “Evolution is as natural as development.” Darwin’s friend Thomas Huxley thought that a bit of imagination was all it took to realise that changes in development, building up over the aeons, are what gives life such diversity.
But for more than a hundred years after Darwin published The Origin of Species, we had very little idea about how embryonic development might happen, let alone the modifications that could turn a leg into a fin, or vice-versa. The study of embryos, heredity and evolution became three separate fields of biology. Scientists made great discoveries in genetics and palaeontology, and came up with the ‘modern synthesis’ of evolution: the idea that an accumulation of small genetic changes can drive large changes in form and function – but we were no clearer on the question of how exactly an embryo develops or a species changes over time.
In the 1970’s, some scientists began to call for bringing embryology and evolutionary biology back together. The palaeontologist Stephen Jay Gould started writing about how changes in developmental processes in an embryo could affect evolutionary processes in a species, and pointing out the gaps on our understanding of evolution. It would, however, take a revolution in embryology to move things forward.
The revolution that eventually happened centred around the humble fruit fly. In the 1980’s a group of incredibly resourceful geneticists discovered the set of genes that control development – master-switches if you like. They then realised that these genes were not just there in flies, but in humans, mice, and in fact other type of animal studied so far. So we now know that eyes develop because the gene that regulates eye development tells them to. It doesn’t matter if they eyes in question are an insect’s compound eyes or a human’s camera eyes – eye development in general is controlled by a gene called Pax6. The way in which these genes are organised, which I have described in another post, is wonderful in its own right. But the fact that they are shared across the animal kingdom was, writes Carrol, “a bombshell”.
At last, biologists were able to compare these developmental genes between different species. If you studied a regulator of limb development in a mouse, that would give you some big clues about limb development in humans. We could get a much clearer idea of how animals were related to each other, and how they might have evolved. For example cave fish have no eyes, but they are just like normal fish – only with a broken Pax6 gene.
Now that the DNA of whole species has been decoded, we know that it’s not differences in our DNA that makes us different – it’s when genes are switched on and off during development. So rather than a blueprint, DNA is more like a set of switches, or the sequence that must be planned out for every major firework display. Each component needs to be set off at the right time, and any change in the sequence will produce a visible difference. In fact, with a large enough set of fireworks, you could produce a number of quite different displays – as different as a human is to a mouse, for instance.
So evo devo has unravelled many of the mysteries of evolutionary biology. It has not by any means solved all the problems – which is good, because it means there is so much more to explore. But when I learned as a student about some of these mechanisms underlying development, and the ways in which modifications in those mechanisms have caused whole new species to evolve, I was filled with a sense of wonder that has never left me. There truly is, to quote Darwin, “grandeur in this view of life…that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved”.