Tree of life

img_3615It must have been exciting to be a scientifically minded man* in London in the 1800’s. The latent potential had been building for centuries; the way the world was perceived was shifting, seemingly disconnected scientific discoveries were stacking up, access to learning was changing. The conditions were right, the stage was set, the Scientific Revolution and Age of Enlightenment had paved the way. Modern science as we recognise it was born; laboratories were set up, scientific journals established, government funding for research became available. The word scientist replaced ‘natural philosopher’ and over the century, leaps in scientific understanding transformed the way people lived with innovation ranging from light bulbs to the periodic table, from batteries to vaccines to the railway. But the thing that fires my imagination more than anything else emerging from this melting pot of scientific thought, is Darwin and Wallace’s theory of natural selection.

I remember coming home from one biology lesson at school, having learned about diffusion; specifically that sugar can move into plant cells, helping the plant to grow. I went straight to the carrot tops I had been growing in a saucer on the kitchen window sill, and tipped a few teaspoons of sugar into the water. Next morning, I was horrified to find four shrivelled orange lumps instead of bushy, green topped carrot cuttings; not at all what I had been led to believe should happen by my biology teacher. It was not until that afternoon, my next biology lesson, that I learned about osmosis; the process that means water moves from a dilute solution to a more concentrated one. Instead of supplying my carrots with life-giving sugars, I had made such a concentrated solution on the outside of them, that I had actually sucked their water out, leaving those shrivelled orange lumps. But I was delighted; that eureka moment when the science explained something I had seen happen. I can only imagine how thrilling it must have been for those 19th Century scientists who read Darwin and Wallace’s theories and realised these men were drawing together numerous threads of scientific observation, coalescing them into a single unifying theory of biology. It was a long while before the theory was accepted by many, but for those who were immediately convinced, it must have been a memorable feeling; TH Huxley, (a contemporary biologist), on reading Darwin’s Origin of Species went as far as to say “How extremely stupid not to have thought of that.”

So many years on from its formulation, and I am still excited by evolutionary theory. It is so incredibly clever, and yet so beautifully simple. At its most basic, it boils down to two related points:

1. All living things evolved gradually, from a common ancestor.

2. Not all individuals in a species are identical, and these individuals are able to hand characteristics down to the next generation. Those individuals with characteristics that give them an advantage are more likely to survive and reproduce, so passing on the beneficial traits. Less well adapted individuals die before they reproduce, so are not represented in the next generation.

This is the basis of evolutionary biology today. Obviously, it very quickly becomes complicated; mechanisms and implications are all still hotly debated, but there is no doubt that Darwin’s book ‘Origin of Species’, which outlined the theory of evolution, changed the study of biology forever.

In addition to the trees I grew up playing round, and the trees I now have the fortune to look after, another kind of tree – the phylogentic tree – has featured prominently in my life. Drawn directly from the theory of evolution, these trees show the evolutionary history of a group of organisms. Starting at a common ancestor at the base of the tree, they show the branching pattern, tracing development over time as species emerge, honed into a particular design by the pressure to fit into the environment they find themselves in. It could be anything – dragons for example. Imagine a few dragons of a particular species are accidentally blown off course and find themselves on an island. Suddenly, individual dragons with longer tongues are at an advantage as they are able to take nectar from otherwise inaccessible flowers on the island – the short tongued dragons begin to die off because they can’t reach the energy-giving nectar and there isn’t much else to eat on the island. It becomes more likely that any dragon reaching reproductive age will have a long tongue. Over successive generations, the population becomes made up of the favoured long tongued dragons. If there are enough of these little changes in the population, the dragons that blew off to the island start to look very different from the group of dragons they left on the mainland, which have also changed to keep up with varying conditions there. (Islands and mainland don’t have to involved, it’s just a good way of picturing things.) Eventually, the two groups will no longer be the same species, both having diverged from the common ancestor. So a branch will divide on the dragon family tree – the tree will grow.

Phylogenetic trees are constructed from all sorts of data and using many different methods, also all hotly debated. But consensus is arrived at and general patterns emerge – and what a wealth of history is held in these trees, what a catalogue of survival, millions of years of varying characteristics and adaptation to change. I love that as we balance here on the tips of the branches, we look back down the boughs of time, at the fascinating swirl and shift of species in the past. It is a reminder that nothing is solid, nothing is permanent, species move forward, ever changing, ever evolving.

I have a terrible habit of forgetting that the natural world I see about me today is not a set of animals and plants that will always be here, and that have always been here. They are a dynamic, shifting force – and in the days of climate change and the sixth extinction I would do well not to forget it. It would be tragic if humankind caused of the loss, not only of the species around now, but of the millions of years of active evolution that has been necessary for these species to exist.  It is impossible to tell how long we will continue to have the privilege of sitting on the tip of this mammal branch we have come to rest on, but with privilege comes responsibility.  How much of this tree of life do we stand to lose over the next century?

Only time will tell…



* I write ‘man’ here on purpose. I imagine being a scientifically minded woman in London in the 1800’s would, in most cases, have been an infuriating, disempowering, destructive experience.