First of all - Happy New year! I still haven't managed my goal of more frequent posts but I will keep trying. This post is about one of the (many) areas of biology I find fascinating.
I could list hundreds of reasons why I chose to study biology because it really is a rich and diverse field which is thoroughly fascinating in many ways. If I had to specify an overarching theme of biology to explain why it sustains my interest, then I would perhaps say the astounding complexity and powerful beauty that underlies biological processes and systems.
Whilst I was considering this recently one thing struck me very strongly, and that is how some very surprising and often highly complicated biological processes are taken for granted by all people; whilst some others which are based on the same underlying biological themes can be challenged and sometimes derided as fanciful.
To give a good example of the kind of thing I mean I will chose a process familiar to everyone – because it is a key moment in how we all came to be here in the first place. I am referring to fertilization; when a sperm entered an egg and produced a single-celled zygote (the initial product of fertilization prior to development into an embryo). Immediately after fertilization a human zygote is roughly around 150 micrometres in diameter (one micrometer is one millionth of a meter); please take a moment to consider the kind of size we are dealing with. However, the key point I want to press home is that the zygote forms the first cell of a new human body. The zygote has one cell only, but importantly that cell contains the full complement of DNA that will go on to produce the embryo and finally a new baby.
It is not my intention to provide a technical account of development despite the fact it is thoroughly absorbing and well worth reading into (although, my level of knowledge would only allow me to give detail to a certain point in any case). What I primarily want to get across is the almost unbelievably complex and ordered process which takes place to turn one tiny cell into a human body with at least 10 trillion cells (estimates vary of the total number of human cells). Please dwell on that difference – one single cell turning into trillions of cells. Whilst a human body is developing the various tissues, blood vessels, nerves, bones, organs and limbs develop in the correct order, in the right place and at the right time of development relative to each other.
Every cell in your body derived from that initial and tiny zygote. That single cell contained the instructions for making every part of a human body. Again, in an almost mind-boggling feat of complexity the relevant instructions were only turned in each appropriate part of the body. That’s why your eyes became eyes and not fingernails – even though the cells in your eyes contain the instructions for making fingernails as well as the instructions for making a heart, a strand of hair, a toe or any part of the body you could name.
I don’t know about you but I find this truly wonderful and amazing and it drives me to learn more about how this transformation takes place. I have been focusing on humans but let’s not forget that the equivalent happens for all other animals, plants or fungi - and for all multicellular life with specialisation of form (as an aside, a great deal can be said about the complexity of bacteria and other unicellular life – but that is off the point here).
Think for a moment how readily this feat of transformation is accepted by us all. We all accept that a human develops in this way. Again, we have no issue with the fact that a tiny seed can develop into a Giant Redwood tree (Sequoia) over 100 metres tall – again note the difference in size and form between the seed and the tree. We are equally familiar with the fact that a caterpillar will pupate and change form quite remarkably into a butterfly or moth that takes to the air; again consider the difference in form between the stages. To me, these facts are all astounding and yet I have barely scratched the surface of biological wonders.
Having made plain the quite astonishing developmental processes which we often take for granted, I am left to wonder why many people will argue that similar transformations of an evolutionary kind are not possible. I have heard many people tell me how absurd it is to suggest that life as we know it arose from simple unicellular organisms. I don’t wish to claim that ontogeny (development) is exactly the same process as evolution because that would be slightly misleading. What I do wish to suggest is that we should be no more incredulous about unicellular life evolving into complex multicellular life over 3,500 million years, than we are at a single fertilized egg becoming a fully formed human in around 9 months. They both involve a complex, step-wise and highly ordered biological process of change over periods of time. Furthermore, understanding development sheds light on evolution.
Recent progress in evolutionary-developmental biology (or Evo Devo) has cast light on how changes in development drive large evolutionary change. It can be shown how changes in the timing of gene activation, or even in the location of where genes are turned on, can produce quite striking differences of form. One interesting example I recently learnt of during lectures is how the webbing in ducks feet are formed. During development bird feet are webbed but the expression (turning on) of certain genes causes the webbing between the toes to die; the toes themselves do not die off because another gene called ‘Gremlin’ is expressed on the toes - this prevents the toe cells from being killed. In Ducks, Gremlin is also turned on in the webbing and therefore it does not die and they are born with aquatic webbed feet. If chicken embryos are manipulated so that Gremlin is expressed in the webbing (before it is killed off) then they too are born with webbed feet. This is quite a complex process and difficult to sum up easily in a few sentences, the take-home message is that by changing where a gene is expressed can change the form of feet quite dramatically – a simple genetic change produces an evolutionary step. We may think ducks and chickens have very different feet but the underlying genetic changes are very small.
If you are intrigued by this idea of ‘Evo Devo’ then I would recommend reading “Endless forms most beautiful” by Sean B Carroll for some greater clarity and insight than I have just provided.
I was inspired to write about this because both evolutionary change and developmental change stir up a sense of awe inside me, and that is a primary reason I have chosen to study biology. I hope that I have been able to give a glimpse into the remarkable complexity and beauty that biology offers and that you may decide to find out a bit more about it. I can only touch upon things in a blog but rest assured there are many more examples I could give – and more interestingly there are many more waiting to be discovered!
Many thanks for reading,
Dominic
