Life Lines 66

THE LIFE CYCLE IS LARGELY UNSEEN

All living things, from viruses to humans have a life cycle. This simply means that the individual of a previous generation produces another individual and that production involves events biologists can describe as the life cycle. We rarely see the fertilization of our own children, although we might imagine it when we are in a procreative stage of our life cycle. My wife Nedra has seen that event thousands of time as an in vitro fertilization embryologist and she used the motto “happiness is two pronuclei” a phrase that would be meaningless to anyone not in the field. That is the moment when a sperm has entered an egg, swelled up its nucleus, and has begun a movement toward a migrating egg nucleus coming toward it. Their union produces the fertilized egg with the potential to form a future human being. That potential resides in the thousands of genes that are in the united nucleus containing the chromosomes of the sperm and the egg. Some nine months later that one cell will be several trillion cells in the form of a crying baby.

Genes begin to turn on at fertilization and change the chemistry of the cell. The process is gradual and largely done by releasing chemicals that diffuse in gradients. They set up left-right, head-tail, and back-belly regions. They segment the embryo. They lead to limb buds and other organs, internally and externally. Each of those events takes dozens or hundreds of genes to set up. What is remarkable is how many of them are being identified by molecular biologists, whose curiosity led to their working out the events for each of these unfolding stages that were once considered a mystery beyond human understanding. I do not doubt some author not yet born will write a popular book at the end of this 21st century describing that journey from one cell to a newborn baby, revealing all those molecular events, like a detailed description of how to knit a sweater or a conductor reading a score of a new symphony. Genes not only make proteins that act as enzymes or serve as building blocks of membranes and other structures in the cell, they make genes that can switch on these signals in the developing embryo to form more complex structures. The genes can make sheets of cells, furrowing here and there, folding in layers, or ballooning out into projections. Mutating these genes leads to abnormal embryos; many of them are self-aborting because vital organs cannot form. Nature can be cruel in its imperfections because those not normal, if not aborted by their errors, may be born with birth defects. We tolerate a lot of rejects in our fumbling efforts to make things, but act surprised that at the molecular level similar errors abound.

Evolution has a similar step-by-step way of building complexity, but it is extended over a huge expanse of time. At the level of our genes and chromosomes that past history is also being revealed, by much the same molecular technology to tell us how it happened. And just as there will be a book of becoming from one cell to an adult, so will there be a book of becoming from our early ancestors (bacteria-like) to the present diversity of organisms that we can actually see with our eyes, from worms on our lawns to relatives on our hammocks. To me that is not a thumb in the eye for those who believe everything we don’t know is a miracle. Rather, I find it exhilarating to see how the universe works, to understand my place in it, and how I came into being. It connects me to all of life and makes me more respectful of those different from me.