I recently turned 44. As with all of one's birthdays, a milestone like this is a chance to gaze backward and assess, as well as an opportunity to look to the future and imagine possibilities. As I do this, though, I'm cognizant of a friend who is being celebrated for turning 60 this year. She's been in my life since before I was born, and she will be around long after I've faded into distant memory. But, in the way that humans do when we describe a feature in the natural world, we've assigned to her a birthday based on when we first recognized how special she was. I'm talking, of course, about deoxyribonucleic acid -- DNA, our blueprint, the hardware/software combination that keeps us on the straight and narrow, controlling our development as we grow from fertilized egg to adult, as well as our biological evolution as a species.
It's now become commonplace to think of something being "written in our DNA," as if it was a simply a hard drive onto which all of our key biological traits were inscribed. In this sector of the disk we have hair color and the ability to roll one's tongue, while in another we have our risk of heart disease and the likelihood of developing myopia. These traits are shuffled and dealt to our offspring, and they are thus passed down through the generations in an endless digital daisy chain. But is that really how this insanely long molecule (six feet of it packed into nearly every one of our microscopic cells) works?
The genome, it turns out, is far more complicated than we first imagined when Watson and Crick described the double helix in 1953. While their discovery (working with colleagues Rosalind Franklin and Maurice Wilkins) of the structure of the molecule immediately suggested a way in which it could be copied -- each complimentary strand serving as a template for synthesizing its partner -- the way in which the simple four-nucleotide code was turned into a fruit fly wing or a human nose remained a mystery. In the 60 years since those heady early days we have come a long way toward understanding the way the language of DNA is translated into action by the cellular machinery, but we still have a long way to go.
What have we learned recently? The announcement of the draft sequence of the human genome in 2000 (and its completion in 2003) marked a significant milestone. For the first time we had a roadmap of our genetic material, even if we didn't understand what most of the key geographic features were and why they were there. The next step came with the first systematic description of human genetic variation in the HapMap project. It is variation in our DNA that underlies many of the biological traits that distinguish us. The massive increase in the power of DNA sequencing over the past several years has yielded thousands of new human genomes for researchers to pore over. They have revealed a wealth of new information about the genetic basis of complex human traits, but the glut of data has also shown how truly complex these traits are. Among other fascinating recent results, it seems that there are more rare genetic variants in the human population than there should be -- likely a consequence of the huge explosion in human population size over the past several thousand years. These rare, poorly understood variants may play a role in our predisposition to diseases like diabetes and hypertension, underlining the fact that there is still much remaining to be discovered even a decade after "completing" the human genome.
It is perhaps fitting that as we apply the latest technology to learn more about our genetic future, we are also gaining ever more insights into our past. DNA is, after all, a historical document -- through our inherited patterns of variation, our genome contains a record of who our ancestors were and where they lived. It contains unexpected stories -- tales of trysts with Neanderthal cousins in Europe and Asia tens of thousands of years ago, for instance -- as well as more mundane details about how you are related to your great-grandparents. More than anything else, though, it has revealed how closely connected we all are to each other. Our species originated in Africa around 200,000 years ago, and it is only in the past 60,000 years or so that we have spread from this continent to populate the world. The superficial characteristics that distinguish people around the globe have arisen quite recently -- only in the past 2,000 human generations or so. When you look at our DNA, we truly are a human family. If it can teach us this simple truth, DNA really has done something amazing.