Monday, June 25, 2007

A paring of Adam's nail

Darwin convinced us that all life on Earth is related by common descent. It was a stunning overturning of the convention wisdom of simultaneous creation.

The task then was to discern the familial relationships of all species of life on Earth, including the many extinct organisms known through the fossil record. Biologists did this on the basis of morphological similarities. A zebra is presumably more closely related to a horse than to a elephant, and so on. By the middle of the last century a family tree of life on Earth had been established to almost everyone's satisfaction.

Then came the most astonishing surprise. In every cell of our bodies we carry a history of our species' past, written in the four-letter genomic code. Forget looking at the morphology of zebras, horses and elephants. Send their DNA unlabeled to the sequencing lab and the genes will tell which creatures are more closely related, and (with a bit of calibrating against the fossil record) how long ago any two species shared a common ancestor.

This is now being done with ever increasing rapidity, and -- voila! -- the tree of life revealed by the DNA is identical to a satisfying extent with the tree established by the generations of zoologists and botanists who followed Darwin. I share almost all of my genes with chimps, and some genes with bacteria. A fleck of my spittle contains the four-billion-year history of Homo sapiens.

This sort of thing sends shivers up my spine, and renews my spirit of relationship with the birds at the window feeder and the peas in my garden. I swat a fly and I'm smushing some of my own genes, genes for making haemoglobin, say -- or near enough to my own to indicate a common ancestor some hundreds of millions of years ago. Our cells sing the unity of creaturedom.