Monday, December 03, 2007

The fire that burns in every cell


Still clearing out my retirement office at the college. Behind a bookcase I find folded up a huge poster that many years ago hung on the wall of my original office: Biochemical Pathways, by Gerhard Michal, 1974, published by Boehringer Mannheim. What you see here is a random 8-inch square of a 5x4 foot diagram. Click to enlarge.

Folded out, it looks for all the world like the blueprint for a vast petrochemical plant -- acres and acres of pipes, valves, reactors, storage tanks, etc. -- turning out hundreds of petrochemical products. And in a sense, I suppose it is. Except this petrochemical plant is contained with a single cell. Well, not necessarily all of it. These various chemical reactions are distributed among animals, plants and microorganisms, some in common, some uniquely.

"What in the world is that?" asked a friend, when I had folded out the poster on the floor of a corridor. "Life," I replied.

More than half-a-century ago, the great Austrian physicist Erwin Schrodinger tried a definition of life in a little book called What Is Life? He was convinced that life would eventually be accounted for by physics and chemistry, and his book helped inspire the biomolecular revolution, of which the poster is a momentary snapshot. The best he could come up with was "an elaborate, coherent, meaningful design traced by the great master."

Well, here it is, the great design. We will have different ideas about who or what is "the great master."

What we know for sure is that life has existed on Earth for nearly 4 billion years, and that all life on Earth (so far described) is related by common descent. As for what got the whole thing going we have only speculations.

Let us assume an ancestral living cell, as simple as the simplest bacterium existing today -- an unnucleated blob of protoplasm enclosed by a membrane. Microscopically small. Autopoietic: that is, capable of maintaining itself by chemical interaction with the environment.

For billions of years, microbes competed for the opportunity to reproduce. Far more failed than succeeded. Most branches on the tree of life were nipped in the bud. A few lucky lineages eased into the future, avoiding the sweeping scythe of death, like the few stalks of grain that remain standing in a harvested field.

In biology textbooks, timelines of the first 3 billion years of life on Earth are mostly blank. Photosynthesis. Respiration. Nucleated cells. Sexual reproduction. It would appear that not much happened. But in fact everything was happening; life was perfecting the complex chemistry that sustains every living creature on Earth today, the reactions we see on the poster.

By the time the first multi-celled organisms appeared about 700 million years ago -- and the timeline of Earth history becomes crowded and familiar -- most of the real work of evolution is finished. The basic chemical machinery of autopoiesis and reproduction is in place. Everything that follows -- apple trees, great horned owls, great blue whales -- will be variations on a theme.

The Biochemical Pathways poster gives us a glimpse of those first 3 billion years, those delicate lineages fingering into the future, inching forward under the great overarching shadow of death, always bearing the residue of the past, teasing self-maintenance from the environment, transforming the Earth's crust, atmosphere and oceans, competing, occasionally turning exploitation into mutual advantage, perfecting metabolic pathways of astonishing complexity.

What is life? Here it is, folded out on the floor of a college corridor.