A fleeting shadow on a canyon wall

By Jeff Mitton

BOULDER, Colo. — I had hiked to “The Wave” in the Paria Canyon-Vermilion Cliffs Wilderness, straddling the border shared by Utah and Arizona, and now the sun was getting low in the sky.

I was taking photographs when I noticed my shadow on the entrance to The Wave. The contrast of a fleeting shadow on the 190-million-year-old Navajo sandstone triggered thoughts of time and persistence that recurred and wandered.

The shadow of a moving human lasts seconds or minutes and disappears when clouds obscure the sun. In comparison, a sandstone cliff seems permanent.

But all things are relative, and the permanence of a sandstone wall is an illusion created by comparison with my shadow. The layered sandstones of the Southwest, including Entrada, Navajo, Kayenta, Wingate and Moenkopi sandstones, were created over millions of years, but wind and water are now eroding them — they are not permanent.

How long does a human live? A long time or a brief moment, depending on your perspective. Average life expectancy is about 75 years, and some lucky and healthy individuals live to celebrate 100 birthdays. That seems like a long time. But Homo sapiens have been around for a minimum of 200,000 years, and we are a young species. An analysis of persistence of fossil species suggests that the average species exists for about 18 million years. On the scale of 18 million years, a human life span is hardly discernible.

Times of quiet reflection are occasionally consumed by thoughts of the purpose of life and the measure of one’s achievements. From the perspective of deep history and the persistence of a lineage, reproduction is probably the most important achievement, for it is not the number of birthdays celebrated but the number of genes passed on to the next generation that matters.

We recognize species such as humans or bluebirds or walruses by their physical forms, but their bodies are merely temporary packages of genes. It is not the packages, but the flow of genes that sustains a lineage. Human generation time varies among societies and across centuries, but 25 years is a reasonable average. We carry genes for a generation.

Following this line of reasoning, a species is less a physical form and more a reproductive system for sustaining a lineage of genes. A species is a reproductive system that keeps going, a braided stream of anastomosing and bifurcating lineages flowing through time.

Failure to reproduce terminates the lineage. Extinction is not a rare tragedy, but perhaps an inevitability, for 99.99 percent of all species are extinct. Extinction is forever.

Although our life span is short, each individual is special, for each individual has a unique genotype (excluding identical twins), a combination of genes that has never occurred before and will never recur. Each unique genotype produces an unprecedented person, usually with some enviable attributes and some unfortunate shortcomings.

I still cast a shadow on a canyon wall. My shadow will not last forever, and neither will the wall. But the human lineage may persist for a long time, even in comparison to canyon walls.

Jeff Mitton (mitton@colorado.edu) is chair of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the July 9 edition of the Daily Camera.