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The Evolutionary Power of Cities and Light

Urbanization and the spread of artificial light are transforming life for all of earth’s species, bringing about a host of unintended consequences.

In 1800, only two percent of the human population lived in cities. A century later, that portion grew to 15 percent. Then, sometime in 2007, a person was born in a city somewhere on the globe who tipped the proportion of Homo sapiens that lives in cities over the 50 percent mark. Despite the fact that cities cover only two to three percent of terrestrial surface area, more than half of humanity is now urban-dwelling. There is no going back.

For a species that spent close to 200,000 years living in grasslands and scrubby forests, hunting and foraging, and using skins, wood, and grasses for shelter, we are increasingly occupying an evolutionarily unfamiliar niche, where the sensory and physical dimensions of a life lived in daily contact with the natural world have been replaced by a whole set of alternate experiences: Cement and traffic, 90-degree corners, bars, sirens, glass, and streetlights increasingly dominate our senses. As far as our genes are concerned, we live in an alien world. Phobias about snakes slithering out of toilet bowls, coyotes snatching children out of strollers, and diseases infiltrating city water supplies reveal the location of our biological roots. The shadow of the wild continues to haunt the psyche of even the most entrenched urbanite.

Alongside us, fast-breeding and opportunistic species are changing their behaviors and their genomes so that they will fit better in the urban world. City-dwelling swallows are evolving shorter wings that allow them to avoid the traffic better, and sparrows and starlings have raised the pitch of their calls to compensate for the background urban noise. Moths are gaining different color patterns so that they have more suitable camouflage in their new concrete habitat. Evolutionary forces are turning city-bound mice into separate subspecies in different city parks, unable to exchange genes with cousins who live a few blocks away.

A second and related agent of evolutionary change is the progressive banishment of darkness from the world at the hands of electric light. Paul Bogard has written poignantly of his deep regret at the “end of night.” He points out that the spread of electricity across many parts of the globe has condemned real darkness to the planet’s history. This lack of night comes with sizable biological consequences. Excessive illumination is disrupting the natural rhythms created by millions of years of the earth’s steady axial rotation.

The first photos of the earth from space taken by lunar-bound astronauts revealed a spectacular blue marble poised in front of a star-speckled expanse. The individuals lucky enough to see the planet from this vantage point were all transformed. American astronaut Edgar Mitchell memorably described his impression of it as “a small pearl in a thick sea of black mystery.” The planet’s finitude, its swirling beauty, and its apparent fragility gave our species its first clear sense of our lack of astral significance. Norman Cousins later remarked that “what was most significant about the lunar voyage was not that man set foot on the Moon but that they set eye on the Earth.”

More recent photographs of the earth taken at night have revealed … [  ]

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