Defenders of Earth

Are humans Gaia's immune system?

The Gaia hypothesis, which holds that Earth is a living organism in its own right, typically has been used to highlight man's role in messing up the environment. But if the latest warning of a possible ecological catastrophe turns out to be accurate, people could end up helping Gaia rather than harming her.

The Gaia hypothesis--named after the Greek word for the Earth goddess, also translated as "Earth Mother"--was devised in the 1970s by atmospheric chemist James Lovelock and microbiologist Lynn Margulis. In Gaia: A New Look at Life on Earth, Lovelock and Margulis wrote, "The entire range of living matter on Earth from whales to viruses and from oaks to algae could be regarded as constituting a single living entity capable of maintaining the Earth's atmosphere to suit its overall needs and endowed with faculties and powers far beyond those of its constituent parts." They said Gaia could be defined as "a complex entity involving the Earth's biosphere, atmosphere, oceans, and soil; the totality constituting a feedback of cybernetic systems which seeks an optimal physical and chemical environment for life on this planet."

Most evolutionary biologists reject the Gaia hypothesis as an unscientific, although poetic, metaphor. But let's not let such quibbles trouble us today.

According to the Gaia hypothesis, the history of life on Earth can be regarded as a progressive modification of the planet's chemistry and temperature by biological organisms acting in ways that enhance their own flourishing. For example, Earth's atmosphere was modified over billions of years by photosynthetic microorganisms from one that was predominantly carbon dioxide and methane into its current oxygen-rich state. This oxygen-rich atmosphere apparently set the stage for the evolution of multicellular life that took off in earnest during the "Cambrian explosion" some 540 million years ago.

In the millions of years following the Cambrian explosion, Gaia took out all the stops, and the earth saw a vast diversification of life and finally the colonization of land by plants and animals. Then, 250 million years ago, the Permian party came to a catastrophic end in which 95 percent of Earth's species were wiped out. Gaia picked herself up and started over. Dinosaurs and flowering plants eventually evolved to dominate the landscape in the Cretaceous Period (146 to 65 million years ago). At the time, our tiny mammalian ancestors were scrambling about the leaf litter, trying to avoid becoming dinosaur snacks. The cornucopia of Cretaceous life came to an abrupt end 65 million years ago, when 70 percent of all species became extinct.

The leading explanation for these mass extinctions is the havoc caused by asteroids slamming into the earth. The asteroid that brought the Cretaceous Period to a close is thought to have been 10 miles wide, creating the 110-mile-diameter Chicxulub crater just off Mexico's Yucatan peninsula. The massive Permian extinction is thought to have been caused by an asteroid 10 times bigger.

It's inevitable that Earth will be struck again. In 1908 a small comet or asteroid, about 165 feet in diameter, exploded over the remote Tunguska region of Siberia, releasing energy equivalent to 15 megatons of TNT, 1,000 times stronger than the atomic bomb dropped on Hiroshima. In 1989 an asteroid measuring a quarter of mile in diameter missed Earth by just 400,000 miles. In 1994 the house-sized asteroid XM1 was spotted only 14 hours before passing within 65,000 miles of Earth, well inside the moon's orbit of 238,000 miles. In June an asteroid the size of a soccer field missed Earth by 75,000 miles.

Scientists estimate that an asteroid with a diameter of a kilometer (0.62 mile) could destroy civilization by drastically changing the earth's climate after impact and kill one-quarter of the world's population. This disaster scenario was popularized in 1998 by two mediocre movies, Deep Impact and Armageddon.

So far astronomers have identified over 26,000 asteroids in our solar system. The 1,700 or so that regularly pass close to Earth's orbit are designated Near Earth Objects, or NEOs. Six hundred NEOs measure more than a kilometer in diameter. The National Aeronautics and Space Administration's Jet Propulsion Laboratory helpfully lists more than a score of upcoming near misses for those who have a morbid interest in such things.

The latest scare occurred last week with the detection of asteroid 2002 NT7, which measures more than a mile in diameter. Initial calculations showed that there was a small probability that it might hit the earth on February 1, 2019. Fortunately, subsequent analysis found that civilization will be spared until at least February 1, 2060, when there is a very tiny chance the asteroid will hit us. In the meantime, NASA and the European Space Agency are increasing their monitoring of NEOs and thinking of ways to deflect or blow up any asteroids that threaten to smash into Earth.

What do asteroid impacts have to do with the Gaia hypothesis? In Gaia: A New Look at Life on Earth, Lovelock asked, "To what extent is our collective intelligence also a part of Gaia? Do we as a species constitute a Gaian nervous system and a brain which can consciously anticipate environmental changes?"

Perhaps Gaia has gotten tired of being whacked by asteroids and having to restart biological evolution over and over again. Perhaps she evolved technologically sophisticated, big-brained mammals who can travel in space as a way of protecting herself from asteroids. Like antibodies that protect the body from invading disease organisms, humans can defend our Earth Mother against extraterrestrial intruders. Just a thought.