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Scientists studying the pollen in core samples see changes in the frequency of fires, the retreat and expansion of species with glaciations, and more. But the biggest change in millennia has come recently, a measure of the march of human technologies. With the spread of agriculture, tree pollen has, spring after spring, grown less common and the pollen of cereals and weeds more so. As we change Earth's climate, species long adapted to the cold will become rarer, and the pollen of new arrivals from warmer regions will increase. As we fly around the world, the pollen of Asian species is turning up in North America, of African species in Hawaii, of Australian species in South Africa.

Pollen has tracked the progress of civilizations before. In the Maya lowlands of Guatemala, the pollen of forest trees was once the most common. Around 4,600 years ago, corn pollen started to appear. By 2,000 years ago, most pollen came from plants associated with agriculture.

Then around a thousand years ago, corn pollen began to disappear. Weed pollen too. Eventually, the pollen of trees came back. In seeing this change, palynologists can infer much of the rest. The birds also came back, as did the bees and even the bats with their long, sticky tongues. Like all records, the record of pollen has biases, but here the big message needs little interpretation. The civilization rose and then faded—the temples giving way to the scramble of roots and the rise of trees with their pendulant flowers and abundant pollen, which was cast once more into the air and onto animals' wiggling backs. Whatever happens to us in the years to come, the pollen will continue to record. It offers no criticism, just testament.

All life, including our own, is improbable, but somehow the lives of plants, dependent on pollen's traffic, are particularly so. And yet they find each other again and again, as they have since before the days of the dinosaurs, when giant dragonflies cruised the air, yellow dust no doubt stuck in their prehistoric hair. 

Ecologist Rob Dunn, author of Every Living Thing, is a professor at North Carolina State University. Martin Oeggerli is a Swiss molecular biologist.
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