National Geographic : 2009 Dec
owers drinking, and deposit that pollen at the same time as they buzz among owers. en they go one further by scraping some collected pollen into tiny baskets, corbiculae, on their rear legs. Once home, the bees store the pollen in pots they make out of wax, keeping it to eat on rainy days. In allowing plants to have sex at a distance, pollen, and ultimately owers, led to explosive diversi cation, turning a brown planet green and then red, yellow, white, orange, and all the rest. Pollen diversi ed too. In the 300,000 pollen-bearing plant species on Earth, there are 300,000 di erent forms of pollen. e great variety in colors, shapes, and textures of the grains has evolved in accordance with each plant s biological particulars. Beetle-pollinated plants tend to have smooth, sticky pollen, the better to adhere to the lumbering beetles backs. Plants pollinated by fast-moving bees or ies may have spiny pollen that lodges easily be- tween the insects hairs. Plants pollinated by bigger animals, such as bats, sometimes have bigger pollen, though not always---perhaps not even most of the time. In the details of pollen s variety, more remains to be explained than is understood. e most recent story in the history of pollen is recorded not by the successes but by the fail- ures. e air, however clear, is full of unsuccess- ful pollen, dri ing in eddies of wind. Billions of grains reach the stratosphere. Even now, as you read, a few grains may rest on your hands or face, or on your cat. Pollen settles and accumu- lates in sediment, layer a er layer, particularly at the bottoms of lakes and ponds. In those layers, where decay is slow, pollen constitutes a history book far outlasting the plants that produce it. Grass pollen in sediment means grasslands, pine pollen indicates pine forests, and so on---an encyclopedia of detail spelled out in the mud. Palynologists take core samples of lake-bottom layers to examine shi s in the species of plants from one layer to the next. Collectively such chronicles of changing plant life can span thousands of years or more. Scientists studying the pollen in core samples see changes in the frequency of res, 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 a er spring, grown less com- mon 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 y around the world, the pollen of Asian species is turning up in North America, of African species in Hawaii, of Australian spe- cies 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. en around a thousand years ago, corn pol- len began to disappear. Weed pollen too. Even- tually, the pollen of trees came back. In seeing this change, palynologists can infer much of the rest. e 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 interpreta- tion. e civilization rose and then faded---the temples giving way to the scramble of roots and the rise of trees with their pendulant ow- ers 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 of- fers no criticism, just testament. All life, including our own, is improbable, but somehow the lives of plants, dependent on pollen s tra c, are particularly so. And yet they nd each other again and again, as they have since before the days of the dinosaurs, when giant dragon ies cruised the air, yellow dust no doubt stuck in their prehistoric hair. j Ecologist Rob Dunn, author of Every Living ing, is a professor at North Carolina State University. Martin Oeggerli is a Swiss molecular biologist.