河流曾经宽而浅地流淌，直到自然进化出的树状植被才将它们变成线状，克里斯多夫·因塔利亚塔报导。

Rivers today have high muddy banks, sandbars and bends. But they didn’t always look that way. Because it wasn’t until the evolution of tree-like plants, some 330 million years ago, that rivers were corralled into their current form. Before that, ancient waters flowed wide and shallow over the land, with little to constrain them other than mountains. So says a study in the journal Nature Geoscience. [Neil S. Davies and Martin R. Gibling, Evolution of fixed-channel alluvial plains in response to Carboniferous vegetation]

Researchers looked through over 400 studies of the Earth’s rock record, and visited nearly 70 field sites. And they found that channel formations in the rock—a signature of modern rivers—didn’t appear until the Carboniferous period, when tree-like plants evolved. That’s because larger plants needed deeper roots, which stabilized river banks and forced rivers into narrower paths. And deep roots helped form sticky clays, which are harder to erode.

All this engineering was to the trees’ advantage, the researchers say. Because river banks provide trees with easy access to water, without the constant risk of flooding. Pretty much what we humans want. Many of our greatest cities formed along river banks—for which we might have trees to thank.

—Christopher Intagliata