Nothing in biology makes sense except in the light of evolution.

—Theodosius Dobzhansky

In medieval Europe, before there was paper, manuscripts were made by writing on parchment and vellum, thin sheets of dried animal skin. Because these were hard to produce, many medieval writers simply reused earlier texts by scraping off the old words and writing on the newly cleaned pages. These recycled manuscripts are called palimpsests, from the Greek palimpsestos, meaning "scraped again."

Often, however, minute traces of the earlier writing remained. This has proved critical in our understanding of the ancient world. Many ancient texts are in fact known to us only by peering beneath the stratum of medieval overwriting to recover the original words. Perhaps the most famous of these is the Archimedes Palimpsest, first written in Constantinople in the tenth century and then cleaned and overwritten three centuries later by a monk making a prayer book. In 1906, a Danish classicist identified the original text as the work of Archimedes. Since then, a combination of X-rays, optical character recognition, and other complex methods have been used to decipher the original underlying text. This painstaking work yielded three mathematical treatises of Archimedes written in ancient Greek, two of them previously unknown and enormously important in the history of science. In such arcane ways we recover the past.

Like these ancient texts, organisms are palimpsests of history— evolutionary history. Within the bodies of animals and plants lie clues to their ancestry, clues that are testimony to evolution. And they are many. Hidden here are special features, "vestigial organs," that make sense only as remnants of traits that were once useful in an ancestor. Sometimes we find "atavisms"—"throwback" traits produced by the occasional reawakening of ancestral genes that have long been silenced. Now that we can read DNA sequences directly, we find that species are also molecular palimpsests: in their genomes is inscribed much of their evolutionary history, including the wrecks of genes that once were useful. What's more, in their development from embryos, many species go through contortions of form that are bizarre: organs and other features appear, and then change dramatically or even disappear completely before birth. And species aren't all that well designed, either: many of them show imperfections that are signs not of celestial engineering, but of evolution.

Stephen Jay Gould called these biological palimpsests the "senseless signs of history." But they are not really senseless, for they constitute some of the most powerful evidence for evolution.

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