Reading the palimpsests of life

Parchment and vellum provided durable writing surfaces for scholars in the early Middle Ages in Europe. In the years before suitable paper and the technical means to print multiple copies of a work were available, it was important that a scribe leave a lasting impression. Parchment and vellum were convenient, easier to work with than stone tablets, and yet still durable. But they were costly to make, so when it came time to write a new book, often the writing on an old work was removed and the new text was lettered in a neat monastical hand on the same surface. In some instances the original writing was washed off, while in others it was physically scraped away. In the former case, faint traces of the original sometimes remain and can be restored with laborious and patient effort so that, centuries later, scholars can read through the superimposed writing to discern what had been written before. These rewritten pages, called palimpsests, are a source of intellectual, religious, and science history, providing important ancient texts that would otherwise not be available. The most famous example is a manuscript copy of a paper by the third-century BC mathematician, Archimedes of Syracuse, one of history’s most brilliant scientists. In his Method of Mechanical Theorems, Archimedes devised a method of infinitesimals that anticipated Isaac Newton’s invention of integral calculus by nearly 2,000 years. He conceptualized geometrical problems in mechanical terms like levers and centers of gravity. For example, as in the faint figures restored in Figure 1B, he divided a geometric structure such as a parabola into essentially an infinite number of small subparts, the areas of which could be added together to approximate the total to answer some question about the structure. No copy of Archimedes’ original presentation of his method survives, so his ideas had to be inferred from their use in his known works. But, as it happens, a tenth-century monk made a parchment copy of Method along with other of Archimedes’ works. In the twelfth-century, however, a monk named Joannes Myronas washed away the original writing and recycled the parchment as a prayer book. Fortunately, the erasure was not complete. Modern methods have restored much of the earlier manuscript. The most complete restoration so far has recently been achieved by a method called X-ray fluorescence, which detects iron molecules in the original scribe’s ink. So we now know what Archimedes said. A phenomenon in evolution somewhat resembles a palimpsest. What we see today evolved from ancestral states, often ‘‘writing over’’ the original state as organisms adopt new functions so that the original state no longer exists. An example has to do with what has made vertebrates what we are today. There is a good vertebrate fossil record largely because what makes fossils hard is also what made vertebrates hard. Bones and other mineralized tissues, initially forming an external skeleton and subsequently an internal skeleton, as well as dentition, are perhaps the most important distinguishing characteristic of our group of animals. In a research project to understand the genetic basis of these evolving traits, we had to confront a problem akin to revealing and reading a palimpsest.