Читать книгу The Tangled Tree: A Radical New History of Life - David Quammen, David Quammen - Страница 18

Somebody did, though not immediately. Seven years passed, during which several other scientists began noodling along various routes that would lead to a similar idea. Two of them were Linus Pauling and Emile Zuckerkandl, who gave their own fancy name to the enterprise—they called it “chemical paleogenetics”—and they converged on it by very different trajectories.

Zuckerkandl was a young Viennese biologist whose family had escaped Nazi Europe via Paris and Algiers. He got to America, did a master’s degree at the University of Illinois (long before Carl Woese would arrive there), then returned to Paris after the war for a doctorate. He found work at a marine laboratory on the west coast of France and studied the molting cycles of crabs, which involve a molecule analogous to hemoglobin. His interest drifted from crustacean physiology to questions at the molecular level, and he hankered to return to America. In 1957 Zuckerkandl finagled a chance to meet Pauling, who by then was a celebrated chemist with the first of his two Nobel Prizes already won. The prize had given Pauling some latitude to expand his own range of concerns, from lab chemistry at the California Institute of Technology to the wider world, and some leverage in pursuing those concerns. He had two in particular: genetic diseases such as sickle cell anemia and the threats posed by thermonuclear weapons, including radioactive fallout from testing. By the late 1950s, Pauling was raising his voice. He initiated a petition against atmospheric nuclear testing that more than eleven thousand scientists signed. He had become, along with Bertrand Russell, the provocative British philosopher, also a Nobel winner, one of the world’s most august peaceniks.

Pauling’s initial encounter with Zuckerkandl coincided with his increasing interest in genetics, evolution, and mutation—most pointedly, the mutations that might be caused by radiation released in weapons tests. His interest in disease led in the same direction, because sickle cell anemia is a problem that results from mutations in one of the genes for hemoglobin. Pauling found Zuckerkandl impressive enough that he offered the younger man a postdoctoral fellowship in chemistry at Caltech. Then, when Zuckerkandl arrived in Pasadena, intending to continue work on the crab-molting molecule, Pauling discouraged that project and said, “Why don’t you work on hemoglobin?”

Pauling suggested further that he take up a newly invented technique—still primitive but promising—that employed electrophoresis (separating molecules by their sizes, using electrical charge) and other methods to “fingerprint” such proteins, distinguishing one variant from another. Comparing protein molecules that way, Pauling figured, might allow researchers to draw some evolutionary conclusions. So Zuckerkandl went to work, learning the technique and applying it to hemoglobin in variant forms. Before long, he could see the close similarity between human hemoglobin and chimpanzee hemoglobin, and that human hemoglobin was less similar to hemoglobin found in orangutans. He could also tell a pig from a shark just by looking at the molecular fingerprints. Of course, there were easier ways to tell a pig from a shark, but never mind. Although it wasn’t such a precise methodology as he might have wished, this sort of molecular comparison was a start.

Over the next half dozen years, Zuckerkandl’s work thrived, and he published a series of papers with Pauling. Some of those were invited contributions to celebratory volumes, Festschriften, in honor of eminent scientists, generally on some occasion such as retirement or a big, round birthday. Such invitations came often because of Pauling’s own eminence, and he recruited Zuckerkandl as coauthor to do much of the thinking and most of the writing. In the meantime, Pauling won his second Nobel, this time the Peace Prize in recognition of his efforts against nuclear weapons proliferation and testing. That one didn’t add to his scientific reputation (in fact, he resigned from his Caltech professorship because university administrators and trustees disapproved of his peace activism), but it certainly helped amplify his public voice. He was a busy man, much in demand. The invitations—to speak, to visit, to contribute scientific papers for ceremonial volumes—continued. Because such papers didn’t normally go through the peer-review filter, they could be a little more bold and speculative than a typical journal article. One of them, written in 1963 to honor a Russian scientist on his seventieth birthday, was titled “Molecules as Documents of Evolutionary History.” Two years later, it was reprinted in English in the Journal of Theoretical Biology, giving it much broader reach and influence. Pauling and Zuckerkandl were wading into the same pond where Francis Crick had dipped his toe.

Their 1963 paper made an important distinction between molecules that carry genetic information—such as DNA or the proteins it encodes—and other molecules, such as vitamins, that cycle through a living creature and out the other end. Information molecules have histories that can be deduced; they have ancestors from which the variant forms, in this creature or that, have descended. Scrutiny of such molecules, wrote Zuckerkandl and Pauling, can tell us three things: how much time has passed since the lineages split, what the ancestral molecules must have looked like, and what were the lines of descent. The first of those three kinds of information became known as the molecular clock, although Zuckerkandl and Pauling hadn’t yet named it. The third kind implied trees.

Zuckerkandl continued reworking and developing these ideas, with Pauling as his coauthor and sponsor. In September 1964, before a distinguished and argumentative symposium audience at Rutgers University, he delivered a long paper that became the definitive version of their shared ideas and that, despite Zuckerkandl having done most of the writing, has been called the “most influential of Pauling’s later career.” In this paper, the two authors offered their memorable metaphor: if the minor changes in molecular variants are proportional to elapsed time over the eons, they said, what you have is “a molecular evolutionary clock.”

It was tentative, a hypothesis. The hypothesis was disputed at the Rutgers symposium and would be controversial in coming years, but it captured attention, it focused thought, and it promised a whole new way of measuring life’s history, if it was right. The molecular clock has since been called “one of the simplest and most powerful concepts in the field of evolution,” and also “one of the most contentious.” Crick himself later judged it “a very important idea” that turned out to be “much truer than people thought at the time.”

Emile Zuckerkandl, meanwhile, moved back to France. Along with Pauling and just a few others, he had helped launch a new scientific enterprise, and when a Journal of Molecular Evolution came into being, in 1971, he was its first editor in chief. His name isn’t familiar to the wider world, as Pauling’s is, but if you say “Zuckerkandl and Pauling” to a molecular biologist today, he or she will think “molecular clock.” Fitting as that may be, it overlooks the other important point: the other metaphor embedded in the long Rutgers paper, where Zuckerkandl wrote that “branching of molecular phylogenetic trees should in principle be definable in terms of molecular information alone.” This was a whole new way of sketching those trees, which rose and spread their branches as the clock ticked.