Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [112]
It appeared that the surface of Mars was far harsher than scientists had reckoned. Ultraviolet light and highly reactive chemicals such as hydrogen peroxide quickly destroyed any organic carbon. The chances of life on Mars seemed low or nil. Lederberg was more optimistic than some of his colleagues, but not by much. It was possible that life on Mars existed only in a few oases, perhaps around hot springs bubbling up from the interior of the planet. But if there was life on Mars, it was far more retiring than the boisterous, all-consuming life on Earth. “We can no longer be confident that no matter where you look you will find life,” Lederberg told reporters.
Viking’s failure was no reason to stop looking for life, Lederberg and others believed. They urged NASA to put together a “son of Viking”—a new probe that could take a new set of instruments to Mars. But NASA was more interested in astronauts, those teeming reservoirs of E. coli. As support for exobiology faded, Lederberg returned to other pressing issues in biology, such as the emergence of new diseases and the threat of biological warfare. His days of professional stargazing were over.
Twenty years later, NASA’s interest in extraterrestrial life grew again. A meteorite from Mars bore strange markings that some scientists suggested were fossils of microbes. The Galileo probe passed by Europa, a moon of Jupiter, and captured images of the ice covering its surface. Perhaps life was lurking underneath. The search for life—now called astrobiology—found new support from NASA, which founded the NASA Astrobiology Institute in 1998.
Today many astrobiologists search for extreme places on Earth where life manages to survive. E. coli is a rugged creature, but scientists have found many other organisms that live in places where it would quickly die: acid-drenched mine shafts, oxygen-free swamp bottoms, the depths of glaciers, superheated water shooting out of hydrothermal vents, the spaces inside crystals of salt. Planets and moons with similar environments might be suitable homes for life.
But as weird as some new species may be, they all share E. coli’s fundamental features. They are membranes wrapped around proteins and DNA. They need sources of carbon and energy in order to grow. And they need liquid water as a medium in which their chemistry can take place. If some of these rugged microbes were carried to an underground hydrothermal system on Mars or perhaps slipped beneath the icy crust of Europa, they might be able to eke out an existence.
Yet scientists are also keenly aware that life on Earth may not be the rule for life in the universe. Our own tinkering with life has made that clear. Expanding E. coli’s genetic code does not kill it, so there’s no reason to think that life on other planets couldn’t use other amino acids to build its proteins. All life on Earth uses the four-letter language of bases to encode information in its genes. But scientists have been able to engineer E. coli with man-made bases—in other words, adding new letters to its alphabet. Synthetic biology blurs into astrobiology.
Life might not even need DNA. Some experiments have suggested that other molecules can take on the same structure, with a backbone carrying information-bearing compounds. They might even be able to replicate themselves accurately. Scientists have even speculated that life may be able to exist without liquid water. Another liquid, such as liquid methane, might serve as its matrix.
No matter what extraterrestrial life might be made of, our discovery of it would change how we think about life in general. It would finally give scientists more than one planet’s worth of life with which to search for the rules of existence. Scientists would probably start studying alien life at its lowest levels, trying to determine how it stores genetic information. But some of the most interesting comparisons would come later. Living things on Earth have more in common than DNA. E. coli and elephants alike can survive in a