Universe a grand tour of modern science Phần 7

languages Other linguists doubted this theory, and saw no logical reason why the evolutionary mechanism that produced the language faculty in the first place should carry through into the diversification of the world’s languages. An analogy was with dancing. Biological evolution provided agile limbs and a sense of rhythm, but it did not follow that every traditional dance had to pass some evolutionary fitness test. ‘The hand that rocks the cradle rules the world’ is an example of a relative clause, which can qualify the subject or object of a sentence. Every headline writer knows that mismanaged relative clauses can become scrambled into nonsense like rocks the cradle rules. In protecting the integrity of relative clauses, there is a trade-off between risky brevity, as in newspaper headlines, and longwinded and pedantic guarantees against ambiguity. Languages vary greatly in the precautions that speakers are expected to take. Relative clauses were a focus of interest for many years for Bernard Comrie of the Max-Planck-Institut fur evolutionare Anthropologie in Leipzig, one of the editors of The World Atlas of Language Structures. He found instances of exuberant complexity that could not be explained in terms of practical advantages. Rather, they seem to reflect the emblematic function of language as a symbol of its speech community. Speakers like having striking features that make their language stand out. ‘By all means let’s agree that the faculty of language evolved in a biological manner,’ Comrie said. ‘But to understand Babel we have to go beyond that kind of explanation and look for historical and social reasons for the proliferation and diversification of languages. Mapping their structures worldwide gives us the chance of a fresh start in that direction.’ I The face-to-face science Along with the flag and the football team, a language is often a badge of national identity. Nations—tribes with bureaucrats—remain the chief engineers of war. Instead of chariots and longships, some of them now have nuclear, biological and chemical weapons. Any light that linguistics can shed on the rationale and irrationalities of nationhood is urgently needed. People are also starting to ask, ‘What language will they speak on Mars?’ The study of language evolution remains at its roots the most humane of all the sciences, in both the academic and the social sense of that adjective. William Labov at Penn cautioned his students against becoming so enraptured by theoretical analysis and technology that they might be carried away from the human issues involved in the use of language. ‘The excitement and adventure of the field,’ he said, ‘comes in meeting the speakers of the language face to face, entering their homes, hanging out on 450 life’s origin E ‘I corners, porches, taverns, pubs and bars. I remember one time a 14-year-old in Albuquerque said to me, ‘‘Let me get this straight. Your job is going anywhere in the world, talking to anybody about anything you want?’’ I said, ‘‘Yeah.’’ He said, ‘‘I want that job!’’’ For related topics concerning language, see Speech and Grammar. For genetic correlations in human dispersal, see Prehistoric genes. For social behaviour, see Altruism and aggression. can trace my ancestry back to a protoplasmal primordial atomic globule,’ boasts Pooh-Bah in The Mikado. When Gilbert and Sullivan wrote their comic opera in 1885 they were au courant with science as well as snobbery. A century later, molecular biologists had traced the genetic mutations, and constructed a single family tree for all the world’s organisms that stretched back 4 billion years, to when life on Earth probably began. But they were scarcely wiser than Pooh-Bah about the precise nature of the primordial protoplasm. In 1995 Wlodzimierz Lugowski of Poland’s Institute of Philosophy and Sociology wrote about ‘the philosophical foundations of protobiology’. He listed nearly 150 scenarios then on offer for the origin of life and, with a possible single exception to be mentioned later, he judged none of them to be satisfactory. Here is one of the top conundrums for 21st-century science. The origin of life ranks with the question of what initiated the Big Bang, as an embarrassing lacuna in the attempt by scientists to explain our existence in the cosmos. In the last paragraph of his account of evolution in The Origin of Species (1859) Charles Darwin remarked, ‘There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one.’ Privately he thought that the divine breath had a chemical whiff. He speculated that life began ‘in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present’. 451 life’s origin By carbon chemistry plus energy, scientists would say nowadays. Since Darwin confided his thoughts in a letter to a friend in 1871, a long list of eminent scientists have bent their minds to the problem in their later years. Two of them (Svante Arrhenius and Francis Crick) transposed the problem to a warm little pond far away, by visualizing spores arriving from outer space. Another (Fred Hoyle) proposed the icy nuclei of comets as places to create and harbour our earliest ancestors, in molten cores. Most investigators of the origin of life preferred home cooking. The Sun’s rays, lightning flashes, volcanic heat and the like may have acted on the gases of the young Earth to make complex chemicals. In the 1950s Harold Urey in Chicago started a student, Stanley Miller, on a career of making toffee-like deposits rich in carbon compounds by passing electrical discharges through gases supposedly resembling the early atmosphere. These materials, it was said, created the primordial soup in the planet’s water, and random chemical reactions over millions of years eventuallycame up with the magic combinations needed for life. Although they were widely acclaimed at the time, the Urey–Miller experiments seemed in retrospect to have been a blind alley. Doubts grew about whether they used the correct gassy ingredients to represent the early atmosphere. In any case the feasibility of one chemical reaction or another was less at issue than the question of how the random chemistry could have assembled the right combination of ingredients in one spot. Two crucial ingredients were easily specified. Nucleic acids would carry inheritable genetic instructions. These did not need to be the fancy double-stranded deoxyribonucleic acid, DNA, comprising the genes of modern organisms. The more primitive ribonucleic acid, RNA, would do. Secondly, proteins were needed to act as enzymes that catalysed chemical reactions. Around 1970, Manfred Eigen at Germany’s Max-Planck-Institut fur biophysikalische Chemie sought to define the minimum requirement for life. He came up with the proposition that the grandmother of all life on Earth was what he called a hypercycle, with several RNA cycles linked by cooperative protein enzymes. Accompanying the hypothesis was a table game played with a pyramidal dice and popper beads, to represent the four chemical subunits of RNA. The aim was to optimize random mutations to make RNA molecules with lots of loops made with cross-links, considered to be favourable for stability in the primordial soup. I Catalysts discovered Darwin’s little pond may have needed to be hot, rather than warm, to achieve the high concentrations of molecules and energy needed to fulfil the recipe for life. Yet high temperatures are inimical for most living things. Students of the 452 life’s origin origin of life were therefore fascinated by heat-resistant organisms found thriving today in volcanic pools, either on the surface or on the deep ocean floor at hydrothermal vents. Perhaps volcanic heat rather than sunlight powered the earliest life, some said. Reliance on the creativity of random chemistry nevertheless remained for decades a hopeless chicken-and-egg problem. The big snag, it seemed, was that you couldn’t reproduce RNA without the right enzymes and you couldn’t specify the enzymes without the right RNA. A possible breakthrough came in 1982. Thomas Cech of Boulder, Colorado, was staggered to find that RNA molecules could act as catalysts, like the protein enzymes. In a test tube, an RNA molecule cut itself into pieces and joined the fragments together again, in a complicated self-splicing reaction. There was no protein present. The chicken-and-egg problem seemed to be solved at a stroke. Soon other scientists were talking about an early RNA World of primitive organisms in which nucleic acids ruled, as enzymes as well as genetic coders. Many other functions for RNA enzymes, or ribozymes, emerged in subsequent research. Especially telling was their role in ribosomes. These are the chemical robots used by every living creature, from bacteria to whales, to translate the genetic code into specified protein molecules. A ribosome is a very elaborate assembly of protein molecules, but inside it lurk RNA molecules that do the essential catalytic work. ‘The ribosome is a ribozyme!’ Cech declared, in a triumphant comment on the latest analyses in 2000. ‘If, indeed, there was an early RNAWorld where RNA provided both genetic information and catalytic function, then the earliest protein synthesis would have had to be catalysed by RNA. Later, the RNA-only ribosome/ ribozyme may have been embellished with additional proteins; yet, its heart of RNA functioned sufficiently well that it was never replaced by a protein catalyst.’ The chief rival to the RNA World by that time was a Lipid World, where lipid means the oily or fatty stuff that does not mix with water. It is well suited, today and at the origin of life, to provide internal membranes and outer coatings for living cells. The packaging could have preceded the contents, according to an idea that traces back to Aleksandr Oparin of Moscow in the 1920s. He visualized, and in later experiments made, microscopic lipid membranes enclosing water rich in various chemicals, which might be nondescript at first. These coacervate droplets, to use the technical term, could be the precursors of cells. As Oparin pointed out, they provided a protected environment where any useful, self-reproducing combinations that emerged from random chemistry could gather. They would not simply disperse in the primordial soup. By the end of the century, progress in molecular science and cell biology had brought two thought-provoking discoveries. One was that some lipids have their 453 life’s origin own hereditary potential. They can make copies of themselves by self-assembly from available molecular components, independently of any genetic system. Also remarkable was the realization that, like protein enzymes and RNA ribozymes, some lipids, too, could act as catalysts for chemical reactions. Doron Lancet of Israel’s Weizmann Institute of Science called them lipozymes. Lancet became the leading advocate of the Lipid World as the forerunner of the origin of life. His computer models showed that diverse collections of lipid molecules could self-assemble and self-replicate their compositions, while providing membranes on which other materials could form, including proteins and nucleic acids. ‘It is at this stage,’ Lancet and his colleagues suggested, ‘that a scenario akin to the RNA World could be initiated, although this does not imply by any means that RNA chemistry was exclusively present.’ I What was the setting? One difficulty about any hypothesis concerning the first appearance of life on the Earth is verification. No matter how persuasive it may be, in theory or even in laboratory experiments that might create life from scratch, there is no very obvious way to establish that one scenario rather than another was what actually happened. Also lacking is clear knowledge about what the planet was like at the time. It was certainly not a tranquil place. Big craters still visible on the Moon mainly record a heavy bombardment by stray material—icy comets and stony asteroids—left over from the origin of the Solar System. It afflicted the young Earth as well as the Moon and continued for 600 million years after our planet’s main body was complete 4.5 billion years ago. In this Hadean Era, as Earth scientists call it, no region escaped untouched, as many thousands of comets and asteroids rained down. As a result, the earliest substantial rocks that survive on the surface are 4 billion years old. Yet it was during this turmoil that life somehow started. Abundant water may have been available, perhaps delivered by icy impactors. Indirect evidence for very early oceans comes from zircons, robust crystals of zirconium silicate normally associated with continental granite. In 1983, Derek Froude of the Australian National University and his colleagues found zircons more than 4.1 billion years old included as grains in ancient sedimentary rocks in Western Australia. By 2001, an Australian–UK–US team had pushed back the age of the earliest zircon fragment to 4.4 billion years. That was when the Earth’s crust had supposedly just cooled sufficiently to carry liquid water, which then interacted with the primitive crust to produce granite and its enclosed zircons. A high proportion of heavy oxygen atoms in the zircon testified to the presence of water. 454 ... - tailieumienphi.vn