Universe a grand tour of modern science Phần 7 docx

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 fu¨r evolutiona¨re 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

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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!’’ ’

E 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.

‘I 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’.

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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 eventually came 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 fu¨r

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

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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

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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.

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‘Our zircon evidence suggests that life could have had several false starts,’ said

Simon Wilde of the Curtin University of Technology in Perth, as proud

possessor of the oldest known chip of the Earth. ‘We can picture oceans and life

beginning on a cooling Earth, and then both being vaporized by the next big

impact. If so, our own primitive ancestors were the lucky ones, appearing just

when the heavy bombardment was coming to an end and somehow surviving.’

The composition of the young Earth’s atmosphere, and chemical reactions there

that could have contributed carbon compounds to the primordial soup, also

remained highly uncertain. In that connection, space scientists saw that Titan, a

moon of Saturn, might be instructive about life’s origin. It has a thick, hazy

atmosphere with nitrogen as its principal ingredient, as in our own air.

Whilst Titan is far too cold for life, at minus 1808C, it possesses many carbon

compounds that make a photochemical smog in the atmosphere and no doubt

litter the surface. So Titan may preserve in deep freeze many of the prelife

chemicals available on the young Earth. In 1997 NASA’s Cassini spacecraft set off

for Saturn, carrying a European probe, Huygens, designed to plunge into the

atmosphere of Titan.

In an exciting couple of hours in 2005, Huygens will parachute down to the

surface. During its descent, and for a short while after it thuds or splashes onto

the surface, the probe will transmit new information about Titan’s appearance,

weather and chemical make-up. The mother ship Cassini will also examine the

chemistry from the outside, in repeated passes.

‘One reason why all attempts to visualize the origin of life remain sadly

inconclusive is that scientists can only guess what the chemistry of the Earth was

like 4 billion years ago, when the event occurred,’ said Franc¸ois Raulin of the

Laboratoire Interuniversitaire des Syste`mes Atmospheriques in Paris, a mission

scientist for Cassini–Huygens. ‘The results of our examination of Titan may lead

us in unexpected directions, and stimulate fresh thinking.’

Whilst the Titan project might be seen as a pursuit of a home-cooking scenario

on another world, other astrochemists took the view that many materials

directly useful for starting life arrived ready-made from space. They would have

come during the heavy bombardment, when comets filled the sky. Even from

those that missed the Earth entirely, huge quantities of carbon compounds

would have rained gently onto the primordial surface in the form of small grains

strewn from the comets’ tails.

I Are we children of the comets?

Whether it was a joke or a serious effort to deceive, no one knows. Someone

took a piece of a meteorite that fell from the sky at Orgueil near Toulouse in

1864, and stuck lumps of coal and pieces of reed on it. The jest flopped. It went

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unnoticed for a hundred years, because there were plenty of other fragments of

that meteorite to examine. In 1964, Edward Anders and his colleagues at

Chicago disclosed the hoax in a forensic examination that identified even the

19th-century French glue.

In reality the Orgueil meteorite had a far more interesting story to tell. A 55-

kilogram piece at France’s Muse´um National d’Histoire Naturelle became the

most precious meteorite in the collection. It contains bona fide extraterrestrial tar

still being examined in the 21st century, with ever more refined analytical

techniques, for carbon compounds of various kinds that came from outer space

and survived the heat and blast of the meteorite’s impact.

Rapid advances in astrochemistry in the closing decades of the 20th century led

to the identification of huge quantities of carbon compounds, of many different

kinds, in cosmic space and in the Solar System. They showed up in the vicinity

of stars, in interstellar clouds, and in comets, and they included many

compounds with rings of carbon atoms, of kinds favoured by living things.

Much of the preliminary assembly of atoms into molecules useful for life may

have gone on in space. Comets provide an obvious means of delivering them to

the Earth. Confirmation that delicate carbon compounds can arrive at the planet’s

surface, without total degradation on the way down, comes from the Orgueil

meteorite. In 2001, after a Dutch–US re-examination of the Paris specimen, the

scientists proposed that this lump from the sky was a piece of a comet.

‘To trace our molecular ancestors in detail is now a challenge in astronomy,

space research and meteoritics,’ said the leader of that study, Pascale

Ehrenfreund of Leiden Observatory. ‘Chemistry in cosmic space, proceeding

over millions of years, may have been very effective in preparing useful and

reactive compounds of the kinds required for life. Together with compounds

formed on the Earth, those extraterrestrial molecules could have helped to

jump-start life.’

Comets now figure in such a wide range of theories about life’s origin, that a

checklist may be appropriate. The mainstream view in the late 20th century was

that, when comets and comet tails delivered huge quantities of loose carbon-rich

material to the Earth’s primordial soup, its precise chemical forms were

unimportant. In Ehrenfreund’s interpretation the molecules did matter, and may

have influenced the direction of subsequent chemistry on the Earth.

Quite different scenarios included the proposal that comets might be vehicles on

which spores of bacteria could hitchhike from one star system to another, or

skip between planets. Or, as Hoyle suggested, the comets might themselves be

the scene of biochemical action, creating new life aboard them. Finally,

according to a German hypothesis, comet grains may have directly mothered

living cells on the Earth.

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