Читать книгу Wonders of Life - Andrew Cohen - Страница 7
ОглавлениеOn Christmas Eve 1968, Frank Borman, Jim Lovell and William Anders became the first humans in history to lose sight of their home planet as they orbited the Moon on board Apollo 8. As Borman looked into the crystal dark, pitted by the faint light of a billion worlds untarnished by atmospheric gases, framed by a virgin lunar surface unseen since its formation 4.5 billion years ago, he described his universe without Earth as a ‘vast, lonely, forbidding expanse of nothing’. On the ninth orbit, the crew made a scheduled live television broadcast in which they chose to read the Genesis creation story back across the quarter of a million miles to Earth.
‘We are now approaching lunar sunrise and, for all the people back on Earth, the crew of Apollo 8 has a message that we would like to send to you:
In the beginning God created the heavens and the Earth.
And the Earth was without form, and void; and darkness was upon the face of the deep.’
The act of reading from the Bible proved controversial, and was challenged in court as a violation of the 1st amendment of the United States Constitution, which prevents the promotion of religion by the federal government, of which NASA is a part. The Supreme Court dismissed the case, on the grounds that it had no jurisdiction in lunar orbit.
While the Genesis story is a myth, I have always found this broadcast moving; not merely because the King James version of the Bible contains some of the greatest prose ever written in the English language, but because it speaks to an ancient, resonant desire to understand our origin and the origin of our home. Why is the Earth a living oasis amid, as far as anyone can tell, a forbidding expanse of nothing? What is special about our pale blue anomaly of a world that makes it home to life?
These questions are complex, and we do not yet have all the answers, but there is a scientific consensus on at least some of the ingredients a planet requires to allow the emergence of life and the evolution of complex organisms capable of taking their first, faltering steps into a wider Universe. Many of those ingredients are common throughout the Solar System and beyond, but we have as yet no evidence for life, simple or complex, beyond Earth. That may be because the emergence of living things required a significant slice of luck and billions of years of relative stability; spacecraft builders may be a rare and precious commodity.
This thought may have been adrift somewhere in Frank Borman’s consciousness, catalysed by his feelings of isolation 400,000 km from home, when he ended the 1968 Christmas broadcast with a phrase I have always found overpowering in its simplicity and depth of meaning. To me, it was an instinctive plea to all of us to value our home – the absolutely necessary platform for the continued existence of, just possibly, the only living civilisation in the Universe:
‘And, from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.’
‘Earth rise’, first observed from Apollo 11 in 1969, gave us a totally new perspective on the planet we call home.
In early September each year, monarch butterflies gather in their millions east of the Rocky Mountains before migrating south to the evergreen forests of central Mexico.
With its vivid orange colour and beautiful markings, the monarch butterfly (Danaus plexippus) is a striking example of the simple aesthetic beauty of life. But as is so often the case in the natural world, the superficial beauty of these butterflies is immeasurably enhanced by a deeper scientific understanding of their life cycle and biochemistry, and the reasons for their form and function.
Each year, as autumn approaches across Canada and the northern United States, millions of monarch butterflies begin preparations for an arduous expedition. To survive the harsh northern winter, they embark on one of nature’s great migrations, travelling up to 4,000 km to warmer domains in the south. It is a vast distance for such a small and seemingly fragile creature to travel, and requires the birth of a special generation of butterflies. An average adult monarch has a life span of little more than four weeks, but, when faced with the journey south, a ‘methuselah generation’ emerges; a generation that lives nearly ten times longer than its parents and grandparents.
Living for up to eight months, these butterflies carry with them the privilege of a longer life and the responsibility of carrying their genes through to the following year. As autumn begins in the forests, fields and meadows of the north, preparation starts for travel. The fading of the northern Sun, a result of Earth’s journey around the Sun coupled with the 23-degree tilt of its axis, causes temperatures to fall and food to become scarce. By early September, the young butterflies sense the shortening days and begin to gorge themselves on nectar, laying down extra layers of fat to increase their resilience. When the temperature approaches the very limits of their tolerance, they take flight. This is no random journey south. Covering up to 100 km a day, half a billion monarchs head towards a very specific location. None of them has travelled the route before, yet their destination has remained the same for thousands of years.
Of the many possible solutions to this annual challenge, the monarch butterflies have evolved into skilled navigators, using time and a star as their guide. From their starting point east of the Rocky Mountains, they journey across the great plains of the central United States into the damp humidity of the south. Along the way they face the same dangers as all long-distance travellers; illness and infection, bad weather and storms are a constant danger, and predatory birds will pick off thousands before they come close to completing their annual voyage.
But every year, despite the daunting distance and difficulties, millions of monarchs arrive in a single small area of evergreen forest in the heart of central Mexico. Populations of monarchs that were living west of the Rockies will have made a similar, though shorter, voyage to safety in southern California.
USING SPECTRAL GRADIENTS TO FIND THE POSITION OF THE SUN: Long wavelengths (green light) dominate the solar hemisphere, and shorter wavelengths (violet) dominate the anti-solar hemisphere.
The monarchs navigate like eighteenth-century explorers, using the position of the Sun in the sky and an internal clock to guide them. Taking a southerly bearing using the Sun is simple if you know the time. At noon in the northern hemisphere, the Sun will always be due south. This can be taken as a definition of noon. You can take a southerly bearing at other times of day if you have a watch. Point the hour hand at the Sun, and the line halfway between the hour hand and the 12 o’clock mark will point due south. The monarchs use a sophisticated version of this technique – known as a time-compensated Sun compass – to maintain their southerly orientation during their migration.
This magnified image of the head of a butterfly clearly shows its long, segmented antennae, its two segmented eyes, and its tightly coiled proboscis – the three most important sensory organs.
The butterflies measure the position of the Sun using their sophisticated eyes, which can detect the polarisation of sunlight, enabling them to ‘see’ the position of the Sun, even through cloud. They are also thought to use ‘spectral gradients’, whereby the precise mixture of colours in any given patch of sky depends on how close it is to the Sun. This is due to the way that different wavelengths of sunlight scatter in the atmosphere, an effect that is most familiar in the reddening of the sky at sunset and sunrise.
The nature of the monarch’s clock is more elusive. Biological clocks are ubiquitous in nature and thought to be a very ancient evolutionary invention. Circadian rhythms, which require the beating of an internal biological clock, are found in every corner of the biosphere, from the most complex of mammals to the simplest of bacteria. It is possible that biological clocks could have emerged as a form of protection against the destructive effects of the Sun’s radiation. An organism’s DNA is most exposed to damage at the point of replication, so restricting cell division to the hours of darkness would have been advantageous. This requires a clock that is synchronised to the rotation of the Earth.
Until recently it was assumed that, in common with other animals, the monarch’s clock must reside in the brain. But an experiment conducted by neurobiologists at the University of Massachusetts Medical School in 2009 revealed that it is instead located in the delicate structure of the antennae. The reason for this unusual location is not known. Timing information from the antenna clock is combined with information on solar position from the eyes in dedicated regions deep within the butterflies’ tiny brains, and this allows them to maintain a southerly bearing on their journey to central Mexico.
For the next five months, a handful of Mexican valleys are home to a billion butterflies, clustering on the firs in such numbers that the forests are painted with a magnificent orange glow. The monarch migration is a powerful example of the way that an organism’s home is not a fixed place, but rather a set of conditions that enable it to survive. If those conditions change, it may be necessary to move.
The monarch is an evocative example of a deep truth in biology. The form and function of an animal cannot be understood in isolation. The monarch’s behaviour and biochemistry are intimately connected with its habitat, the behaviour of countless other animals and plants, and the constantly shifting seasons driven by the dynamics of the Solar System. I find it simultaneously trivial and wonderful to observe that there would be no monarch butterflies as we know them if our planet’s spin axis were not tilted; there would be no seasons, and no evolutionary imperative for migrations. The reason for the tilt is undoubtedly pure chance – a relic of our planet’s formation and history stretching back over 4.5 billion years. Jupiter and Mercury have virtually no tilt, while Uranus rotates on its side.
This poses a series of interesting questions: What are the factors that make Earth a home to such a bewilderingly rich and complex ecosystem? What is the minimal set of ingredients necessary for life to evolve, and how widespread are these ingredients in the Universe beyond Earth? Is the emergence of complex living things such as monarch butterflies, fir trees and human beings an inevitable consequence of the laws of physics, or does it rely on a home whose existence is so improbable that Earth and its living ecosystem is a rare, even unique, corner of the Milky Way galaxy, itself one of billions of galaxies in the observable Universe?
The behaviour and biochemistry of monarch butterflies cannot be understood in isolation either from their habitat or from the shifting seasons.
It is difficult to do justice in a few short paragraphs to Mexico – or, as it is more correctly known, the United Mexican States. An intense and colourful country of contradictions, it is both welcoming yet occasionally frightening, peaceful yet troubled. It has a striking veneer of colonial architecture and customs, but the magnificent architecture of its great indigenous civilisations is intact and imposing, and their ancient mythology makes a vibrant contribution to twenty-first-century global culture. What schoolchild isn’t fascinated by the Aztecs, and which New Age conspiracy theorist doesn’t read infinitely too much into the Mayans’ fascination with the creation of complex and far-reaching calendars?
Physically, Mexico covers almost 2 million sq km and is home to 112 million people. Bordering the United States of America to the north, the Pacific Ocean to the south and west, the Gulf of Mexico to the east, and Guatemala, Belize and the Caribbean Sea to the southeast, it is a land of tremendous geological and climatic variation – from lowland rainforests to pine savannahs; from fertile grasslands to high volcanic mountain ranges. Its position – straddling the Tropic of Cancer and bounded by two of the world’s great oceans – also makes it one of the most biodiverse countries on Earth. Even though it covers only 1 per cent of the land area of our planet, it is home to over 200,000 different species – at the last count, 10 per cent of Earth’s bank of life. There are 707 species of reptiles, 438 species of mammals, 290 species of amphibians and over 26,000 species of flora. This is why we chose Mexico to tell the story of the ingredients that make our world such a comfortable home for life.
HOT SPOTS OF HIGH ENDEMISM AND SIGNIFICANT THREAT OF IMMINENT EXTINCTION
Mexico is one of the most biodiverse countries on Earth. Even though it covers only 1 per cent of the land area of our planet, it is home to over 200,000 different species – 10 per cent of Earth’s bank of life.
Surface water is often scarce in the rainforests of the Yucatan peninsula, Mexico. Yet this region is one of the most biodiverse on Earth.
Cenotes (a type of sinkhole) mark the edge of a massive crater, formed 65 million years ago when an asteroid, measuring some 10 km in diameter, smashed into Earth.
The ocellated turkey (Meleagris ocellata) resides primarily in the rainforests of Mexico’s Yucatan peninsula. Only the male – shown here – has such striking plumage.
The Mexican beaded lizard (Heloderma horridum) is just one of the 707 species of reptile known to exist in Mexico.
The cenotes of the Yucatan peninsula contain remarkably clear water, which has been filtered through the porous limestone above over many thousands of years.
We began filming in the tropical rainforests of the Yucatan peninsula, where accessible water resources can be unexpectedly scarce. Large areas of the Yucatan are devoid of rivers and streams because the bedrock, composed mainly of limestone, is porous. There is a large subterranean source of fresh water, however, contained in a complex, stratified aquifer. Fortunately for the occupants of the peninsula, this underground water source is easily accessible through a series of sinkholes known as cenotes. The cenotes lead into vast networks of subterranean caverns dissolved out of the limestone over many thousands of years and flooded by the clean waters of the aquifer. The Mayans built their civilisation around cenotes, many of which lie in a strange, semicircular arc centred on a small village called Chicxulub. They mark out the edge of a giant crater, formed 65 million years ago when an asteroid 10 km in diameter smashed into Earth. Known as the Cretaceous-Paleogene extinction event (or the K-T extinction), this impact is the most widely accepted theory for the cause of the mass extinction of the dinosaurs.
The Mayans built their civilisation around cenotes, many of which lie in a strange, semicircular arc centred on a small village called Chicxulub.
The water in the cenotes is exceptionally clear because it is filtered slowly through the porous rocks of the Yucatan before emerging after thousands of years to flood this subterranean world. Diving into the clear darkness of these underground wells is a unique experience and a welcome respite from the heat and insects of the forest. As you journey deeper into the cave systems, the sunlight fades to darkness but an abundance of life can still be found. This is typical of what we find in even the most extreme conditions on the planet. Remove light, heat, soil, plants, insects, and even oxygen, and life still thrives. But one ingredient is, as far as we know, absolutely essential for life to exist.
Cenotes contain an abundance of life, and taking a dive into their crystal-clear, yet dark, world is a unique experience.
Water arguably exhibits the most complex behaviour of any known substance. This may come as a shock, because the ubiquitous familiarity of its chemical signature – H2O – is the stuff of the most basic of classroom chemistry lessons. Yet this familiarity hides a deep complexity that we are only now beginning to understand. The complexity doesn’t lie in the structure of water molecules themselves of course: each molecule is made of three atoms – two hydrogen atoms and one oxygen atom. From chemistry lessons gone by, you might recall that the two hydrogen atoms are covalently bonded to a single atom of oxygen. Oxygen has eight electrons around its nucleus, six of which are in the outer shell; these are known as ‘valence electrons’. Four of these are paired together, leaving two lone electrons that would dearly like to pair up with electrons from other atoms.1 Each hydrogen atom has a single electron, which it readily shares with the electron-hungry oxygen, and the result is a molecule of water.
However, this simple tetrahedral arrangement of a central oxygen atom surrounded by two pairs of electrons and two hydrogen atoms is deceptive, because the structure allows for tremendously complex behaviour when water molecules come together in large numbers. And, as we shall see, this unique behaviour may well make water a prerequisite for the existence of life, not only on Earth, but anywhere in the Universe. Perhaps unsurprisingly, given its dominance in our lives, scientists have been attempting to unlock its secrets for over three hundred years.
1 For those who don’t like such anthropomorphic language, it is energetically favourable for electrons with opposite spins to pair up in the available energy levels around a nucleus, and there are four available upper energy levels around the oxygen nucleus for the six electrons to occupy.
MOLECULAR GEOMETRY: Tetrahedral electron pair geometry
We often take water for granted, yet it is a remarkably complex substance, and without it there would be no life, not only on Earth, but anywhere in the Universe.
