yufan/voice_David_Long · Datasets at Hugging Face

text string	wpm float64	snr float64	loudness float32
Chapter one, The Infinite Variety.	19.05	44.39	0.015514
It is not difficult to discover an unknown animal. Spend a day in the tropical forest of South America, turning over logs, looking beneath bark, sifting through the moist litter of leaves.	122.37	40.11	0.041854
Followed by an evening shining a mercury lamp on a white screen, and one way or another, you will collect hundreds of different kinds of small creatures, moths, caterpillars, spiders, long nosed bugs, luminous beetles.	139.17	38.6	0.049352
Harmless butterflies disguised as wasps, wasps shaped like ants, sticks that walk leaves that open wings and fly. The variety will be enormous, and one of these creatures is quite likely to be undescribed by science.	132.6	42.72	0.045917
The difficulty will be to find specialists who know enough about the groups concerned to be able to single out the new one. No one can say just how many species of animals there are in these greenhouse, humid, dimly lit jungles.	147.37	59.69	0.048527
They contain the richest and the most varied assemblage of animals and plant life to be found anywhere on earth. Not only are there many major categories of creatures, monkeys, rodents, spiders, hummingbirds, butterflies.	126.16	59.63	0.060475
But most of those types exist in many different forms. There are over 40 different species of parrot, over 70 different monkeys, 300 hummingbirds and 10s of 1000s of butterflies. If you're not careful, you can even be bitten by a hundred different kinds of mosquito.	133.72	61.71	0.048294
In 1832, a young Englishman, Charles Darwin, 24 years old, and the naturalist on H. M. S. Beagle, a brig sent out by the Admiralty in London on a surveying voyage round the world, came to such a forest outside Rio de Janeiro.	140.07	56.29	0.057404
In one day, in one small area, he collected 68 different species of small beetle. That there should be such a variety of species of one kind of creature astounded him. He had not been searching specially for them.	142.34	66.28	0.058405
So that as he wrote in his journal, it is sufficient to disturb the composure of an entomologist is mind to look forward to the future dimensions of a complete catalog, the conventional view of his time.	136.78	57.32	0.063388
Was that all species were immutable and that each had been individually and separately created by God. At the time, Darwin was far from being an atheist. He had, after all, taken a degree in divinity at Cambridge University, but he was deeply puzzled by this enormous multiplicity of forms.	141.64	57.29	0.06611
During the next 3 years, the Beagle sailed down the east coast of South America, rounded Cape Horn, and came north again up the coast of Chile. The expedition then sailed out into the Pacific, until a 1000 kilometers from the mainland, they came to the lonely archipelago of the Galapagos.	142.06	62.46	0.073317
Here Darwin is questions about the creation of species recurred, for in these islands he found fresh variety. He was fascinated to discover that the Galapagos animals bore a general resemblance to those he had seen on the mainland.	130.65	58.01	0.070419
But differed from them in detail, there were cormorants, black, long necked diving birds, like those that fly low along Brazilian rivers. But here in the Galapagos, their wings were so small and with such stunted feathers that they had lost the power of flight.	139.97	63.97	0.061039
There were iguanas, large lizards, with a crest of scales along their backs. Those on the continent climbed trees and ate leaves here on the islands where there was little vegetation. One species fed on seaweed and clung to the rocks among the surging waves with unusually long and powerful claws.	134.74	62.02	0.066371
There were tortoises very similar to the mainland forms, except that these were many times bigger giants that a man could ride. The British vice governor of the Galapagos told Darwin that even within the archipelago there was variety.	133.33	63.64	0.059155
The tortoises on each island were slightly different, so that it was possible to tell which island they came from. Those that lived on relatively well watered islands where there was ground vegetation to be cropped, had a gently curving front edge to their shells just above the neck.	168.1	56.5	0.06579
But those that came from arid islands and had to crane their necks in order to reach branches of cactus or leaves of trees, had much longer necks and a high peak to the front of their shells that enabled them to stretch their necks almost vertically upwards.	158.15	56.4	0.058631
The suspicion grew in Darwin is mind that species were not fixed forever. Perhaps one could change into another. Maybe 1000s of years ago, birds and reptiles from continental South America had reached the Galapagos.	114.19	65.78	0.053752
Unintentional passengers on the rafts of vegetation that float down the rivers and out to sea. Once there, they had changed as generation succeeded generation to suit their new homes until they became.	127.66	60.85	0.060097
Their present species, the differences between them and their mainland cousins were only small. But if such changes had taken place, was it not possible that over many 1000000s of years, the cumulative effect on a dynasty of animals could be so great?	131.7	64.11	0.066048
That they could bring about major transformations. Maybe fish had developed muscular fins and crawled onto the land to become amphibians. Maybe amphibians in their turn had developed watertight skins and become reptiles.	115.99	63.96	0.065975
Maybe even some ape like creatures had stood upright and become the ancestors of mankind. In truth, the idea was not a wholly new one. Many others before Darwin had suggested that all life on Earth was interrelated.	123.58	66.98	0.062879
Darwin's revolutionary insight was to perceive the mechanism that brought these changes about. By doing so, he replaced a philosophical speculation with a detailed description of a process supported by an abundance of evidence that could be tested and verified.	119.58	60.27	0.067143
And the reality of evolution could no longer be denied. Put briefly, his argument was this, all individuals of the same species are not identical in one clutch of eggs from, for example, a giant tortoise. There will be some hatchlings, which, because of their genetic constitution, will develop slightly longer necks than others.	125.14	74.07	0.060925
In times of drought, they will be able to reach leaves and so survive. Their brothers and sisters with shorter necks will starve and die, so those best fitted to their surroundings will be selected and be able to transmit their characteristics to their offspring.	143.31	63.46	0.060602
After a great number of generations, tortoises on the arid islands will have longer necks than those on the watered islands. One species will have given rise to another. This concept did not become clear in Darwin is mind until long after he had left the Galapagos.	134.48	66.16	0.058257
For 25 years, he painstakingly amassed evidence to support it. Not until 1859, when he was 48 years old, did he publish it, and even then he was driven to do so only because another younger naturalist, Alfred Wallace.	131.09	56.89	0.073867
Working in Southeast Asia had formulated the same idea. He called the book in which he set out his theory and detail on the origin of species by means of natural selection or the preservation of favored races in the struggle for life.	137.38	58.99	0.067311
Since that time, the theory of natural selection has been debated and tested, refined, qualified and elaborated. Later discoveries about genetics, molecular biology, population dynamics and behavior have given it new dimensions.	99.07	63.38	0.061433
It remains the key to our understanding of the natural world, and it enables us to recognize that life has a long and continuous history during which organisms, both plant and animal, have changed generation by generation.	133.33	58.23	0.076195
As they colonized all parts of the world, there are now two direct sources of evidence for this history. One can be found in the genetic material in the cells of every living organism. The other lies in the archives of the earth, the sedimentary rocks.	141.54	66.5	0.059951
The vast majority of animals leave no trace of their existence after their passing, their flesh decays, their shells and their bones become scattered and turned to powder, but very occasionally one or 2 individuals out of a population of many 1000s have a different fate.	143.3	68.75	0.062675
A reptile becomes stuck in a swamp and dies. Its body rots, but its bones settle into the mud. Dead vegetation drifts at the bottom and covers them. As the centuries pass and more vegetation accumulates, the deposit turns to peat.	138.75	63.46	0.060389
Changes in the sea level cause the swamp to be flooded and layers of sand to be deposited on top of the peat. Over great periods of time, the peat is compressed and turned to coal. The reptile is bones still remain within it.	163.27	64.52	0.07038
The great pressure of the overlying sediments and the mineral rich solutions that circulate through them cause chemical changes in the calcium phosphate of the bones. Eventually they are turned to stone, but they retain the outward shape that they had in life, albeit sometimes distorted.	137.11	59.29	0.071757
On occasion, even their detailed cellular structure is preserved so that you can look at sections of them through the microscope and plot the shape of the blood vessels and the nerves that once surrounded them. In rare cases, even the color of skin or feathers can be detected.	160.66	57.99	0.072125
The most suitable places for fossilization are in seas and lakes where sedimentary deposits that will become sandstones and limestones are slowly accumulating on land where for the most part rocks are not built up by deposition but broken down by erosion deposits such as sand dunes are only very rarely created and preserved.	137.76	56.88	0.068846
In consequence, the only land living organisms likely to be fossilized are those that happen to fall into water. Since this is an exceptional fate for most of them, we're never likely to know from fossil evidence anything approaching the complete range of land living animals and plants that has existed in the past.	153.41	52.49	0.068687
Water living animals such as fish, molluscs, sea urchins and corals are much more promising candidates for preservation. Even so, very few of these perished in the exact physical and chemical conditions necessary for fossilization.	121.53	63.56	0.062281
Of those that did, only a tiny proportion happen to lie in the rocks that outcrop on the surface of the ground today, and of these few, most will be eroded away and destroyed before they are discovered by fossil hunters. The astonishment is that in the face of these adverse odds.	166.4	57.41	0.067517
The fossils that have been collected are so numerous and the record they provide so detailed and coherent. How can we date them? Since the discovery of radioactivity, scientists have realized that rocks have a geological clock within them.	129.78	73.82	0.062219
Several chemical elements decay with age, producing radioactivity in the process. Potassium turns into argon, uranium into lead, rubidium into strontium. The rate at which this happens can be estimated.	99.34	66.51	0.059374
So if the proportion of the secondary element to the primary one in the rock is measured, the time at which the original mineral was formed can be calculated. Since there are several such pairs of elements decaying at different speeds, it's possible to make cross checks.	153.85	64.88	0.062284
This technique, which requires extremely sophisticated methods of analysis, will always remain the province of the specialist, but anyone can date many rocks in a relative way by simple logic. If rocks lie in layers and are not grossly disturbed, then the lower layer must be older than the upper.	138.31	57.64	0.067291
So we can follow the history of life through the strata and trace the lineages of animals back to their beginnings by going deeper and deeper into the earth's crust, the deepest cleft that exists in the earth's surface.	134.25	67.01	0.063715
Is the Grand Canyon in the western United States the rocks through which the Colorado River has cut its way still lie roughly horizontally layer upon layer red brown and yellow sometimes pink in the early light sometimes blue in the shadow distance.	140.07	60.45	0.067562
The land is so dry that only isolated juniper trees and low scrub freckle the surface of the cliffs, and the rock strata, some soft, some hard, are clear and stark. Most of them are sandstones or limestones that were laid down at the bottom of the shallow seas that once covered this part of North America.	155.31	59.2	0.069778
When they are examined closely, breaks in the succession can be detected. These represent times when the land rose, the sea drained away, and the seabed became dry, so that the deposits that had accumulated on it were eroded away.	150.09	61.21	0.075113
Subsequently, the land sank again, seas flooded back and deposition restarted. In spite of these gaps, the broad lines of the fossil story remain clear. A mule will carry you in an easy day's ride from the rim to the very bottom of the canyon.	131.58	71.89	0.051387
The 1st rocks you pass are already some 200000000 years old. There are no remains of mammals or birds in them, but there are traces of reptiles close by the side of the trail. You can see a line of tracks crossing the face of a sandstone boulder.	161.34	69.71	0.0672
They were made by a small 4 footed creature, almost certainly a lizard like reptile running across a beach. Other rocks at the same level elsewhere contain impressions of fern leaves and the wings of insects.	138.2	64.08	0.065797
Halfway down the canyon, you come to 400000000 year old limestones. There are no signs of reptiles to be found here, but there are the bones of strange armored fish. An hour or so later, and a 100000000 years earlier, the rocks contain no sign of backbone animals of any kind.	137.65	64.08	0.065031
There are a few shells and worms that have left behind a tracery of trails in what was the muddy sea floor 3 quarters of the way down. You are still descending through layers of limestone, but now there is no sign of fossilized life.	149.5	62.84	0.056937
Whatsoever, by the late afternoon, you ride at last into the lower gorge where the Colorado River runs green between high rock walls. You are now well over a vertical kilometer below the rim.	129.77	67.6	0.070182
And the surrounding rocks have been dated to the immense age of 2000000 years. Here you might hope to find evidence for the very beginning of life, but there are no organic remains of any kind.	136.36	63.49	0.062007
The dark, fine grained rocks lie not in horizontal layers like all those above, but are twisted and buckled and riven with veins of pink granite. Are signs of life absent?	120.16	62.79	0.055841
Because these rocks and the limestones directly above are so extremely ancient that all such traces have been crushed from them. Could it be that the 1st creatures to leave any sign of their existence were as complex as worms and molluscs?	154.98	58.84	0.058815
For many years, these questions puzzled geologists all over the world. Rocks of this antiquity were carefully searched for organic remains. One or 2 odd shapes were found, but most authorities dismissed these as patterns produced by the physical processes of rock formation that had nothing whatever to do with living organisms.	130.49	61.39	0.060942
Then, during the 1950s, the searchers began to use high-powered microscopes on some particularly enigmatic rocks around 1600 kilometers northeast of the Grand Canyon, ancient rocks of about the same age as those beside the Colorado River.	107.71	65.88	0.055551
Outcrop on the shores of Lake Superior, some of them contain seams of a fine grained flint like substance called chert. This was well known during the 19th century because the pioneers used it in their flintlock guns.	142.06	65.87	0.056598
Here and there it contains strange white concentric rings a metre or so across. Were these merely eddies in the mud on the bottom of the primeval seas, or could they have been formed by living organisms?	132.62	66.3	0.060179
No one could be sure, and the shapes were given the noncommittal name of stromatolite, a word derived from Greek, meaning no more than stony carpet. But when researchers cut sections of these rings.	132.3	63.95	0.064254
Ground them down into slices so thin that they were translucent and examined them through the microscope. They found preserved in the chert the shapes of simple organisms, each no more than one or two hundredths of a millimeter across.	151.23	61.78	0.066395
Some resembled filaments of algae, others, while they were unmistakably organic, had no parallels with living organisms, and some looked to be identical with the simplest form of life existing today.	123.75	60.69	0.057809
Bacteria, it seemed almost impossible to many people that such tiny things as micro organisms could have been fossilized at all. That relics of them should have survived for such a vast period of time seemed even more difficult to believe.	137.82	70.61	0.052989
The solution of silica which had saturated the dead organisms and solidified into chert was clearly as fine grained and durable a preservative as exists. The discovery of the fossils in the gunflint chert stimulated further searches not only in North America but all over the world.	137.83	58.53	0.063216
And other micro fossils were found in church in Africa and Australia. Some of these astonishingly predated the gun flint specimens by a billion years. And some scientists now claim to have found fossils from around four billion years ago.	125.2	60.02	0.061533
Not long after the formation of the Earth. But if we want to consider how life arose, fossils can't help us. For the origin of life involved the interaction of molecules which leave no fossil traces.	136.36	62.98	0.056589
To understand what scientists think happened, we have to look back beyond even the earliest microfossils to a time when the Earth was completely lifeless. In many ways, the planet then was radically different from the one we live on today.	137.12	66.44	0.059892
There were seas, but the way the land masses lay bore no resemblance in either form or distribution to modern continents. Volcanoes were abundant, spewing noxious gases, ash and lava.	118.34	60.85	0.058347
The atmosphere consisted of swirling clouds of hydrogen, carbon monoxide, ammonia and methane. There was little or no oxygen. This unbreathable mixture allowed ultraviolet rays from the sun to bathe the earth surface with an intensity that would be lethal to modern animal life.	117.02	64.13	0.068448
Electrical storms raged in the clouds, bombarding the land and the sea with lightning. Laboratory experiments were made in the 1950s to discover what might happen to these particular chemical constituents under such conditions.	120.35	61.88	0.063505
Such gases mixed with water vapor were subjected to electrical discharge and ultraviolet light. After only a week of this treatment, complex molecules were found to have formed in the mixture, including sugars, nucleic acids and amino acids, the building blocks of proteins.	129.52	59.8	0.066689
We now know that such simple organic molecules can be found throughout the universe, including on interstellar bodies such as comets, but amino acids are not life, nor are they even necessary for life to exist.	139.53	58.73	0.073044
The experiment proved little about the origin of life. All forms of life that exist today share a common way of transmitting genetic information of telling cells what to do. It is a molecule called deoxyribonucleic acid.	114.02	66.2	0.058133
Or DNA, for short, its structure gives it two key properties. First, it can act as a blueprint for the manufacture of amino acids, and second, it has the ability to replicate itself with this substance.	126.98	65.24	0.04959
Molecules had reached the threshold of something quite new. These two characteristics of DNA also characterize even the simplest of living organisms, such as bacteria and bacteria. Besides being the simplest form of life, we know are also among the oldest fossils we have discovered.	125.7	62.4	0.053731
The ability of DNA to replicate itself is a consequence of its unique structure. It's shaped like 2 intertwined helices. During cell division, these unzip, splitting the molecule along its length into 2 separate helices.	108.36	70.59	0.052634
Each then acts as a template to which other simpler molecules become attached until each has once more become a double helix. The simple molecules from which the dna is mainly built are of only 4 kinds, but they are grouped in trios.	132.92	65.84	0.055879
And arranged in a particular and significant order on the immensely long dna molecule. This order specifies how the 20 or so different amino acids are arranged in a protein. How much is to be made in what tissue.	133.56	61.93	0.066997
And when a length of DNA bearing such information for a protein or for how a protein should be expressed is called a gene occasionally the DNA copying process involved in reproduction.	114.49	71.08	0.057108
May go wrong. A mistake may be made at a single point, or a length of DNA may become temporarily dislocated and be re inserted in the wrong place. The copy is then imperfect.	130.77	71.07	0.057752
And the protein it will create may be entirely different. Changes in the dna sequence can also be induced by chemicals or radiation when this occurred in the 1st organisms on earth.	120.98	65.82	0.056059
Evolution began for such hereditary changes brought about by mutation and errors are the source of variations from which natural selection can produce evolutionary change. Because all life shares DNA as the hereditary material, it's possible to compare DNA sequences in different organisms and show how they're related.	109.09	58.84	0.057291
Such is the progressive technology that it is now also possible to sequence all the dna in an organism in a matter of hours using a device the size of a mobile phone. The 1000000s of dna sequences that have been established stored in databases and compared show us unequivocally that just as darwin predicted all life on earth shares a common ancestor.	132.21	59.65	0.063412
Because parts of our DNA accumulate mutations at a constant rate like a molecular clock, we can use DNA sequences to estimate when two species split apart. In general, genetic and fossil timings agree with each other, although genetic data do sometimes throw up surprises.	128.21	58.97	0.064482
Using this method we can estimate that the last universal common ancestor of all life on earth commonly known as LUCA Leuka and basically a population of simple bacteria lived around four billion years ago.	109.89	61.1	0.060465
Everything we can see around us can trace its ancestry back to that group of cells. Such vast periods of time baffle the imagination, but we can form some idea of the relative duration of the major phases of the history of life if we compare the entire span from these 1st beginnings until today with one year.	139.93	62.42	0.05316
That means that roughly each day represents around 10000000 years. On such a calendar, the gunflint fossils of algae like organisms, which seemed so extremely ancient when they were 1st discovered, are seen to be quite latecomers in the history of life, not appearing until the 2nd week of August.	139.08	59.16	0.059921
In the Grand Canyon, the oldest worm trails were burrowed through the mud in the 2nd week of November, and the 1st fish appeared in the Limestone Seas a week later. The little lizard will have scuttled across the beach during the middle of December.	154.64	61.89	0.069158
And humans didn't appear until the evening of 31st of December. But we must return to January. The bacteria fed initially on the various carbon compounds that had taken so many 1000000s of years to accumulate in the primordial seas, producing methane as a by product.	126.55	73.28	0.052617
Similar bacteria still exist today all over the planet, and that was all there was for around 5 or 6 months of our year. Then, in the early summer of the year of life, so sometime over 2000000000 years ago, bacteria developed an amazing biochemical trick.	129.4	63.83	0.060682
Instead of taking readymade food from their surroundings, they began to manufacture their own within their cell walls, drawing the energy needed to do so from the sun. This process is called photosynthesis.	124.06	66.59	0.062268
One of the ingredients required by the earliest form of photosynthesis is hydrogen, a gas that is produced in great quantities during volcanic eruptions. Conditions very similar to those in which the early photosynthesizing bacteria lived can be found today	116.45	65.95	0.056124
In such volcanic areas as Yellowstone in Wyoming here a great mass of molten rock lying only a thousand meters or so down in the earth's crust heats the rocks on the surface in places the groundwater is well above boiling point.	138.16	62.85	0.055026
It rises up channeled through the rocks under decreasing pressure until suddenly it flashes into steam and water spouts high into the air as a geyser. Elsewhere, the water wells up into steaming pools.	133.6	62.92	0.059081
As it trickles away and cools, the salts it gathered from the rocks on its way up, together with those derived from the molten mass far below, are deposited to form rimmed and buttressed basins surrounded by tiers of terraces.	159.36	54.04	0.069656
In these scolding mineral laden waters, bacteria flourish, some grow into matted filaments and curds, others into thick leathery sheets. Many are brilliantly colored, their intensity of hue varying during the year as the colonies wax and wane.	112.26	67.79	0.072941
The names given to these pools hint at the variety of the bacteria and the splendour of the effects they produce emerald pool, sulfur cauldron, beryl spring, far holed falls, morning glory pool, and a particularly rich one with several species of bacteria.	117.81	62.23	0.051424

Chapter one, The Infinite Variety.

19.05

44.39

0.015514

It is not difficult to discover an unknown animal. Spend a day in the tropical forest of South America, turning over logs, looking beneath bark, sifting through the moist litter of leaves.

122.37

40.11

0.041854

Followed by an evening shining a mercury lamp on a white screen, and one way or another, you will collect hundreds of different kinds of small creatures, moths, caterpillars, spiders, long nosed bugs, luminous beetles.

139.17

38.6

0.049352

Harmless butterflies disguised as wasps, wasps shaped like ants, sticks that walk leaves that open wings and fly. The variety will be enormous, and one of these creatures is quite likely to be undescribed by science.

132.6

42.72

0.045917

The difficulty will be to find specialists who know enough about the groups concerned to be able to single out the new one. No one can say just how many species of animals there are in these greenhouse, humid, dimly lit jungles.

147.37

59.69

0.048527

They contain the richest and the most varied assemblage of animals and plant life to be found anywhere on earth. Not only are there many major categories of creatures, monkeys, rodents, spiders, hummingbirds, butterflies.

126.16

59.63

0.060475

But most of those types exist in many different forms. There are over 40 different species of parrot, over 70 different monkeys, 300 hummingbirds and 10s of 1000s of butterflies. If you're not careful, you can even be bitten by a hundred different kinds of mosquito.

133.72

61.71

0.048294

In 1832, a young Englishman, Charles Darwin, 24 years old, and the naturalist on H. M. S. Beagle, a brig sent out by the Admiralty in London on a surveying voyage round the world, came to such a forest outside Rio de Janeiro.

140.07

56.29

0.057404

In one day, in one small area, he collected 68 different species of small beetle. That there should be such a variety of species of one kind of creature astounded him. He had not been searching specially for them.

142.34

66.28

0.058405

So that as he wrote in his journal, it is sufficient to disturb the composure of an entomologist is mind to look forward to the future dimensions of a complete catalog, the conventional view of his time.

136.78

57.32

0.063388

Was that all species were immutable and that each had been individually and separately created by God. At the time, Darwin was far from being an atheist. He had, after all, taken a degree in divinity at Cambridge University, but he was deeply puzzled by this enormous multiplicity of forms.

141.64

57.29

0.06611

During the next 3 years, the Beagle sailed down the east coast of South America, rounded Cape Horn, and came north again up the coast of Chile. The expedition then sailed out into the Pacific, until a 1000 kilometers from the mainland, they came to the lonely archipelago of the Galapagos.

142.06

62.46

0.073317

Here Darwin is questions about the creation of species recurred, for in these islands he found fresh variety. He was fascinated to discover that the Galapagos animals bore a general resemblance to those he had seen on the mainland.

130.65

58.01

0.070419

But differed from them in detail, there were cormorants, black, long necked diving birds, like those that fly low along Brazilian rivers. But here in the Galapagos, their wings were so small and with such stunted feathers that they had lost the power of flight.

139.97

63.97

0.061039

There were iguanas, large lizards, with a crest of scales along their backs. Those on the continent climbed trees and ate leaves here on the islands where there was little vegetation. One species fed on seaweed and clung to the rocks among the surging waves with unusually long and powerful claws.

134.74

62.02

0.066371

There were tortoises very similar to the mainland forms, except that these were many times bigger giants that a man could ride. The British vice governor of the Galapagos told Darwin that even within the archipelago there was variety.

133.33

63.64

0.059155

The tortoises on each island were slightly different, so that it was possible to tell which island they came from. Those that lived on relatively well watered islands where there was ground vegetation to be cropped, had a gently curving front edge to their shells just above the neck.

168.1

56.5

0.06579

But those that came from arid islands and had to crane their necks in order to reach branches of cactus or leaves of trees, had much longer necks and a high peak to the front of their shells that enabled them to stretch their necks almost vertically upwards.

158.15

56.4

0.058631

The suspicion grew in Darwin is mind that species were not fixed forever. Perhaps one could change into another. Maybe 1000s of years ago, birds and reptiles from continental South America had reached the Galapagos.

114.19

65.78

0.053752

Unintentional passengers on the rafts of vegetation that float down the rivers and out to sea. Once there, they had changed as generation succeeded generation to suit their new homes until they became.

127.66

60.85

0.060097

Their present species, the differences between them and their mainland cousins were only small. But if such changes had taken place, was it not possible that over many 1000000s of years, the cumulative effect on a dynasty of animals could be so great?

131.7

64.11

0.066048

That they could bring about major transformations. Maybe fish had developed muscular fins and crawled onto the land to become amphibians. Maybe amphibians in their turn had developed watertight skins and become reptiles.

115.99

63.96

0.065975

Maybe even some ape like creatures had stood upright and become the ancestors of mankind. In truth, the idea was not a wholly new one. Many others before Darwin had suggested that all life on Earth was interrelated.

123.58

66.98

0.062879

Darwin's revolutionary insight was to perceive the mechanism that brought these changes about. By doing so, he replaced a philosophical speculation with a detailed description of a process supported by an abundance of evidence that could be tested and verified.

119.58

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And the reality of evolution could no longer be denied. Put briefly, his argument was this, all individuals of the same species are not identical in one clutch of eggs from, for example, a giant tortoise. There will be some hatchlings, which, because of their genetic constitution, will develop slightly longer necks than others.

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In times of drought, they will be able to reach leaves and so survive. Their brothers and sisters with shorter necks will starve and die, so those best fitted to their surroundings will be selected and be able to transmit their characteristics to their offspring.

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After a great number of generations, tortoises on the arid islands will have longer necks than those on the watered islands. One species will have given rise to another. This concept did not become clear in Darwin is mind until long after he had left the Galapagos.

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For 25 years, he painstakingly amassed evidence to support it. Not until 1859, when he was 48 years old, did he publish it, and even then he was driven to do so only because another younger naturalist, Alfred Wallace.

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Working in Southeast Asia had formulated the same idea. He called the book in which he set out his theory and detail on the origin of species by means of natural selection or the preservation of favored races in the struggle for life.

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Since that time, the theory of natural selection has been debated and tested, refined, qualified and elaborated. Later discoveries about genetics, molecular biology, population dynamics and behavior have given it new dimensions.

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It remains the key to our understanding of the natural world, and it enables us to recognize that life has a long and continuous history during which organisms, both plant and animal, have changed generation by generation.

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As they colonized all parts of the world, there are now two direct sources of evidence for this history. One can be found in the genetic material in the cells of every living organism. The other lies in the archives of the earth, the sedimentary rocks.

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The vast majority of animals leave no trace of their existence after their passing, their flesh decays, their shells and their bones become scattered and turned to powder, but very occasionally one or 2 individuals out of a population of many 1000s have a different fate.

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A reptile becomes stuck in a swamp and dies. Its body rots, but its bones settle into the mud. Dead vegetation drifts at the bottom and covers them. As the centuries pass and more vegetation accumulates, the deposit turns to peat.

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Changes in the sea level cause the swamp to be flooded and layers of sand to be deposited on top of the peat. Over great periods of time, the peat is compressed and turned to coal. The reptile is bones still remain within it.

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The great pressure of the overlying sediments and the mineral rich solutions that circulate through them cause chemical changes in the calcium phosphate of the bones. Eventually they are turned to stone, but they retain the outward shape that they had in life, albeit sometimes distorted.

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On occasion, even their detailed cellular structure is preserved so that you can look at sections of them through the microscope and plot the shape of the blood vessels and the nerves that once surrounded them. In rare cases, even the color of skin or feathers can be detected.

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The most suitable places for fossilization are in seas and lakes where sedimentary deposits that will become sandstones and limestones are slowly accumulating on land where for the most part rocks are not built up by deposition but broken down by erosion deposits such as sand dunes are only very rarely created and preserved.

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In consequence, the only land living organisms likely to be fossilized are those that happen to fall into water. Since this is an exceptional fate for most of them, we're never likely to know from fossil evidence anything approaching the complete range of land living animals and plants that has existed in the past.

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Water living animals such as fish, molluscs, sea urchins and corals are much more promising candidates for preservation. Even so, very few of these perished in the exact physical and chemical conditions necessary for fossilization.

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Of those that did, only a tiny proportion happen to lie in the rocks that outcrop on the surface of the ground today, and of these few, most will be eroded away and destroyed before they are discovered by fossil hunters. The astonishment is that in the face of these adverse odds.

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The fossils that have been collected are so numerous and the record they provide so detailed and coherent. How can we date them? Since the discovery of radioactivity, scientists have realized that rocks have a geological clock within them.

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Several chemical elements decay with age, producing radioactivity in the process. Potassium turns into argon, uranium into lead, rubidium into strontium. The rate at which this happens can be estimated.

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So if the proportion of the secondary element to the primary one in the rock is measured, the time at which the original mineral was formed can be calculated. Since there are several such pairs of elements decaying at different speeds, it's possible to make cross checks.

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This technique, which requires extremely sophisticated methods of analysis, will always remain the province of the specialist, but anyone can date many rocks in a relative way by simple logic. If rocks lie in layers and are not grossly disturbed, then the lower layer must be older than the upper.

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So we can follow the history of life through the strata and trace the lineages of animals back to their beginnings by going deeper and deeper into the earth's crust, the deepest cleft that exists in the earth's surface.

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Is the Grand Canyon in the western United States the rocks through which the Colorado River has cut its way still lie roughly horizontally layer upon layer red brown and yellow sometimes pink in the early light sometimes blue in the shadow distance.

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The land is so dry that only isolated juniper trees and low scrub freckle the surface of the cliffs, and the rock strata, some soft, some hard, are clear and stark. Most of them are sandstones or limestones that were laid down at the bottom of the shallow seas that once covered this part of North America.

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When they are examined closely, breaks in the succession can be detected. These represent times when the land rose, the sea drained away, and the seabed became dry, so that the deposits that had accumulated on it were eroded away.

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Subsequently, the land sank again, seas flooded back and deposition restarted. In spite of these gaps, the broad lines of the fossil story remain clear. A mule will carry you in an easy day's ride from the rim to the very bottom of the canyon.

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The 1st rocks you pass are already some 200000000 years old. There are no remains of mammals or birds in them, but there are traces of reptiles close by the side of the trail. You can see a line of tracks crossing the face of a sandstone boulder.

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They were made by a small 4 footed creature, almost certainly a lizard like reptile running across a beach. Other rocks at the same level elsewhere contain impressions of fern leaves and the wings of insects.

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Halfway down the canyon, you come to 400000000 year old limestones. There are no signs of reptiles to be found here, but there are the bones of strange armored fish. An hour or so later, and a 100000000 years earlier, the rocks contain no sign of backbone animals of any kind.

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There are a few shells and worms that have left behind a tracery of trails in what was the muddy sea floor 3 quarters of the way down. You are still descending through layers of limestone, but now there is no sign of fossilized life.

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Whatsoever, by the late afternoon, you ride at last into the lower gorge where the Colorado River runs green between high rock walls. You are now well over a vertical kilometer below the rim.

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And the surrounding rocks have been dated to the immense age of 2000000 years. Here you might hope to find evidence for the very beginning of life, but there are no organic remains of any kind.

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The dark, fine grained rocks lie not in horizontal layers like all those above, but are twisted and buckled and riven with veins of pink granite. Are signs of life absent?

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Because these rocks and the limestones directly above are so extremely ancient that all such traces have been crushed from them. Could it be that the 1st creatures to leave any sign of their existence were as complex as worms and molluscs?

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For many years, these questions puzzled geologists all over the world. Rocks of this antiquity were carefully searched for organic remains. One or 2 odd shapes were found, but most authorities dismissed these as patterns produced by the physical processes of rock formation that had nothing whatever to do with living organisms.

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Then, during the 1950s, the searchers began to use high-powered microscopes on some particularly enigmatic rocks around 1600 kilometers northeast of the Grand Canyon, ancient rocks of about the same age as those beside the Colorado River.

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Outcrop on the shores of Lake Superior, some of them contain seams of a fine grained flint like substance called chert. This was well known during the 19th century because the pioneers used it in their flintlock guns.

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Here and there it contains strange white concentric rings a metre or so across. Were these merely eddies in the mud on the bottom of the primeval seas, or could they have been formed by living organisms?

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No one could be sure, and the shapes were given the noncommittal name of stromatolite, a word derived from Greek, meaning no more than stony carpet. But when researchers cut sections of these rings.

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Ground them down into slices so thin that they were translucent and examined them through the microscope. They found preserved in the chert the shapes of simple organisms, each no more than one or two hundredths of a millimeter across.

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Some resembled filaments of algae, others, while they were unmistakably organic, had no parallels with living organisms, and some looked to be identical with the simplest form of life existing today.

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Bacteria, it seemed almost impossible to many people that such tiny things as micro organisms could have been fossilized at all. That relics of them should have survived for such a vast period of time seemed even more difficult to believe.

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The solution of silica which had saturated the dead organisms and solidified into chert was clearly as fine grained and durable a preservative as exists. The discovery of the fossils in the gunflint chert stimulated further searches not only in North America but all over the world.

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And other micro fossils were found in church in Africa and Australia. Some of these astonishingly predated the gun flint specimens by a billion years. And some scientists now claim to have found fossils from around four billion years ago.

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Not long after the formation of the Earth. But if we want to consider how life arose, fossils can't help us. For the origin of life involved the interaction of molecules which leave no fossil traces.

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To understand what scientists think happened, we have to look back beyond even the earliest microfossils to a time when the Earth was completely lifeless. In many ways, the planet then was radically different from the one we live on today.

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There were seas, but the way the land masses lay bore no resemblance in either form or distribution to modern continents. Volcanoes were abundant, spewing noxious gases, ash and lava.

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The atmosphere consisted of swirling clouds of hydrogen, carbon monoxide, ammonia and methane. There was little or no oxygen. This unbreathable mixture allowed ultraviolet rays from the sun to bathe the earth surface with an intensity that would be lethal to modern animal life.

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Electrical storms raged in the clouds, bombarding the land and the sea with lightning. Laboratory experiments were made in the 1950s to discover what might happen to these particular chemical constituents under such conditions.

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Such gases mixed with water vapor were subjected to electrical discharge and ultraviolet light. After only a week of this treatment, complex molecules were found to have formed in the mixture, including sugars, nucleic acids and amino acids, the building blocks of proteins.

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We now know that such simple organic molecules can be found throughout the universe, including on interstellar bodies such as comets, but amino acids are not life, nor are they even necessary for life to exist.

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The experiment proved little about the origin of life. All forms of life that exist today share a common way of transmitting genetic information of telling cells what to do. It is a molecule called deoxyribonucleic acid.

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Or DNA, for short, its structure gives it two key properties. First, it can act as a blueprint for the manufacture of amino acids, and second, it has the ability to replicate itself with this substance.

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Molecules had reached the threshold of something quite new. These two characteristics of DNA also characterize even the simplest of living organisms, such as bacteria and bacteria. Besides being the simplest form of life, we know are also among the oldest fossils we have discovered.

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The ability of DNA to replicate itself is a consequence of its unique structure. It's shaped like 2 intertwined helices. During cell division, these unzip, splitting the molecule along its length into 2 separate helices.

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Each then acts as a template to which other simpler molecules become attached until each has once more become a double helix. The simple molecules from which the dna is mainly built are of only 4 kinds, but they are grouped in trios.

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And arranged in a particular and significant order on the immensely long dna molecule. This order specifies how the 20 or so different amino acids are arranged in a protein. How much is to be made in what tissue.

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And when a length of DNA bearing such information for a protein or for how a protein should be expressed is called a gene occasionally the DNA copying process involved in reproduction.

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May go wrong. A mistake may be made at a single point, or a length of DNA may become temporarily dislocated and be re inserted in the wrong place. The copy is then imperfect.

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And the protein it will create may be entirely different. Changes in the dna sequence can also be induced by chemicals or radiation when this occurred in the 1st organisms on earth.

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Evolution began for such hereditary changes brought about by mutation and errors are the source of variations from which natural selection can produce evolutionary change. Because all life shares DNA as the hereditary material, it's possible to compare DNA sequences in different organisms and show how they're related.

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Such is the progressive technology that it is now also possible to sequence all the dna in an organism in a matter of hours using a device the size of a mobile phone. The 1000000s of dna sequences that have been established stored in databases and compared show us unequivocally that just as darwin predicted all life on earth shares a common ancestor.

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Because parts of our DNA accumulate mutations at a constant rate like a molecular clock, we can use DNA sequences to estimate when two species split apart. In general, genetic and fossil timings agree with each other, although genetic data do sometimes throw up surprises.

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Using this method we can estimate that the last universal common ancestor of all life on earth commonly known as LUCA Leuka and basically a population of simple bacteria lived around four billion years ago.

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Everything we can see around us can trace its ancestry back to that group of cells. Such vast periods of time baffle the imagination, but we can form some idea of the relative duration of the major phases of the history of life if we compare the entire span from these 1st beginnings until today with one year.

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That means that roughly each day represents around 10000000 years. On such a calendar, the gunflint fossils of algae like organisms, which seemed so extremely ancient when they were 1st discovered, are seen to be quite latecomers in the history of life, not appearing until the 2nd week of August.

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In the Grand Canyon, the oldest worm trails were burrowed through the mud in the 2nd week of November, and the 1st fish appeared in the Limestone Seas a week later. The little lizard will have scuttled across the beach during the middle of December.

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And humans didn't appear until the evening of 31st of December. But we must return to January. The bacteria fed initially on the various carbon compounds that had taken so many 1000000s of years to accumulate in the primordial seas, producing methane as a by product.

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Similar bacteria still exist today all over the planet, and that was all there was for around 5 or 6 months of our year. Then, in the early summer of the year of life, so sometime over 2000000000 years ago, bacteria developed an amazing biochemical trick.

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Instead of taking readymade food from their surroundings, they began to manufacture their own within their cell walls, drawing the energy needed to do so from the sun. This process is called photosynthesis.

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One of the ingredients required by the earliest form of photosynthesis is hydrogen, a gas that is produced in great quantities during volcanic eruptions. Conditions very similar to those in which the early photosynthesizing bacteria lived can be found today

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In such volcanic areas as Yellowstone in Wyoming here a great mass of molten rock lying only a thousand meters or so down in the earth's crust heats the rocks on the surface in places the groundwater is well above boiling point.

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It rises up channeled through the rocks under decreasing pressure until suddenly it flashes into steam and water spouts high into the air as a geyser. Elsewhere, the water wells up into steaming pools.

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As it trickles away and cools, the salts it gathered from the rocks on its way up, together with those derived from the molten mass far below, are deposited to form rimmed and buttressed basins surrounded by tiers of terraces.

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In these scolding mineral laden waters, bacteria flourish, some grow into matted filaments and curds, others into thick leathery sheets. Many are brilliantly colored, their intensity of hue varying during the year as the colonies wax and wane.

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The names given to these pools hint at the variety of the bacteria and the splendour of the effects they produce emerald pool, sulfur cauldron, beryl spring, far holed falls, morning glory pool, and a particularly rich one with several species of bacteria.

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