Based on a transcript written by the brilliant Sir David Attenborough

Our Earth is the only known planet that sustains life, and it does so in abundance.The sheer number and variety of plants and animals is astonishing. Estimates of the number of different species vary from 6 million to 100 million, and there are often a multitude of variations on a single pattern. For example, there are nearly 200 different kinds of monkey, 315 hummingbirds, nearly 1000 bats, and around 350,000 species of beetle. Not to mention a quarter of a million different species of flowering plant!

Even in one small English woodland, you might see four or five different kinds of finches! Why should there be such dazzling variety, and how can we make sense of such a huge range of living organisms?

200 years ago, a man was born who was to explain this incredible diversity of life, and in doing so he revolutionised the way in which we see the world and our place in it. His name was Charles Darwin.

The Bible explains how this wonderful diversity came about:

'...on the third day after the creation of the world - God created plants. On the 5th day, fish and birds and then, on the sixth day mammals and finally man...'

'...and when God had finished, he said to Adam and Eve, "Be fruitful and multiply, and replenish the earth and subdue it, and have dominion over the fish of the sea, and over the fowl in the air and over every living thing that moveth upon the earth"...

This made it clear that according to the bible, humanity could exploit the natural world as they wished, and this explanation was believed - literally - by most of western Europe for the best part of 2000 years!

In 1831, a British survey ship 'The Beagle' set off on a voyage around the world. On board, as a companion to the captain, was a 22 year old Charles Darwin. They crossed the Atlantic and made landfall on the coast of Brazil. There, the sheer abundance of tropical nature astonished the newcomer. As a boy, Darwin was a fanatical collector of insects and here he was enthralled almost to the point of ecstasy. In one day - and only in a small area, he discovered 69 different species of beetle. In his journal, he wrote

'...its enough to disturb the composure of the entomologists mind to contemplate the future dimension of a complete catalogue...'

They went south, rounded Cape Horn and so reached the Pacific. Then in September 1835 - after they had been away for four years, they landed on the little known islands of the Galapagos. Here they found creatures that existed nowhere else in the world. Cormorants that had lost the power of flight, and lizards that swam out to the surf to graze on the bottom of the sea.

Darwin, who had studied botany and geography at Cambridge university, collected specimens of the animals and plants, and as usual, when he went ashore to investigate he described what he found in his journal.

'...my servant and self were landed a few miles to the north-east in order that I might examine the district, mentioned above - as resembling chimneys, volcanic chimneys presumably, the comparison would have been more exact if I had said the iron furnaces near Wolverhampton...'

The British resident in the Galapagos claimed that he knew from the shape of the giant tortoises shell, which island it had come from. If it had a rounded front to its shell, it came from a well watered island where it fed on lush ground plants. Whereas one from a drier island had a peak at the front which enabled it to reach up to higher vegetation. Were these tortoises - each on their separate islands - different species, and if so, was each one a separate act of divine creation?

The differences that Darwin had noticed amongst these Galapagos animals were of course all tiny, but if they could develop, wasn't it possible that over the thousands or millions of years a whole series of such differences might add up to one revolutionary change?

On his voyage home, Darwin had time to ponder on such things. Could it be that species were not fixed for all time, but could in fact slowly change? On his return, he sorted out his specimens and sent them off to relevant experts so that each could be identified and classified. Most of the mammal bones and fossils he sent to Richard Owen.

Owen was one of the most brilliant zoologists of his time. He was the first to recognise dinosaurs and was indeed the very man who invented their name. Owen would later become the creator, and then director of the Natural History Museum in London.

Many of the specimens that Darwin collected are still preserved and treasured at the Natural History Museum amongst the 17 million other specimen housed in the museum that Owen founded.

Soon after his return from his voyage, Darwin made his home at Downe House in Kent. Here, he wrote an account of his travels and worked on detailed scientific treatises about corals and barnacles, and the geology and fossils of South America. But he also pondered deeply on what he had seen in the Galapagos and elsewhere. Maybe species were not fixed!

Every day, Charles Darwin took a walk down to a small spinney which was planted at the end of his garden, and it was here that he came to ponder on the problems of natural history, including that mystery of mysteries. How could one species turn into another?

He noted that most, if not all, animals produced many more young than was required than live and breed themselves. A female blue tit for example may well lay a dozen eggs a year, and perhaps 50 or so in her lifetime. Yet only two of her chicks needed to survive and breed themselves in order to maintain the numbers of the blue tit population. Those survivors of course, are likely to be the healthiest and best suited to their particular environment. Their characteristics are then inherited so perhaps over many generations, and particularly if there are environmental changes, species may well change. Only the fittest survived and that was the key. Charles Darwin called this - Natural Selection!

This observation would explain the differences that he had noted in the finches that he had brought back from the Galapagos. They were very similar except for their beaks.

One species had a thin, delicate beak which it used to catch insects, while another - on the other hand - came from an environment where there was a lot of nuts, has a big heavy beak that enables it to crack them. So maybe over the vastness of geological time, particularly is species were invading new environments, those changes would amount to very radical changes indeed.

Darwin drew a sketch in his notebooks to illustrate his idea, showing how a single ancestral species might give rise to several different ones, and wrote above it a tentative:

I think!

Now, Darwin had to prove his theory and he spent years gathering abundant and convincing evidence. He was an extraordinary letter writer. In fact he wrote as many as a dozen letters every day to scientists and naturalists all over the world.

He also realised that when people had first started domesticating animals, they had been doing experiments for him for centuries. All domestic dogs are descended from a single ancestral species, the wolf! Dog breeders selected those pups which had the characteristics which happened to please them. Nature of course, selects those young animals that are best suited to a particular environment, but the process is essentially the same and in both cases it has produced an astonishing variety.

In effect many dog breeds could be considered to be different species, because they do not and indeed they cannot interbreed. For purely mechanical reasons, there is no way a Pekingese could mate with a Great Dane! Of course, it's true that if you used artificial insemination you could get crosses between almost any breed of dog, but that is because human beings have been selecting dogs for only a few centuries. Nature has been selecting between animals for millions of years, tens of millions of years, in fact even hundreds of millions of years. So what might have started out as we would have considered breeds, have now become so different, they are species.

Darwin, sitting in Downe House, wrote to pigeon fanciers and rabbit breeders asking all kinds of detailed questions about their methods and results. He himself, being a country gentleman and running an estate knew about breeding horses, sheep and cattle and he also conducted careful experiments on plants in his greenhouse. But Darwin knew that species could appear without Divine intervention would appal society in general and it was also contrary to the beliefs of his wife Emma who was a devout christian. Perhaps for that reason, he was keen to keep the focus of his work scientific.

He made a point of not being drawn in public about his religious beliefs, but in the latter part of his life he withdrew from attending church. Perhaps because he feared that his theory would cause outrage in some quarters, he delayed publishing it year after year after year - but he wrote a long abstract of it. Then, on July 5th 1844, he wrote this letter to his wife:
My dear Emma, I have just finished this sketch of my species theory, I therefore write this in case of my sudden death that you would devote £400 to its publication.

He then goes on to list his various naturist friends who would be asked to edit and check it, and he ends the letter charmingly:

My dear wife, yours affectionately, C.R. Darwin.

He continued to accumulate evidence and refine his theory for the next fourteen years, but then his hand was forced. In June 1858, 22 years after he got back from the Galapagos, he received a package from a naturalist who was working in what is now Indonesia. His name was Alfred Russel Wallace.

Wallace had been corresponding with Darwin for some years, but this package was different! It contained an essay that set out exactly the same idea as Darwin's - of evolution by natural selection! The idea had come to Wallace as he lay in his hut semi delirious in a malarial fever. But although his idea of natural selection was the same as Darwin's he had not spent twenty years gathering the mountain of evidence to support it as Darwin had done, but whose idea was it?

In the end the senior members of the Linnean Society decided that the fairest thing was for a brief outline of the theory from each of them would be read out one after the other at a meeting of the society in Burlington House, London. The Linnean, then as now, was a place where scientists studying the natural world held regular meetings to present and discuss papers about their observations and thoughts.

The one held on July 1st 1858 was attended by only about 30 people, and neither of the authors were present. Wallace was 10,000 miles away in the East Indies, and Darwin was ill and devastated by the death a few days earlier of his infant son,so he was still at his home in Kent. As a consequence, the two papers had to be read by the secretary and as far as we can tell they made very little impression on anyone!

Darwin spent the next year writing out his theory in detail, then he sent the manuscript to his publisher John Murray whose firm, then as now, had offices in Piccadilly, London. Murray was the great publisher of his day, and dealt with the works of Jane Austen, and Lord Byron.

Darwin regarded his work as simply as a summery, but even so, it is 400 pages long! It was published on November 24th 1859, and changed forever how humans see themselves and the world in which we live.

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