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Darwin’s book Darwin had been working for several years on his major “species” book, but was still years away from finishing it. On July 20, 1858, he decided to write an abstract of this work for quicker publication, to be called An Abstract of an Essay on the Origin of Species and Varieties Through Natural Selection, a title the publisher (John Murray) rightly considered too clunky. The full title of the book in its first edition was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, but by the sixth edition this had been shortened to just The Origin of Species. Title Page of the First Edition of On the Origin of Species. “Races” in the long subtitle (typical of Victorian books) had the meaning of “varieties” and not the modern meaning. The first use of “races” in the book is a reference to “races of cabbage.” Darwin is identified as the author of his journal about the voyage of the Beagle. Review of On the Origin of Species On December 26, 1859, a very favorable review of Darwin’s book appeared in the Times of London. The Times was a very conventional, very orthodox newspaper (satirized by Anthony Trollope as “Jupiter Olympus” – the voice of the gods), which ran only one or two book reviews a month, and it made no sense that if they reviewed On the Origin of Species they would not severely attack it. Darwin was greatly surprised, and wondered who had written it. The review sounded to him as though it had been written by Huxley, but it was inconceivable, given Huxley’s reputation, that they would ever ask him to write for them. Darwin wrote to Huxley about the review …. “The author is a literary man & German scholar. – He has read my book attentively; but what is very remarkable, it seems that he is a profound naturalist. He knows my Barnacle book, & appreciates it too highly. – Lastly he writes & thinks with uncommon force & clearness; & what is even still rarer his writing is seasoned with most pleasant wit… Who can it be? Certainly I should have said that there was only one man in England who could have written this essay & that you were the man. But I suppose that I am wrong, & that there is some hidden genius of great calibre. For how could you influence Jupiter Olympus & make him give 3 ½ columns to pure science. The old Fogies will think the world will come to an end. Well whoever the man is, he has done great service to the cause, far more than by a dozen reviews in common periodicals. … If you should happen to be acquainted with the author for Heaven-sake tell me who he is.” 5 Separate Theses of Darwin’s Views of Evolution These are the major theses of what has often been called “Darwinism.” (1) Evolution as such. This is the thesis that the world is not constant or recently created nor perpetually cycling but rather is steadily changing and that organisms are transformed in time. (2) Common descent (3) Multiplication of species (4) Gradualism (5) Natural selection Examples: A common ancestor of chimpanzees and bonobos (formerly “pygmy chimpanzees” or “hippie chimpanzees) split into these two lines several million years ago. A common ancestor of chimpanzees, bonobos, and humans split about 5 to 7 million years ago. Many splits seem to have occurred on the hominid line, with only one species remaining today, Homo sapiens. The apple maggot (Rhagoletis pomonella) originally fed on hawthorn apples, but in North America has split since 1800 into two species, one feeding on hawthorns and one on apples, which the maggot also found palatable. (1) Evolution as such (2) Common descent. This is the thesis that every group of organisms descended from a common ancestor and that all groups of organisms, including animals, plants, and microorganisms, ultimately go back to a single origin of life on earth. (3) Multiplication of species (4) Gradualism (5) Natural selection The Fundamental Tree of Life (1) Evolution as such (2) Common descent (3) Multiplication of species. This thesis explains the origin of the enormous organic diversity. It postulates that species multiply, either by ‘splitting’ into daughter species or by ‘budding,’ that is, by the establishment of geographically isolated founder populations that evolve to new species. (4) Gradualism (5) Natural selection Humans and their nearest relatives (1) Evolution as such (2) Common descent (3) Multiplication of species (4) Gradualism. According to this thesis, evolutionary change takes place through the gradual change of populations and not by the sudden (saltational) production of new individuals that represent a new type. Many of the older “Darwinians,” including Huxley, preferred saltation; for one thing, it was thought that the Earth was not old enough for such gradual processes to have occurred. (5) Natural selection (1) Evolution as such (2) Common descent (3) Multiplication of species (4) Gradualism (5) Natural selection. According to this thesis, evolutionary change comes about through the abundant production of genetic variation in every generation. The relatively few individuals who survive, owing to a particularly well-adapted combination of inheritable characters, give rise to the next generation. The Natural Selection Argument Fact 1. Every population has such high fertility that its size would increase exponentially if not constrained. This was pointed out by both William Paley and Thomas Malthus, and is clear from our own observations of plants and animals. Think of a single tomato plant, and all the tomatoes it produces, and all the seeds in each tomato. Think of a pine tree, and all the pine cones it produces each year, all the seeds on each cone, and all the new pine tree seedlings that can result therefrom – or think of a redbud tree, or a maple tree, or almost any kind of weed. Think of a queen honeybee and all the bees it can give birth to each year. Think of all the children a human female could give birth to in her lifetime. Fact 2. The size of populations, except for temporary annual fluctuations, remains stable over time (observed steady-state stability). This we see from our own observations again. A gardener who plants a few tomato plants one year doesn’t necessarily find thousands of tomato plants in the garden the next year. A homeowner with a prolific crabapple tree doesn’t find tens of thousands of crabapple seedlings on his property the next year (or at least, will get rid of most of them!). Rabbits breed quickly don’t usually multiply as wildly as they could, in principle. Fact 3. The resources available to every species are limited. This point was made by Malthus, but is evident from our own observations, whether we are talking about plants or animals. Inference 1. There is intense competition (struggle for existence) among the members of a species. This point was made by Malthus with regard to human populations, de Candolle with regard to all organisms. It is most evident for animals, but is also true for plants and bacteria. Fact 4. No two individuals of a population are exactly the same (population thinking). (This was evident to animal breeders, who deliberately chose the best specimens to propagate, to horticulturists, for the same reason, and, in general, to all taxonomists – but not generally appreciated by non-scientists.) Inference 2. Individuals of a population differ from each other in the probability of survival (i.e., natural selection). This inference was made by Darwin, for populations of all types of organisms, but had previously been noted by others, usually with respect to a particular species or variety. Fact 5. Many of the differences among the individuals of a population are, at least in part, heritable. This information came from animal breeders (breeders of pigeons, livestock, etc.) Inference 3. Natural selection, continued over many generations, results in evolution. This was Darwin’s conclusion from the five facts and two inferences listed above. This line of reasoning makes natural selection pretty unavoidable and thus pretty certain. What is not clear is how important it is in nature and at what rate it occurs. Also, there could be other types of selection, and Darwin identified one: sexual selection (to be discussed later). Chapter Titles in On the Origin of Species 1. Variation under Domestication 2. Variation under Nature 3. Struggle for Existence 4. Natural Selection 5. Laws of Variation 6. Difficulties on Theory 7. Instinct 8. Hybridisation 9. On the Imperfection of the Geological Record 10. On the Geological Succession of Organic Beings 11. Geographical Distribution 12. Geographical Distribution continued 13. Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs 14. Recapitulation and Conclusion Chapter 1: Variation under Domestication Darwin’s first chapter discusses “artificial selection” – the breeding of better horses, pigeons, plants, etc. by subjecting them to new conditions and deliberately choosing to reproduce the best stock or the individuals with the desired characteristics. This was a familiar topic to the British. Darwin argues at length that it is not possible to distinguish between new breeds produced by artificial human selection and the new species produced in nature by natural selection. Individuals of a domestic variety differ more from one another than do individuals in a wild species. More species occur in populous genera than in the less populous – an unusual fact of no significance if one assumes special creation, but understandable under the theory of natural selection. Pigeons “Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons. I have kept every breed which I could purchase or obtain, and have been most kindly favoured with skins from several quarters of the world, more especially by the Honourable W. Elliot from India, and by the Honourable C. Murray from Persia. Many treatises in different languages have been published on pigeons, and some of them are very important, as being of considerable antiquity. I have associated with several eminent fanciers, and have been permitted to join two of the London Pigeon Clubs. … “The diversity of the breeds is something astonishing. Compare the English carrier and the short-faced tumbler, and see the wonderful difference in their beaks, entailing corresponding differences in their skulls. The carrier, more especially the male bird, is also remarkable from the wonderful development of the carunculated skin about the head, and this is accompanied by greatly elongated eyelids, very large external orifices to the nostrils, and a wide gape of mouth. The short-faced tumbler has a beak in outline almost like that of a finch; and the common tumbler has the singular and strictly inherited habit of flying at a great height in a compact flock, and tumbling in the air head over heels. … “The runt is a bird of great size, with long, massive beak and large feet; some of the sub-breeds of runts have very long necks, others very long wings and tails, others singularly short tails. The barb is allied to the carrier, but, instead of a very long beak, has a very short and very broad one. The pouter has a much elongated body, wings, and legs; and its enormously developed crop, which it glories in inflating, may well excite astonishment and even laughter. The turbit has a very short and conical beak, with a line of reversed feathers down the breast; and it has the habit of continually expanding slightly the upper part of the oesophagus. … “The Jacobin has the feathers so much reversed along the back of the neck that they form a hood, and it has, proportionally to its size, much elongated wing and tail feathers. The trumpeter and laugher, as their names express, utter a very different coo from the other breeds. The fantail has thirty or even forty tailfeathers, instead of twelve or fourteen, the normal number in all members of the great pigeon family; and these feathers are kept expanded, and are carried so erect that in good birds the head and tail touch; the oil-gland is quite aborted. Several other less distinct breeds might have been specified.” Variability of pigeons: A carrier pigeon and a pouter pigeon Chapter 1 continued Darwin, who had discussed pigeons with many pigeonbreeders, described their immense variability, but concluded that “Great as are the differences between the breeds of the pigeon, I am fully convinced that all are descended from the rock-pigeon Columba livia.” Pigeon breeders, who thought they were just perfecting each breed, did not believe Darwin. Columba livia The wild rock-pigeon or rock-dove. Variations in domestic fowl: Hamburg fowl (upper left); Spanish fowl (upper right); Polish fowl (bottom) Chapter 2: Variation under Nature In Chapter 2, Darwin showed that animal and plant populations in the wild exhibit considerable variation. Some variations (like monstrosities and variations due to the environment) are not inheritable, but others are, and slight variations can build up into large differences. Darwin stated that variations could exist between varieties in a species and between species, but the distinction between species and varieties is difficult to ascertain and probably not meaningful. This was proved by the fact that botanists and zoologists often disagreed in classifying different organisms as members of different species or members of different varieties: “few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges.” Chapter 2 continued … Darwin regarded the line between species and varieties as somewhat arbitrary, although variation between species would have to be greater than variation between varieties. But larger samples of organisms of a species exhibited greater variation, so species or genera that extended over greater geographical areas would exhibit greater variation (have more species or subspecies) than those confined to a smaller area. “I look at individual differences, though of small interest to the systematist, as of high importance for us, as being the first step towards such slight varieties as are barely thought worth recording in works on natural history. And I look at varieties which are in any degree more distinct and permanent, as steps leading to more strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to species. … I attribute the passage of a variety, from a state in which it differs very slightly from its parent to one in which it differs more, to the action of natural selection in accumulating (as will hereafter be more fully explained) differences of structure in certain definite directions. Hence I believe a well-marked variety may be justly called an incipient species …” Chapter 3: Struggle for Existence This chapter describes the constant struggle for existence in nature. Darwin mentions Malthus as an inspiration for this insight. Darwin describes the constant struggle for existence among organisms, mainly between individuals of the same species but also to some extent between individuals of different species. Nature itself also puts pressure on individuals through natural disasters, epidemics, changes in climate, etc. Individuals with better characteristics predominate, leading to wonderful adaptations found in nature: the woodpecker’s beak that allows it to catch insects to eat, the structure of a parasite allowing it to attach to its host and feed on it, the ability of a beetle to dive underwater for food, seeds that can be carried far away by the wind, etc. Species with larger populations are more likely to be able to adapt and survive by leaving more progeny. Darwin points out that the struggle for existence is easy to overlook or forget about, despite its overwhelming importance: “We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that though food may be now superabundant, it is not so at all seasons of each recurring year.” “Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection.” Chapter 4: Natural Selection Chapter 4 fleshes out the operation of natural selection, a metaphorical term to suggest that nature can act on organisms in a way that results in changes like those of an animal breeder. Darwin’s argument is based on the facts we discussed earlier: (1) Organisms produce many more offspring than necessary just to replace the parent organisms; (2) resources to support life are limited, leading to competition (the “struggle for existence”); (3) individual organisms have different traits, which influence their survival and reproduction rates; (4) organisms with the best such traits leave the most progeny, and over many generations these traits come to predominate and may lead to new species: i.e., evolution. Chapter 4 contains the only diagram in On the Origin of Species, a “tree of life” diagram, reproduced below. Chapter 5: Laws of Variation This is the least successful chapter in On the Origin of Species, and does not contain much of value. Darwin really doesn’t know what the laws of variation are, and he knows he doesn’t, because he says, “Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents.” The chapter ends: “Whatever the cause may be of each slight difference in the offspring from their parents – and a cause for each must exist – it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive.” Chapter 6: Difficulties on Theory Darwin wanted to answer objections to his theory before the critics could make them. He addresses two in this chapter. “Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?” Darwin points out that as organisms become better adapted and win in the struggle for existence, the transitional forms must lose and disappear. This doesn’t happen instantaneously, of course, so organisms are usually not perfectly adapted to their way of life. The existence of organisms that are not perfectly adapted is not compatible with the concept of a creator who created perfectly adapted organisms. “Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some animal with wholly different habits? Can we believe that natural selection could produce, on the one hand, organs of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, organs of such wonderful structure, as the eye, of which we hardly as yet fully understand the inimitable perfection?” Darwin admitted he could not give complete answers to any specific questions of this sort, but that with greater knowledge of present and former organisms the evolution of such traits and organs would be understood. “To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.” Darwin went on to point out numerous example of things that look like steps in the evolution of the eye in a variety of different animals, very close to our current understanding of how, in fact, the eye evolved. He adds, “If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case.” In recent years “intelligent design” advocates have listed such complex organs, but evolutionary biologists have been successful in determining how they evolved, often from organs that originally served other purposes. “We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind. Numerous cases could be given amongst the lower animals of the same organ performing at the same time wholly distinct functions.” Darwin cites, as an example, the swimbladder of fish, whose purpose was to permit flotation at different depths in the ocean, but which clearly evolved into the lung of the higher vertebrates, which is used for respiration. “I can, indeed, hardly doubt that all vertebrate animals having true lungs have descended by ordinary generation from an ancient prototype, of which we know nothing, furnished with a floating apparatus or swimbladder.” Chapter 7: Instinct This chapter deals with another difficulty Darwin had stated at the beginning of Chapter 6: “Thirdly, can instincts be acquired and modified through natural selection? What shall we say to so marvellous an instinct as that which leads the bee to make cells, which have practically anticipated the discoveries of profound mathematicians?” Darwin was writing about the hexagonal shape of the wax cells made by the honeybee. He also mentions birds that lay their eggs in other birds’ nests (avoiding having to care for them) and certain ants that act as slaves to other ants, and other examples. Darwin indicated that instinct is difficult to define, but that it related to habitual activities that were not learned. He was sure that instinct was also the result of natural selection, which then acted on behavior as well as physical characteristics. Instinctive behavior would have developed very slowly, over long periods of time. Many chapters in On the Origin of Species were greatly abridged compared to what they would have been had Darwin written his “big species book” and not been rushed into writing On the Origin of Species. He returned to instinct in other books, notably The Expression of the Emotions in Man and Animals, since he regarded emotions as instincts that evolved through natural selection. Chapter 8: Hybridisation In Chapter 8 Darwin discussed hybridization, the crossing of two species (i.e., one parent from each species). It was thought at the time that if two individuals were of the same species, their offspring would be fertile, but if they were of different species, their offspring would be sterile. The hybrids are usually sterile, and when they are fertile the offspring of two hybrids are usually sterile. Darwin argued that this may just be because of dissimilarities in their reproductive organs, not because the individuals were or were not of the same species or same variety. Often, however, the reasons for sterility were not known. Darwin believed their was no good way to distinguish between species and varieties. Chapter 9: Imperfection of the Geological Record The theory of natural selection indicates that there were intermediate forms between existing species and their ancestral species, which normally would be extinct. These might occur as fossils in the geological record but few were known in Darwin’s time. He argued that their absence is not a disproof of the theory. Gaps in the fossil record are to be expected because the intermediate fossils may have been destroyed or just not yet discovered. What about forms intermediate between two existing related species? They may never actually have existed, according to the theory of evolution through natural selection, only forms intermediate between each of them and their common ancestor. In many cases, intermediate forms were never very numerous or widespread, as natural selection was constantly operating on these forms. Darwin accepted Lyell’s theory of uniformitarianism, that geological processes had been slowly but constantly acting on the earth’s surface, in many cases probably removing some of the intermediate forms. Indeed, millions of years of strata have disappeared completely in many parts of the world. “[I]t is highly important for us to gain some notion, however imperfect, of the lapse of years. During each of these years, over the whole world, the land and the water has been peopled by hosts of living forms. What an infinite number of generations, which the mind cannot grasp, must have succeeded each other in the long roll of years! Now turn to our richest geological museums, and what a paltry display we behold!” Modern geology was still a young science when On the Origin of Species was published barely 75 years after James Hutton’s first published work. Since its time there has been much more work carried out and many new fossils discovered. Many intermediate forms have been discovered, but, clearly, not all. Many parts of the world that were not explored in Darwin’s time have been since. Most remarkably, think of all the paleoanthropological studies that have been carried out on the African continent and that have begun to fill in the “tree of life” beginning with the common ancestor of chimpanzees and humans. This particular “difficulty” cited by Darwin is not presently regarded as a difficulty. Chapter 10: On the Geological Succession of Organic Beings Darwin referred back to his “tree of life” in chapter 4 to describe his picture of a “succession of types” as new species constantly appear on earth, evolve and flourish, and finally decline and become extinct, perhaps leaving living descendants, but perhaps bringing their line to an end. Darwin believed that the geological record of the succession of organic beings better agreed with the slow and gradual modification of organic beings, through “descent with modification” that with the immutability of species, which the record clearly contradicts. Chapters 11 and 12: Geographical Distribution and Geographical Distribution continued In these chapters Darwin discusses a variety of topics relating evolution and speciation to biogeography: 1. Similar climates in different parts of the world may have similar or dissimilar species – there is no hard and fast rule. 2. Large land masses separated from each other, and islands, and separated bodies of water, all have their own generally very different species. 3. When species can migrate to another land or water area, they may evolve into different species: migration and natural selection both affect the nature of the species found. 4. Darwin believed species originated in one place and then migrated to other places, rather than appearing (being created) differently in different places. 5. Some species migrate easily (birds and plants whose seeds can migrate) while others do not (such as most land animals). 6. Geological processes caused land and water levels to fluctuate, affording opportunities for species to evolve due to the appearance and disappearance of water barriers. Darwin discussed the effect of the ice ages in leading to new and different species and the extinction of some. Chapter 12 of The Origin of Species continues the discussion of the effect of geographical distribution and separation by focusing on island life. The Galápagos Islands are used as the prime example, because Darwin had visited them on the voyage of the HMS Beagle and written about their species of animals, which exhibited the effects of natural selection. “I have carefully searched the oldest voyages, but have not finished my search; as yet I have not found a single instance, free from doubt, of a terrestrial mammal (excluding domesticated animals kept by the natives) inhabiting an island situated above 300 miles from a continent or great continental island; and many islands situated at a much less distance are equally barren.” Chapter 13: Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs. In Chapter 13 Darwin shows that his theory explains many different observations about organisms: their similarities (internal, not necessarily external, because unrelated species may have developed similar adaptations due to similar environments), their morphology (structure), embrology, and their rudimentary and atrophied structures (relics of once-useful organs). The naturalists’ grouping of organisms into various taxa (species, genera, etc.) is the result of a relationship due to descent with modification (“evolution” in modern language) from common ancestors. Darwin insists that assuming the existence of a divine plan adds nothing to our understanding of natural relationships. The evolutionary relationships are evident in homologous structures: “What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?” “The real affinities of all organic beings are due to inheritance or community of descent. The natural system is a genealogical arrangement, in which we have to discover the lines of descent by the most permanent characters, however slight their vital importance may be.” Chapter 14: Recapitulation and Conclusion In his concluding chapter, Darwin summarized the implications of his argument, confidently adding that “young and rising naturalists” would share his vision and reject the prejudices that led many biologists to cling to older ideas. This chapter includes Darwin’s only use of the word evolved (or any variations thereof) – it’s the last word of the book. Remember that Darwin never spoke of “evolution,” rather of “descent with modification.” “Although I am fully convinced of the truth of the views given in this volume under the form of an abstract, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. It is so easy to hide our ignorance under such expressions as the ‘plan of creation,’ ‘unity of design,’ etc., and to think that we give an explanation when we only restate a fact. Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject my theory. A few naturalists, endowed with much flexibility of mind, and who have already begun to doubt on the immutability of species, may be influenced by this volume; but I look with confidence to the future, to young and rising naturalists, who will be able to view both sides of the question with impartiality. Whoever is led to believe that species are mutable will do good service by conscientiously expressing his conviction; for only thus can the load of prejudice by which this subject is overwhelmed be removed.” Chapter 14 continued Darwin’s last chapter refers to the whole book as “one long argument.” He summarizes his theory and its difficulties, and states that he believes it will lead to a “revolution in natural history.” The closing paragraph of On the Origin of Species: “It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.” Closing paragraph, continued … “These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows.” Closing paragraph, concluded … “There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” The last word of The Origin of Species is the only example in the book of the word “evolved” or any words with the same root. Presumably he tried to avoid using the word because Lamarck and Chambers had used it and been criticized. Evidence for a Common Ancestor of All Life on Earth 1869: Nucleic acids were first isolated by Friedrich Miescher. 1944: Avery et al. identify DNA was identified as the genetic material of all life – conceivably there could have been many different genetic materials, or possibly each species could have had a different genetic material. (Maybe currently unknown species might have genetic materials different from DNA.) However, all known life uses the same polymer, polynucleotide (DNA or RNA), for storing species specific information. All known organisms base replication on the duplication of this molecule. 1953: Crick and Watson discover the molecular structure of DNA – a double helix. The DNA used by living organisms is synthesized using only four nucleosides (deoxyadenosine, deoxythymidine, deoxycytidine, and deoxyguanosine) out of the dozens known (at least 102 occur naturally). LUCA: Last Universal Common Ancestor Working backwards in time, all life apparently had at least one common ancestor, perhaps more. Perhaps there were several early life forms – maybe some with different genetic material, not the DNA or RNA we are familiar with. But if so, all but one disappeared (unless there are other life forms on Earth, with different genetic material, that we have not yet discovered). Of the common ancestors of all known life on Earth, the last one – before the first – is the Last Universal Common Ancestor, or LUCA. In 1864 Darwin complained that Wallace always spoke of “Darwin’s theory” without giving himself credit, and Wallace wrote back: “As to the theory of Natural Selection I shall always maintain it to be yours and yours only. You had worked it out in details I had never thought of, years before I had a ray of light on the subject, and my paper would never have convinced anybody or been noticed as more than an ingenious speculation, whereas your book has revolutionized the study of natural history, and carried away captive the best men of the present age.” William Irvine, in Apes, Angels, and Victorians, called Wallace “so retiring, so reassuring, so generous.” Darwin’s only reference to humans – apart from occasional mentions like their hand bones – in the first edition of On the Origin of Species was this: “Light will be thrown on the origin of man and his history.” In later editions this became “Much light will be thrown …” He apparently felt there was enough in his book already to disturb many readers without discussing humans – but this was the subject most readers were interested in. It was not long, however, before Huxley and Lyell wrote books on this subject: Huxley’s Evidence as to Man's Place in Nature and Lyell’s Geological Evidences of the Antiquity of Man, both appearing in 1863. Darwin eventually dealt with the topic of humans and their history in The Descent of Man, published in 1871.