The Ancestor's Tale: A Pilgrimage to the Dawn of Life is a popular science book by Richard Dawkins and Yan Wong in which the history of life is retraced in reverse chronological order. A growing band of species meet their most recent common ancestors (concestors). First published in 2004, it was updated in 2016 to reflect recent discoveries. There are new tales, including one written during Dawkins's pilgrimage to the Galápagos.[1] The phylogenetic trees in the second edition are based on OneZoom.[2]
The epigraph is from Mark Twain: "History doesn't repeat itself, but it rhymes." The authors contend that “Evolution rhymes, patterns occur. And this doesn't just happen to be so. It is so for well-understood reasons: Darwinian reasons, mostly, for unlike human history or even physics, biology already has its grand unifying theory.”[4]:1 They emphasize that no living species is ancestral to any other, but that all share a common ancestor, some more recently than others. Evidence for this fact is that the genetic code for translatinggenes into proteins is universal. More closely related species have more similar genes and proteins. Genes and proteins act as molecular clocks that allow us to determine when species split. They use concestor, coined by Nicky Warren,[4]:11 to describe the most recent common ancestor at each rendezvous point.[4]
It is patterned on Geoffrey Chaucer's The Canterbury Tales, in which travelers take turns telling tales. The pilgrimage progresses through extant species, with a few exceptions: “Animals such as the dodo, which survived into historical times and whose DNA is still available to us, are treated as honorary members of the modern fauna starting off on our pilgrimage. … Since we are responsible for their recent extinction, it seems the least we can do. The other honorary pilgrims, exceptions to the rule that dead man tell no tales, really are men (or women). Since we human pilgrims are directly seeking our own ancestors, fossils that might plausibly be considered candidates for being our ancestors are deemed members of our human pilgrimage and we shall hear tales from these shadow pilgrims, for example the Handyman, Homo habilis.” The pilgrims do not speak in the first person, as the authors think that "twee."[4]:10
The book was revised in 2016 in light of recent discoveries. Notably, "The Denisovan's Tale" replaces "The Neanderthal's Tale" and "The Elephant Bird's Tale" has been updated. "The Mudskipper's Tale" was present in the first edition but not the second. "The Armadillo's Tale", about biogeography, is now "The Sloth's Tale." In 2005, Dawkins wrote three tales during a pilgrimage to the Galápagos.[5][6][1] The third is reprinted in the second edition.
The Tasmanian's Tale is about the identical ancestors point, from which all living people trace exactly the same set of ancestors. The authors look at the small, isolated population of Tasmania and the last surviving native Tasmanian, Truganini. They point out that she and other humans alive today share a recent common ancestor.
Eve's Tale is about coalescent theory. The authors introduce mitochondrial DNA, which is inherited from the mother and allows us to trace matrilineal ancestry (in humans, to Mitochondrial Eve). The Y-chromosome is inherited from the father, and allows us to trace patrilineal ancestry (in humans, to Y-chromosomal Adam.) They introduce haplotypes.
The Orangutan's Tale is about parsimony. The authors introduce the principle of parsimony and its use in construction of cladograms. They discuss the possibility that the ancestors of the Great Apes were in Asia, where one line stayed and gave rise to Orangutans, where they are found today ("Orang utan" is Malay for man of the forest.) The other line went to Africa and gave rise to Gorillas, Chimpanzees and humans. They find this more parsimonious than the alternative. Orangutan is the last of the Great Apes to join the pilgrimage. (See also Orangutan–human last common ancestor.)
The Gibbon's Tale is about phylogenetics. The authors note that the genetic code is degenerate. As a result, “some DNA codes are synonymous: they specify exactly the same amino acid.”[4]:158Synonymous substitutions are invisible to evolution but not to biologists. Since they are not subject to selection, they show greater variation between species. The same is true of pseudogenes. Coding regions are conserved sequences. Rare genomic changes are unlikely to have arisen independently, and are likely due to common descent. The authors note that “literary scholars use the same techniques as evolutionary biologists in tracing the ancestries of texts. And – almost too good to be true – one of the best examples happens to be the work of the Canterbury Tales Project.”[4]:159 Gibbon is the last ape to join the pilgrimage. (See also Gibbon–human last common ancestor.)
Old World monkeys, being in the same Catarrhini clade as apes, are closer cousins to apes than to New World monkeys. Old World monkeys are sometimes called the 'tailed apes'. It is not known if the actual common ancestor had a tail or not.
The Howler Monkey’s Tale is about gene duplication and how it creates new genes. When a gene is copied, it can evolve a new function. This happened in the evolution of color vision in primates. Old World Primates (including humans) are trichromats, with three opsins (red, green and blue), each coded for by a specific gene. The red and green opsin genes are similar, and adjacent on the X chromosome. They are derived from a recent duplication. Mutations that improved the function of the new gene were selected for until the opsin could detect a different wavelength.
Most New World monkeys are dichromats, with either red or green genes on the X chromosome. In some species, both red and green alleles are present in the population as a polymorphism. In Howler monkeys, both genes were put on the X chromosome through a translocation. As often happens in evolution, the same end was reached via different means.
The Tarsier's Tale is about its enormous eyes. Unlike most nocturnal mammals, they lack a tapetum lucidum to reflect light from the back of the eye for a second exposure on the retina. The ancestor of the tarsier was a diurnal animal which lost the tapetum lucidum. In 2013, Archicebus, the oldest primate fossil found so far, was discovered in China.[10]An artist's reconstruction of Archicebus achilles.
The Aye-Aye's Tale is about island ecology. In Madagascar, a small strepsirrhinefounding population evolved to fill available niches. Today, it is home to as many as one hundred species of lemur. Lemurs, kin to lorises and pottos, are found only on this island. “Like virtually everything else that lives on Madagascar, it does not exist anywhere else on Earth.”[4]:199Douglas Adams was writing of the aye-aye, but Madagascar is also home to six of the world’s eight species of baobabs. With a landmass 1/1000 of Earth's total land area, it accounts for 4% of all species of flora and fauna.
The Colugo's Tale is about how genetic evidence has made us rethink existing phylogenies. Case in point: in the earlier edition, colugos and treeshrews joined us at the same point. But genetic evidence from indels suggests that these creatures (colloquially called “flying lemurs” due to their patagium) are closer cousins to primates than to treeshrews.
Rodents make up more than all the other mammals by individuals and have been carried to every corner of the earth. The large connections coming in are dormice, pikas, squirrels.
The Mouse's Tale is about epigenetics. Mice and humans have about the same number of genes. The phenotypic difference comes from the way the genes are used. Borrowing from Matt Ridley, the authors liken genes to words, which can be put together in very different ways. From a similar vocabulary, very different books can be written. From similar genes, very different organisms can be made.
The Beaver's Tale is about Dawkins's concept of The Extended Phenotype. An organism's phenotype is typically defined as its physical traits, coded for by its genotype. But beavers are hard-wired by evolution to build dams, so it could be considered an extension of its phenotype (so, by extension, could the beaver lake.) Better beaver genes make better beaver bodies and better beaver dams.
The Hippo's Tale is about cetacean evolution. All cetaceans, including whales, dolphins and porpoises, are descendants of land-living mammals of the Artiodactyl order, as evidenced by their vestigial pelvic and leg bones, by fossils such as Pakicetus and Basilosaurus and the fact that they have to come to the surface to breathe air. Like all mammals, they produce milk and, like most mammals, live young. The surprise is that whales are the closest living relatives of hippos. So close is the molecular resemblance that whales are now grouped with hippos as Cetartiodactyla. The story illustrates how a species can flip into evolutionary overdrive when it enters into a new environment, while its closest relatives remain unchanged for a long time in their static environment.[11]
The Seal's Tale is about why a sex ratio of 50:50 (males to females) is found in most animals. If females are less common than males, then females will have a reproductive advantage. A tendency to produce females will be favored, until females are more common. The same is true if the sexes are reversed. A 50:50 ratio is evolutionarily stable, as R. A. Fisher found with his principle. This led to further work by Robert Trivers on parental investment.
In a harem-based (polygynous) system such as that of elephant seals, where 4 per cent of males account for 88 per cent of all copulations, the 50:50 sex ratio seems to produce an excess of males. Fisher would note that, while a male is less likely to reproduce, if he does, he will do so in spades. The 50:50 ratio is still evolutionarily stable. The elephant seal also typifies sexual dimorphism, as a bull elephant seal can grow to be three times the size of a cow seal, thanks to sex-limited genes which exist in both male and female bodies, but remain turned off in females.
The Sloth's Tale is about biogeography. Charles Darwin and Alfred Russel Wallace, who independently discovered the engine of evolution, noticed patterns in the geographic distribution of species. Why do volcanic islands have endemic species of birds but not mammals? (Bats are the exception that proves the rule.) Why are those birds similar to species on the nearest mainland? Why are some species specific to certain continents? Why are sloths found only in South America? Alfred Wegener's theory of continental drift, the basis of plate tectonics, provided the missing pieces of the puzzle. Evidence comes from seafloor spreading. Moving away from a mid-ocean ridge in either direction, the rocks get older.
The southern continents were once joined in the super-continent of Gondwana. After South America split from Gondwana, the Xenarthrans could evolve in “splendid isolation” (as G. G. Simpson put it). This included the extinct ground sloths (fossils of which were found by Darwin). The isolation ended 3 million years ago when the Isthmus of Panama formed. Species traveled between North and South America in both directions in the Great American Interchange. This was worked out by Wallace. In Indonesia, where he had his epiphanies, he discovered the line that bears his name.
Placental mammals meet the marsupials (from the Latin for pouch, which they use to carry young.) Most modern marsupials are found in Australia and New Guinea, though they originally evolved in North and South America. Evidence points to a single species of opossum-like marsupial migrating from South America to Australia 55 million years ago via Antarctica, when they were part of Gondwana. After Australia separated, its marsupials evolved to fill the niches occupied by mammals on other continents.
The Marsupial Mole's Tale is about convergent evolution. The marsupial mole is not a mole, but resembles one as it has evolved to fill a similar niche. Many Australian marsupials are doppelgängers of placental mammals on other continents. There are marsupials mirroring mice (Antechinus), flying squirrels (Petaurus breviceps and Petaurus gracilis), rabbits (Macrotis), anteaters (the numbat), groundhogs (the wombat) and wolves (the Thylacine). Evolution is sufficiently powerful to shape similar adaptations in similar environments, even in species separated in time and space.
There are three extant genera of monotremes (Greek, single hole): the short-beaked echidna (found throughout Australia and New Guinea), the long-beaked echidna (in New Guinea) and the platypus (in Eastern Australia and Tasmania). They have mammalian features such as warm-bloodedness, hair and mammary glands. The middle ear is mammalian, with three ossicles (the hammer, stirrup and anvil). But the cochlea is less coiled than in other mammals. They are clearly reptilian in other respects, notably a cloaca which is used to lay eggs.
The Duckbill's Tale is about how seemingly “primitive” animals have evolved complex adaptations. The platypus, reptilian in some respects (a cloaca and eggs) has had precisely the same time to evolve as other mammals, even if it does not resemble our concestor 15. On its large bill, it has evolved a highly developed form of electroreception served by 40,000 electric sensors and 60,000 mechanical push rods, which aid it in search of prey. In humans, the brain dedicates a disproportionally large fraction of cells to the two hands, as illustrated by the Penfield homunculus. When the same somatotopic map is drawn for the platypus brain, the bill dominates. An aside is “What the Star-Nosed Mole Said to the Duck-Billed Platypus”. The mole is not a monotreme, but its sensory apparatus is at least as peculiar as the platypus’s.
"Reptile" is not a true clade name. Dinosaurs, unfortunately extinct, are joined by their only surviving descendants, the birds.
The Lava Lizard's Tale is about geology. Dawkins describes his pilgrimage to the Galápagos island of Santiago, visited by Darwin in 1835. A volcano erupted there one day in 1897. The events of that day are written in the island’s rocks, and we can read them. Dawkins describes the unique flora and fauna of the archipelago, and the camouflage used by the lava lizard.
The Galapagos Finch’s Tale is about natural selection and how it can produce rapid evolutionary change. Peter and Rosemary Grant have found this through their long-term study of Darwin’s finches. In 1977, the island of Daphne Major was struck by drought, which created selective pressure for longer beaks capable of cracking the tough seeds of Tribulus: “The average beak length after the drought was 11.07 mm long compared with 10.68 mm before. The mean depth of beak had similarly gone up from 9.42 to 9.96.”[4]:313
In 1982-1983, there was a flood brought about by El Niño. The selective pressure changed to favor smaller beaks that could eat the smaller seeds of Cacabus. The finches again evolved accordingly. In 2015, the Grants, along with researchers at Uppsala University, “located a number of DNA regions that co-varied with beak shape and homed in on a gene called ALX1… The ALX1 gene tree, deduced from Galapagos finch populations, shows branching patterns indicative of natural selection.”[4]:315[12]
The Peacock's Tale is about sexual selection and how it can produce rapid evolutionary change. The peacock's plumage evolved because generations of peahens preferred plumage. R. A. Fisher found that genes making males have a trait are linked to genes for females preferring that same trait, leading to runaway evolution. Dawkins describes this in The Blind Watchmaker.
The Dodo’s Tale is about island biogeography. Many birds colonizing oceanic islands convergently evolve flightlessness. Since these islands lack predators, wings aren’t selected for, and become vestigial. This happens even if wings would be useful in the future. This was sadly true in the case of the dodo, as wings would have proved useful when men arrived in Mauritius with clubs. On a happier note, biologists led by Alan Cooper have analyzed mitochondrial DNA from dodo specimens, and found they were related to pigeons.
The Elephant Bird's Tale is about island gigantism. Many species colonizing oceanic islands evolve rapidly in size, as seen in the extinct elephant bird of Madagascar and the moa of New Zealand. In the first edition, Dawkins speculated that extant ratites (rheas of South America, emus of Australia, cassowaries of Australia and New Guinea, kiwis of New Zealand and the ostrich, now of Africa) evolved flightlessness convergently rather than inheriting it through a single flightless ancestor. Molecular evidence, from Cooper’s study of Elephant Bird DNA, confirms this.[13]
Mammals and reptiles (the amniotes) join the amphibians to meet the ancestor of all land vertebrates with four feet, the tetrapod. Amphibians include frogs, toads, salamanders, newts and caecilians. While amniotes either give live births or lay waterproof eggs, the amphibians retain the ancestral practise of laying eggs in water. Unlike the waterproof skin of amniotes, the amphibian skin allows body water to evaporate through it, restricting amphibians to land areas with access to fresh water. Almost no amphibians live in saltwater which explains why they are seldom found on islands. The concestor probably already had settled on five digits on each limb.
The Salamander's Tale is about speciation. When a species inhabits a geographic ring, “there is potentially gene flow around the ring. Potentially.”[4]:343-344Robert C. Stebbins studied the Ensatina salamanders, which inhabit a ring surrounding California’s Central Valley. Around the ring, neighbouring populations of Ensatina can interbreed. But the plain Ensatina eschscholtzii on the western end cannot interbreed with the blotched Ensatina klauberi on the eastern end. Moving westward round the ring, populations become progressively plainer until they resemble Ensatina eschscholtzii. Moving eastward, populations become gradually more blotched until they resemble Ensatina klauberi. The authors write that ring species “are only showing us in the spatial dimension something that must always happen in the time dimension.”[4]:346
The Narrowmouth's Tale is about parapatric speciation. Gastrophryne olivacea (Great Plains narrow-mouth toad) and Gastrophryne carolinensis (eastern narrow-mouth toad) are closely related. The most noticeable difference is in their mating calls. The former species has adapted to the western United States, the latter the eastern. But their ranges meet in the middle. It is there that their mating calls differ most sharply. Theodosius Dobzhansky theorized that this was due to reinforcement: natural selection favours adaptations that make interbreeding less likely. This causes character displacement, where two related species differ more in areas where they overlap.
The Axolotl's Tale is about metamorphosis, by which larvae change into dramatically different adult forms for reproduction. A frog has genes for making a tadpole. John Gurdon dramatically demonstrated this in 1962 by transplanting the nucleus of an adult frog into an enucleated cell, which developed into a tadpole. This work won him the Nobel Prize in Physiology or Medicine in 2012.
Most salamanders undergo metamorphosis. They pass through a larval stage with “long, feathery external gills” which they lose as adults. Axolotls exhibit pedomorphosis, a process which enables juveniles to become sexually mature without developing into their usual adult forms. Axolotls remain aquatic and retain the gills. Julian Huxley found that with a treatment of thyroxine, it is possible to induce an axolotl to develop into a mature salamander. On the other hand, the newt first develops from tadpole into a land-based salamander, but later reverts to its juvenile form, returning to the water to reproduce.
Despite their morphological similarities, the lungfish and coelacanth are very different genetically, as expected of species which lived separately for more than 400 million years. One lungfish has the record for largest genome at 133 billion base pairs compared to our 3 billion base pairs.
The Lungfish's Tale is about the evolution of tetrapods. The authors trace transitional fossils documenting this transformation: Eusthenopteron, Panderichthys, Tiktaalik, Acanthostega and Ichthyostega. They speculate about selection pressures might that drove fish to land. For many years, the favored idea was that of Alfred Romer, that fish who could move from a drying pond to a deeper one had an advantage.
Among the six extant species of lungfish, the South American lungfish (Lepidosiren paradoxa) and four species of African lungfish (in the genus Protopterus) do something similar today, using their lungs to make it through dry seasons. (The Australian lungfish, Neoceratodus forsteri, inhabits weedy waters and uses its lung to supplement its gills.) But the Devonian does not seem to have been defined by drought, and Romer’s idea has fallen out of favor. But Steven Balbus notes that Earth’s moon was once much closer, and its tidal forces much stronger. It is possible that this concestor was moving from tide pool to tide pool. (There are other reasons to come to land. Mudskippers feed on land, and some spend more time on land than in water). The lungfish’s eponymous organ is likely homologous to ours, inherited from concestor 18.
The Coelacanth's Tale is about so-called “living fossils”, of which the coelacanth is a celebrated example. The authors describe its discovery by Marjorie Courtenay-Latimer. However, even species that show little phenotypic change undergo molecular evolution, as revealed by genetic analysis. The reasons are seen later in the pilgrimage.
The Leafy Sea Dragon's Tale is about the plasticity of animal forms. Evolution has molded many teleosts into strange shapes. The leafy sea dragon hangs motionless among kelp, which it resembles. Shrimpfish have been elongated to resemble strands of seaweed. The snipe eel has been stretched, while the gulper eel sports grotesquely large jaws. The ocean sunfish, weighing up to two tons, resembles a vast disc or millstone, as its Latin name, Mola mola, suggests. Teleosts can control buoyancy because of a unique adaptation, described in the next tale.
The Pike's Tale is about the evolution of the swim bladder. Contrary to common belief, it is not the progenitor of the lung. The bony-fish ancestor possessed a primitive lung which, in teleosts, was co-opted as the swim bladder, used to maintain hydrostatic equilibrium. In some teleosts, it is used as resonating device. Such exaptation is common in evolution.
The Cichlid's Tale is about adaptive radiation. Cichlids in the African Great Lakes have fast formed new species. Lake Victoria, about 100,000 years old, has as many as 450 cichlid species. Lake Victoria cichlids are distinct from Lake Malawi cichlids which are distinct from Lake Tanganyika cichlids. Yet within each lake, similar adaptations have convergently evolved. The authors quote George W. Barlow on the cause of speciation: “Genes flow but not much.” In allopatric speciation, a physical barrier disrupts gene flow, one reason islands have been so important in evolution. But any isolated environment could be considered an island. To a fish, a lake is an island, if not an archipelago (since not all parts are equally habitable.) A reef (defined by the Oxford English Dictionary as ‘a narrow ridge or chain of rocks, shingle, or sand, lying at or near the surface of the water’) within the lake could be an ‘island’. Indeed, genes seem to flow within reefs but not between them.
The authors recount how, by constructing an "unrooted haplotype network" with computational phylogenetics of mitochondrial DNA, Erik Verheyen, Walter Salzburger, Jos Snoeks and Axel Meyer were able to infer the time and the location of each major speciation event. Geologic evidence shows that Lake Tanganyika was once split into three lakes. Genetic evidence shows splitting into three species around the same time. All the species are ultimately derived from a single species flock from Lake Kivu. As in The Galapagos Finch’s Tale, different lines of evidence corroborate one another.[14]
The Blind Cave Fish's Tale is about Dollo's Law that evolution cannot be reversed. An exception seems to come from species living in caves. Many species, having colonized caves, have convergently evolved similar adaptations, including the loss of skin pigmentation and eyes. Different populations of Mexican tetra (Astyanax mexicanus) have separately ventured into caves, and each has evolved white skin and blindness. This can be explained by opportunity cost: resources spent on building eyes cannot be used for other things. In a cave, resources spent on eyes can be used for things more useful in that environment, and the eyes become vestigial. Another possibility is that, in the absence of selective pressure, deleterious mutations accumulate. Eyes won’t be selected for if they’re not useful in the present, even if they may be in the future. Natural selection is blind.
The Flounder's Tale is about how evolution tinkers with existing body plans. When flatfish hatch, they are symmetric, with an eye on either side. But evolution has fashioned them for a life lying flat on the ocean floor. During development, the eye on the lower (blind) side migrates to the upper side. This happens whether they lie on their left (plaice) or right (turbot). This asymmetry results in distortions in the skull. Dawkins discussed this in his Royal Institution Christmas Lectures, on “Designed and Designoid Objects.”[15]
Sharks, rays and the related animals are supported by a cartilaginous skeleton that never ossifies to become bone. Their skin is covered in dermal denticles, tiny scale-like protrusions, from which teeth may have evolved. Sharks lack a swim bladder for buoyancy and instead rely on constantly swimming, retaining urea in their blood, and having large livers with plenty of oil to remain afloat. Concestor 21 was an ancestor to all gnathostomes, animals with lower jaws, a structure which evolved from the gill arches.
Jawless and limbless fishes, the lampreys and hagfish, join the pilgrimage to meet the concestor of all vertebrates. The jawless fish and the concestor 22 are borderline vertebrates. Unlike the rest of vertebrates, they retain the notochord, a stiffening cartilage rod running along the back of an animal, well into adulthood. In all other vertebrates, the vestigial notochord appears in the embryo briefly and is replaced by segmented, articulate backbones in adults. On the other hand, both the jawless fish and the jawed fish share characteristics common to all members of the phylum Chordata at some time in their life cycle, including the notochord, pharyngeal slit, and the post-anal tail.
The Lamprey's Tale is a variation on the theme of “The Howler Monkey’s Tale”, about how gene duplication creates genetic novelty. In vertebrates, haemoglobin, the protein that carries oxygen in the blood, is composed of four globin chains, two alpha and two beta. The genes coding for haemoglobin are paralogous, derived from duplication. (In humans, they are coded for by gene clusters on chromosomes 11 and 16, respectively.) An ancestor globin gene from an ancient vertebrate split into two genes, which ended up on two different chromosomes. Both alpha and beta further split into more independently evolving genes.
We can make a prediction: Lampreys and hagfish are ancient and predate this gene split. Jawless fish should not possess split globin genes. But they should be present in the genomes of bony fish, amphibians, reptiles, birds and mammals. That is what is found.
The lancelet and sea squirt have been switched in the second edition based on DNA studies. A sea squirt resembles a sedentary bag of seawater anchored to a rock. It feeds on food particles strained from water. Anatomically, the sea squirt looks very different from the joining pilgrimage of all vertebrates and protochordates, that is, until its larvae are examined. The sea squirt larva looks and swims like a tadpole. It possesses a notochord and a dorsal nerve tube and moves by undulating its post-anal tail from side to side. Vertebrates may have branched off from ancient sea squirt larvae via neoteny, in a process reminiscent of The Axolotl's Tale. But recent DNA analysis on larvaceans favours Darwin's initial interpretation, that one branch of ancient tadpole-like protochordates evolved a new metamorphosis stage to turn into sedentary sea squirts. Sea squirts have some of the fastest recorded rates of molecular evolution.
Lancelets are a text book example of a chordate. Equipped with a notochord, a nerve tube on the dorsal side and gill slits, they typify the phylum Chordata. But lancelets are not primitive nor our remote ancestor. They are as modern as all other members in the pilgrimage.
The Lancelet's Tale continues to develop the theme introduced in The Duckbill's Tale, that all living animals have had equal time to evolve since the first concestor, and that no living animal should be described as either lower or more primitive. The authors extend this concept to apply to fossils as well. Even though it is tempting to label fossils as our remote ancestor, they are more accurately described as our distant cousins who have been frozen in time.
Non-chordate animals
From the lancelets onward, the authors provide dates under duress stating that "dating becomes so difficult and controversial that my courage fails me".
This diverse group includes the echinoderms, along with some organisms labelled "worms" and even Xenoturbella, which until 2016 could not be classified at all, but analysis of its genes finally established its position as a distant relative of the echinoderms.[16][17]
The protostomes join the deuterostomes, and this joint is the originator of the kingdom Animalia. Protostomes (meaning 'mouth first') and deuterostomes (meaning 'mouth second') are based on the way animal embryos diverge after gastrulation where the blastula (a hollow ball of cells) indents to form a cup.
In the sub-kingdom of protostomia, the indentation eventually becomes the mouth. In deuterostomia which includes humans, the indentation eventually becomes the anus; the mouth is formed later. This ancestor is sometimes referred to as Urbilaterian. This brings in the Insecta which represent three-quarters of all animal species on Earth.
The Ragworm's Tale is about the evolution of left-right symmetry in bilaterians. Dawkins discusses the evolution of eyes, thought to have happened as many as 40 times convergently. Despite the diversity of eyes, the same genes control their development in different species, for reasons seen in The Fruit Fly's Tale.
The Brine Shrimp's Tale is about its proclivity for swimming upside down, which it shares with fairy shrimp. Dawkins suggests that natural selection favors individuals who learn a behavior, and that genes “catch up.” He asks how it is that animals come to distinguish up from down. It seems to be an instinct, as Dawkins’s graduate work showed. The moral of the story is that evolutionary changes may start as learned behaviors that are reinforced by natural selection.
The Leaf Cutter's Tale is a variation on the theme of the Farmer’s Tale. Fungus-growing ants of the genus Atta independently invented agriculture, long before humans, by domesticating fungi and harvesting their gongylidia. Several species of ants have independently domesticated aphids. The moral of the story is delayed gratification as the basis for agriculture.
The Fruit Fly's Tale is about embryology. It is a variation on the theme of The Mouse’s Tale, about how genes control development. A handful of Hox genes control development in almost all animals. The body plans of mice, men and fruit flies are found in Hox genes, laid down in the same order on chromosomes in all species since Concestor 26.
The Rotifer's Tale is about the evolution of sex. The bizarre bdelloid rotifer reproduces via parthenogenesis. Through mitosis, they produce eggs which yield genetically identical daughters. Mark Welch and Matthew Meselson have found that rotifers have been asexual for millions of years. John Maynard Smith called this “an evolutionary scandal.” The authors write that sex may be the real scandal. Why do we do it? One answer comes from the Red Queen hypothesis, that organisms are engaged in an evolutionary arms race against parasites. Sex (specifically, recombination during meiosis) creates the variation that is the raw material for natural selection.
The Barnacle's Tale is about how appearances can be deceiving, as Darwin discovered through long study. Barnacles, as their larva stage reveals, are crustaceans. Perhaps the most bizarre barnacle is Sacculina, which is as a parasitic castrator of crabs.
Still under debate on how this group fits in due to a long period of molecular evolution similar to the Gibbons Tale. These flatworms lack an anus or a coelom. The organs do not sit in a coelom but a parenchyma and is the reason for the name of the group.
The Jellyfish's Tale discusses how some underwater organisms migrate between different depths due to day and night cycles.
The Polypifer’s Tale is about the formation of coral reefs. Corals are symbionts with zooxanthellae, photosynthetic algae, and as such grow in shallow waters. How do barrier reefs form in deep ocean? Darwin discussed this in his first scientific book, The Structure and Distribution of Coral Reefs. He suggested that the seafloor subsided. His theory still stands, and the subsidence is explained by plate tectonics.
It is not completely clear whether Ctenophora should be placed here as an outgroup to all animals and actually at rendezvous 31. But this would mean that they either independently invented muscle, nerves, cell layers or that the sponges lost them: only 100 species but quite numerous. DNA studies are also complicated by incomplete lineage sorting like with the Gibbon.
The last animal of the chain. Do not move but have a coordinated movement between cells. Also, seems to be two lines of sponges based on molecular data. Sponge cells are totipotent.
The Sponge's Tale is about multicellularity. In 1907, Henry Van Peters Wilson separated a sponge into its component cells with a sieve. The sponges reassembled themselves. The authors suggest this sheds light on the origin of metazoans.
Non-animal eukaryotes
There are essential differences between the 1st and 2nd editions of the book in this section. Another rendezvous has been added (#33), and the unknown rendezvous has been partially resolved.
The Choanoflagellate's Tale is about the evolution of multicellularity. Choanoflagellates are the closest living relatives of the multicellular animals, and can form temporary colonies from a free-living unicellular stage. Sponges have choanocytes, cells that resemble single-celled choanoflagellates, providing an indication about how multicellularity may have evolved. This common ancestor is sometimes called urmetazoan and several theories have been developed on its evolution.
The acronym comes from the letters of the four genera that were first known. These are single-cell parasites of fish and other freshwater animals. DNA sequencing has added about 50 species. Of course, this concestor could not have been a parasite of a fish.
'Amoeba' is a description rather than a classification because many unrelated eukaryotes exhibit an amoeboid form.
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Very large group of light harvesters and their kin: excavates, SAR supergroup, 20 species of single-celled glaucophytes, over 4,000 species of red algae, and hundreds of thousands of species of green plants.
Gometrical considerations of constructing the most efficient supply tube network in tissues dictate a scaling exponent of 3/4 for such different structures as cauliflower and our brain.
The Humped Bladderwort's Tale is about the C-value paradox, that the size of an organism’s genome does not correspond to complexity. In 1983, Barbara McClintock won the Nobel Prize in Physiology or Medicine for her discovery of transposons, genetic elements that cut and paste themselves through the genome. The C-value paradox is resolved by differences in the number of transposons. The Humped Bladderwort has managed to delete most of its parasitic DNA. This may be because a paucity of phosphorus, a key element of DNA, created selective pressure to economize production of the molecule. In primates (including humans), the most common parasitic element is Alu. Presumably, it emerged between Concestors 9 and 10.
The Mixotrich's Tale is about symbiosis. Mixotricha paradoxa translates to ‘unexpected combination of hairs’, so named by J. L. Sutherland because she thought it had both cilia and flagella, not thought possible in protists. In fact, they are not cilia, but bacteria, specifically spirochaetes, which serve Mixotricha as ‘galley slaves’ in S. L. Tramm’s phrase. Mixotricha is itself a symbiont, helping its host digest cellulose. It lives only in the termite Mastotermes darwiniensis.
Great Historic Rendezvous
This is a significantly shorter section in the second edition. The authors describe the critical beginnings of eukaryotic cells and describe the endosymbiotic theory proposed by Lynn Margulis.
Prokaryotes
Prokaryotes (Greek, before kernel) lack a cell nucleus. They can move genetic material between unicellular and multicellular organisms via horizontal gene transfer.
Carl Woese proposed Archaea as a distinct Kingdom, apart from Eukarya and Bacteria. Archaea have metabolic pathways more closely related to eukaryotes such as the enzymes involved in transcription and translation.
However, as seen in “The Rhizobium’s Tale”, wheels seem to have evolved only once, in bacteria. Similarly, syntactic language seems to have evolved only once, in humans.
He concludes that science gives meaning to human existence: “Not only did evolution happen, it eventually led to beings capable of comprehending the process, and even of comprehending how they came to comprehend it.”[4]:699
The Guardian thought it was awkward to move backward in time starting from humans and thought this required linguistic gymnastics with new definitions of before and after a certain evolutionary point.[19]Matt Ridley, in the same publication, appreciated the approach of a Chaucerian Pilgrim traveling backwards and the perspective of not seeing other animals as failures.[20]
Jody Hey notes that Dawkins “writes engagingly on evolutionary topics. With a highly self-assured style, he effortlessly draws insightful connections among disparate notions, trapping the curiosity of readers before they know what’s coming.” However, he says “An unfortunate editorial oversight is seen in the text’s occasional straying into political commentary. Worse still, Dawkins at one point chastises Richard Lewontin, the great population geneticist, for sometimes interjecting politics into scientific discourse. This little touch of hypocrisy is hard to miss if you read the entire volume. But such lapses amount to a few dozen words in a weighty, truly wonderful book.”[21]
Steve Jones calls it “a rigorous and impressively complete account of the Tree of Life… The Ancestor's Tale achieves the almost impossible: it makes biology (not biochemistry, brain science, or bird-watching, but biology as a whole) interesting again. Everyone possessed of a cell nucleus should read it, and ponder their own unimportance. One mystery remains: what did the star-nosed mole say to the duck-billed platypus?”[3]
↑Lamichhaney S, Berglund J, Almén MS, Maqbool K, Grabherr M, Martinez-Barrio A, Promerová M, Rubin CJ, Wang Zamani N, Grant BR, Webster MT, Andersson L (11 February 2015). "Evolution of Darwin's finches and their beaks revealed by genome sequencing". Nature. 518 (7539): 371–5. Bibcode:2015Natur.518..371L. doi:10.1038/nature14181. PMID25686609. S2CID4462253.
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