The Ancestor's Tale (subtitled "A Pilgrimage to the Dawn of Life") is a 2004 popular science book by Richard Dawkins, with contributions from Dawkins' research assistant Yan Wong. It follows the path of humans backwards through evolutionary history, meeting humanity's cousins as they converge on common ancestors. The book was nominated for the 2005 Aventis Prize for Science Books.
The narrative is structured as a pilgrimage, with all modern animals following their own path through history to the origin of life. Humans meet their evolutionary cousins at rendezvous points along the way, the points at which the lineage diverged. At each point Dawkins attempts to infer, from molecular and fossil evidence, the probable form of the most recent common ancestor and describes the modern animals that join humanity's growing travelling party. This structure is inspired by Geoffrey Chaucer's The Canterbury Tales.
The pilgrimage visits a total of 40 "rendezvous points" from rendezvous zero, the most recent common ancestor of all of humanity, to rendezvous 39, eubacteria, the ancestor of all surviving organisms. Though Dawkins is confident of the essential shape of this phylogenetic taxonomy, he enters caveats on a small number of branch points where a compelling weight of evidence had not been assembled at the time of writing.
At each rendezvous point, Dawkins recounts interesting tales about cousin animals which are about to join the band of pilgrims. Every newly recruited species, genus or family has its own peculiar features to offer as amusement for readers. For instance, Dawkins discusses why the axolotl never needs to grow up, how new species come about, how hard it is to classify animals, and why our fish-like ancestors decided to move on to land. These peculiar features are studied and analyzed using a newly introduced tool or method from evolutionary biology, carefully woven into the tale to illustrate how the few simple assumptions of Darwinian evolution can explain all diversity in nature.
Even though the book is best read sequentially, every chapter can also be read independently as a self-contained tale with an emphasis on a particular aspect of modern biology. As a whole, the book elaborates on all major topics in evolution. The Ancestor's Tale can be considered an encyclopedia on evolution disguised as a collection of fascinating stories.
Dawkins also tells personal stories about his childhood and time at university. He talks with fondness about a tiny bushbaby he kept as a child in Malawi (Nyasaland). He described his surprise when he learned that the closest living relatives to the hippos are the whales.
The book was produced in two hardback versions: a British one with extensive colour illustrations (by Weidenfeld & Nicolson), and an American one with a reduced number of black-and-white illustrations (by Houghton Mifflin). Paperback versions and an abridged audio version (narrated by Dawkins and his wife Lalla Ward) have also been published.
Dawkins coined the term concestor for the most recent common ancestor at each rendezvous point. At each rendezvous point, we meet the concestor of ourselves and the listed species or collection of species. This does not mean that the concestor was much like those creatures; after the "rendezvous", our fellow "pilgrims" have had as much time to evolve and change as we have. Only creatures alive at the time of the book's writing join us at each rendezvous point. Except for a few special cases, numerous extinct species and families such as the dinosaurs are excluded from the pilgrimage.
The Farmer's Tale describes the Neolithic Revolution
Great Leap Forward
The Cro-Magnon's Tale describes the Great Leap Forward.
Human pilgrims join their evolutionary cousins, chimpanzees and bonobos.
The Gorilla's Tale considers human's changing attitude towards the great apes, ending with a discussion on racism, speciesism and the Great Ape Project.
The Orangutan's Tale introduces the principle of parsimony and its use in construction of family tree (cladogram) of species. Orangutan is the last of the great apes to join the pilgrimage.
The Gibbon's Tale further elaborates on neighbor-joining, parsimony and textual criticism techniques used to construct cladograms. When simple principle of parsimony proves inadequate to handle 'long branch attraction' problems caused by convergence and reversion, the phylogenetic tree (phylogram) and computational phylogenetic methods such as maximum likelihood analysis are introduced. The tale ends with yet another example of trans-specific polymorphism: sexual dimorphism; the sex-determiningSRY has never been in female bodies long since gibbons and humans diverged. This serves to highlight the fact that different phylogenetic trees can be created by tracing different sets of genes; the one mainstream 'species tree' is nothing more than a summary of multitude of gene trees, a 'majority vote' among gene trees. Gibbon is the last ape to join the pilgrimage.
Old World Monkeys
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'.
New World Monkeys
The Howler Monkey's Tale calls attention to the critical role of gene duplication in evolution. While our remote vertebrate ancestors possessed trichromatic vision, our nocturnal, warm-blooded, mammalian ancestors lost one of three cones in the retina at the time of dinosaurs. This is why fish, reptiles and birds are trichromatic while all mammals with the exception of apes and New World monkeys are strictly handicapped dichromats. Because color vision is of paramount importance to diurnal animals that eat ripe fruits, apes and New World monkeys regained tri-color vision independently via chromosomal translocation. In apes, trichromacy resulted from true duplication of the opsin gene. New World monkeys first achieved trichromacy in its female population by producing two alleles (green and red) for the same locus for the opsin gene on the X-chromosome, an example of polymorphism. Its males, with only one copy of the X-chromosome, remained dichromats with either a green or a red opsin, an example of heterozygote advantage. Howler's monkey, a type of New World monkey, took this one step further and achieved trichromacy for both sexes when its X-chromosome gained two loci to house both the green allele and the red allele. New World monkeys are the last simians (also known as 'higher primates' or anthropoids) to join the pilgrimage.
Tarsier is the last haplorrhine to join the pilgrimage. A nocturnal animals, the tarsier has two enormous eyes each as large as its brain. Unlike other nocturnal mammals, however, tarsier eyes do not contain tapetum lucidum which reflects light from the back of the eye for a second exposure on the retina to maximize light capture. From this we can infer that the common ancestor of all haplorrhine must have been a diurnal animal which shed the tapetum lucidum to eliminate blurry images caused by reflected light. When the tarsier became a nocturnal animal, it enlarged its eyes to compensate for the lack of tapetum lucidum.
Lemurs, Bushbabies and their kin
The pilgrimage meets with the rest of the prosimian cousins: the lemurs, pottos, bushbabies, and lorises. The Aye-Aye's Tale showcases the strange lemurs which are only found on the island of Madagascar. Madagascar was originally part of the Gondwana supercontinent which included present Africa continent and Indian subcontinent. Gondwana broke off into drifting blocks of land, some of which became Africa, India and Madagascar. As an estranged island, Madagascar became a speciation hotbed. For instance a small founding population of strepsirrhine primates (possibly rafted in from neighboring continent) flourished and diversified into all niches of the ecosystem, in the absence of monkeys. The story reminds us how Madagascar, with a land mass 1/1000 of Earth's total land area, ends up housing unique species that account for 4% of all species of animals and plants. Lemurs and their kin are the last of the primates to join the pilgrimage.
Colugo and Tree Shrews
The Cretaceous–Tertiary extinction event occurred at 65 million years ago, due to both large scale volcanic activities in the Deccan traps over a period of time, and the final asteroid impact event which created the Chicxulub Crater. The sudden temperature change and sunlight reduction caused massive disruptions to Earth's ecosystem. As a result, all dinosaurs except the birds, as well as numerous other species went extinct. The disappearance of dinosaurs made it possible for many different species of shrew-like, nocturnal insectivores to evolve into hippos, lions, elephants, etc. to fill the new ecological voids, an example of evolutionary radiation. One of these shrew-like creatures was the concestor of the current pilgrimage party and the new joiners, the colugos and tree shrews.
The tree shrews resemble the squirrels. The colugos resemble flying squirrels. In both cases, the resemblance is only superficial, due to convergence; the squirrels are rodents and will meet us further down the pilgrimage. At the present, scientists are not yet sure about the exact relationships among the tree shrews, the colugos and us. Dawkins provisionally accepts the view that they join forces first, before meeting the pilgrimage party. This would place our concestor 9 at a time before K-T boundary which marked the extinction of the dinosaurs. The Colugo's Tale warns us that even though the general structure of the family tree is sound, some of the details could change as more evidences become known.
Rodents and Rabbitkind
Rodents comprise the largest number of species in mammalia, more than 40 percent of all mammalian species. Members include rats, mice, lemmings, beavers, squirrels, etc. The Mouse's Tale explains how mammals possess similar and relatively small genomes in the order of 30,000 genes, yet each animal exhibits distinct features and surprising complexities. Dawkins debunks the popular description of genome as blueprints which give rise to the misconception that the more complex the animal, the more complex the blueprint ought to be. Instead, genes in a genome should be thought of as words or sentences in a language, and embryonic development over time is akin to 'order' of words and sentences in a book. While the number of genes are limited, the endless number of 'orders' by which similar genes in mice and humans are deployed during embryonic development can generate astonishing complexity and distinguish a mouse from a man.
The Beavers's Tale revisits the key insights that Dawkins contributed to the field of evolution in his book The Extended Phenotype. A beaver's body is known as a phenotype, an external and visible manifestation of the internal and hidden genotype. In the same way the beaver body is regarded as an expression of its genes, beaver dams or beaver lakes can be considered 'extended phenotypes' of the same beaver genes. Better beaver genes make better beaver bodies, beaver dams and beaver lakes. In other words, beaver genes are selected not only by the fitness of beaver bodies, but also by the effectiveness of beaver dams and beaver lakes they produce.
An extremely diverse group of animals join the pilgrimage, including Carnivora (dogs, cats, bears and seals), Perissodactyla (horses, tapirs and rhinos), Cetartiodactyla (deer, cattle, pigs and hippos), Chiroptera (bats), Insectivora (moles and shrews), etc. Some of them fly, others swim, and yet many of them gallop. Half of them are predators which hunt the other half of the group. The only thing they share in common is that they join up with one another before the group joins us to meet concestor 11. This group of animals belong to the Laurasiatheria clade as all of them originated from the supercontinent of Laurasia.
The Hippo's Tale is really the whale's tale. All cetaceans, including whales, dolphins and porpoises, are descendants of land-living mammals of the Artiodactyl order (even-toed ungulate animals). Both cetaceans and artiodactyl are now classified under the super-order Cetartiodactyla which includes both whales and hippos. In fact, whales are the closest living relatives of hippos; they evolved from a common ancestor at around 54 million years ago. This story illustrates how a species can flip into evolutionary overdrive when it enters into a new environment, while its closest relatives remain unchanged for long time in their static environment.
The Seal's Tale illustrates how a sex ratio of 50:50 (males to females) is found in most sexually reproducing animals from both monogamous and polygamous species. In a harem-based (polygynous) system such as that of elephant seals where 4 percent of males account for 88 percent of all copulations, the actual sex ratio of 50:50 seems to produce an excess of males who consume resources but end up leaving no offsprings. This puzzle is solved by the concept of parental expenditure proposed by R.A. Fisher. More importantly, the elephant seal 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 degree of sexual dimorphism is correlated with the harem size, which allows us to draw inferences about our immediate human ancestors: they were probably mildly polygynous.
The Armadillo's Tale reminds us of the aye-aye's tale, except that instead of Madagascar, the speciation hotbed is the continent of South America. This continent broke off from Gondwana in Early Cretaceous period, then joined North America which broke off from Laurasia. During its long period of isolation, South America was host to marsupials which flourished and took up all carnivorous niches. The placental mammals (including armadillo) and now-extinct ungulates evolved to fill the rest of the ecosystem. When South America joined North America during the Great American Interchange at 3 million years ago, animals and plants cross the Isthmus of Panama in both directions, introducing new species to new land and driving some local species to extinction. Jaguars and other carnivorous placental mammals were introduced to South America, while armadillos migrated to North America.
The pilgrimage party is joined by the last of the placental mammals: elephants, elephant shrews, dugongs, manatees, hyraxes, aardwolves, etc. They all hail from Africa, as hinted by the name of their clade, Afrotheria. The concestor we greet at this point, as well as those we met earlier at rendezvous 12 and 11, all look like insectivorous shrews.
The entire band of placental mammals meet up with the other great group of mammals, the marsupials. Even though present-day marsupials are mostly found in Australia and New Guinea, they originally flourished and diversified for a period of time in South America. Evidence points to the migration of a single species of opossum-like marsupial from South America to Australia before 55 million years ago, when it was still possible to make the journey through Antarctica before Australia pulled too far away from Gondwana. Once settled in the isolated Australia, the founding marsupials quickly evolved into distinct species and, for the next 40 million years, took up the entire range of 'trades' previously occupied by dinosaurs, in the absence of any placental mammals.
The Marsupial Mole's Tale again highlights the wonders that convergent evolution can create. Despite great evolutionary distance between marsupial moles in Australia and the golden moles in Africa, they are remarkably similar in terms of phenotypes, with the exception that the marsupial moles sport a pouch as all marsupials do. There are also marsupial mice (Dasyuridae), marsupial flying squirrels (Sugar Glider) and marsupial wolf (Thylacine), not to mention the equivalent of antelopes and gazelles, the kangaroos and wallabies which despite great differences in shape, cover the same range of diet and way of life as their African counterparts.
The monotremes are the last of the mammals to join us, and we meet a concestor for the first time in the then-contiguous supercontinent of Pangea. The monotremes consititutes only a few genera: Platypus, Short-beaked Echidna and Long-beaked echidna. They are mammals and have typical mammalian features such as warm-bloodedness, hair and milk production. But they resemble reptiles and birds in their possession of the cloaca and their egg-laying mode of reproduction.
The Duckbill's Tale warns us about the fallacy of labeling a half-mammal and half-reptile animal such as the duckbill platypus as primitive. The platypus has exactly the same time to evolve as the rest of mammals, even if it does resembles our concestor 15 on the surface. In fact, it has evolved a highly developed form of electroreception served by 40,000 electric sensors, and 60,000 mechanical push rods, both on its large bill to aid it in search of crustaceans in the mud. In human, the brain dedicates disproportionally large amount of cells to the two hands as illustrated by the Penfield brain map, or Penfield homunculus. When the same somatotopic map is drawn for platypus brain, the bill is served by equally prominent percentage of the brain.
The pilgrims are about to join their reptile cousins, after marching for 130 million unbroken years from the last mammal concestor 15 who looks like a shrew to the reptile concestor 16 who looks like a lizard. In these 130 million years, mammal-like reptiles flourished, even before dinosaurs roamed the Earth. But like the 99 percent of all species that ever existed, all branches of mammal-like reptiles are now extinct, so they cannot join us in our pilgrimage.
The term reptile is not a true clade name, as it fails to include birds which share a common ancestry. The terms reptile and fish are known as grades which only make sense scientifically when used in the now-discredited theory of progressive evolution (Orthogenesis). Progressive evolution proposes that species evolve independently, in a parallel, progressive direction from fish grade through amphibian grade via reptile grade towards mammal grade. From a cladistic point of view, turtles, lizards, snakes, crocodiles, dinosaurs and birds are all members of the clade Sauropsids which is what Dawkins adapts instead of the term reptile. Dinosaurs, unfortunately as extinct species, cannot join us. But their only surviving descendants, the birds, take their place in the pilgrimage.
The Galapagos Finch's Tale addresses a surprising question: why doesn't evolution go much faster than it does? Studies on Galapagos finches show that the Medium Ground-finch (Geospiza fortis) could grow to be as large as the Large Ground-finch (Geospiza magnirostris), if 23 consecutive drought years put pressure on birds to grow larger beaks to better handle bigger and tougher seeds. But such extreme evolutionary speed is not observed in nature, given the geological time life has had on Earth. This is mostly because the follows major trends over geological timescale, while short-term pressures tend to cancel out one another.
The Peacock's Tale is the quintessential illustration of sexual selection; the peacock's tail is the ultimate example of non-utilitarian phenotype which appears to be an anomaly in natural selection, as it is a hindrance to peacock's survival in its natural habitat. These arbitrary spurts of evolution can be explained by sexual selection, a special case of natural selection. The runaway explosion of the extravagant tail is created by lockstep dimorphic selections of male's genes for pretty tails and of female's genes for preferring such tails. The preference for pretty tails, in turns, is rooted in their use by males as tokens of underlying fitness; an advertisement of health. Sexual selection often complements other natural selection forces, and helps explain why human became bipedal to free the two hands for tool making and wielding, attained a larger brain with artistic abilities, and shed body hair to advertise lack of ectoparasites during the course of human evolution.
The Dodo's Tale illustrates how evolution optimizes genes for the present environment, how it has no foresight, and how it marches blindly sometimes to the detriment of the species. The dodo, originally a flying pigeon related to the Rodrigues Solitaire, reached the remote island of Mauritius and shed its flying powers due to the lack of competition and natural predators to become a Flightless bird. The reallocation of resources away from building of massive breast muscles for flying allowed the dodo to flourish, but ultimately lead to their extinction when European sailors and their carnivorous pets finally arrived.
The Elephant Bird's Tale demonstrates how enigmatic distributions of genetically close species in completely separate continents can be explained and corroborated by evidences of continental drift and seafloor spreading. The tale recounts the diaspora of a large group of flightless ratite birds from the then unbroken Gondwana; Moa ended up in New Zealand, rhea in South America, emu in Australia, cassowary in New Guinea, kiwi in New Zealand by island hopping, and ostrich in Africa by way of Asia and Europe. Radioactive dating of and magnetic striping studies on continuously formed crust around rifts such as the Mid-Atlantic Ridge allow Paleobiogeographers to piece back a coherent story of these birds' dispersion based on both phylogenetic tree and plate tectonics.
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 practice 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.
The Salamander's Tale uses examples of ring species to illustrate how a continuous series of interbreeding animals in the spatial dimension is conceptually equivalent to that in the time dimension. The Ensatina salamanders in the Central Valley in California form a continuous ring (actually a horseshoe shape) around the valley. Any two neighboring population of Ensatina around the horseshoe can interbreed, but the plain Ensatina eschscholtzii on the western end of the horseshoe cannot interbreed with the large blotched Ensatina klauberi on the eastern end. Larus gulls form another ring species which starts at Herring Gull in Great Britain and ends at Lesser Black-backed Gull in north-western Europe. Dawkins likens both ring species in space to the ring in time that unites humans and chimpanzees via generations of ancestors over 6 million years, with concestor 1 in the midpoint.
The Narrowmouth's Tale shows how speciation may still continue via parapatric speciation, when two closely related toad species meet again after initial geographical isolation. Gastrophryne olivacea (Great Plains narrowmouth toad) and Gastrophryne carolinensis (Eastern narrowmouth toad) are closely related and can interbreed when their habitats overlap. But reinforcement, a selection process which increases reproductive isolation via character displacement, causes both species to differentiate their mating calls from each other by shifting pitch and duration in opposite directions; the more the two populations overlap, the more distinct their mating calls become.
The Axolotl's Tale is about metamorphosis, a biological process which turns juveniles or larvae into drastically dissimilar adult forms for reproduction, and about pedomorphosis, another process which enables juveniles of some species to become sexually-mature without ever developing into their usual adult forms. Species which undergo metamorphosis include butterflies, barnacles and salamanders. Species which exhibit neoteny, a type of pedomorphosis, include human, ostrich, pekingese and axolotl. A text book example of neoteny, the axolotls are members of the Tiger Salamander complex, yet they become sexually-mature in larva form, remaining aquatic and gilled. With a treatment of thyroxine, it is possible to induce an axolotl to develop into a salamander, demonstrating that axolotl genome still retains information on its lost adult form. On the other hand, newt, a type of salamander, first develops from tadpole into land-based salamander, but later reverts back to its juvenile tadpole form, and returns to the water to reproduce. The axolotl's tale reminds us that paedomorphosis often allows species to break out of an evolutionary dead end by sudden changes.
The Queensland lungfish (Australian lungfish) and Coelacanth are two of the most famous living fossils; they resemble ancient fossils and unlike most species, seemingly refused to continue to evolve for the past 400 million years. The lungfish joins the pilgrimage to meet concestor 18, before the coelacanth joins at rendezvous point 19. Instead of looking like members from the pilgrimage, the lungfish actually resembles coelacanth and concestor 19, the lobe-finned fish Sarcopterygii. Despite their morphological similarities, however, the lungfish and coelacanth are very different genetically, as expected of species which lived separately for more than 400 million years. Because genes do not stop evolving, the molecular DNA of these two species show greater evolutionary distance from each other than to DNA of the rest of the pilgrimage. The Lungfish's Tale reminds us that the rate of morphological changes is not always obviously correlated with that of genetic change.
The current pilgrimage consisting of all descendants of lobe-finned fish is joined by the equally successful ray-finned fishes which includes sturgeon, paddlefish, eel, herring, carp, salmon, trout, seahorse, cod, etc. to meet concestor 19, the bony fish. Of all ray-finned fishes, most belong to the large infraclass teleostei.
Some teleost fishes evolved unfishy shapes to cope with their chosen ecological niches. The leafy sea dragon, for instance, abandons the typical streamline fish shape which works so well for the majority of fishes. Instead, it adopts a leafy shape to hang motionless in kelp forest, pretending to be a piece of seaweed. The razorfish takes up an elongated, laterally compressed body, together with a long, flattened snout. It swims in a head-down vertical stance, allowing it to hide amongst tall spines of a sea urchin. The snipe eel is ridiculously thin, while the gulper eel sports jaws which look disproportionally large for its body. Lastly, the ocean sunfish resembles a huge, two-ton disc or millstone, as its Latin name, Mola mola, suggests. The Leafy Sea Dragon's Tale demonstrates how animal shapes are malleable, ever changing to meet the requirements of each animal's way of life.
The Pike's Tale highlights a special organ which gives teleost fishes superior buoyancy control, the swim bladder. Contrary to common assumptions, swim bladder is not a precursor to lungs in human and other lobe-finned fishes. Instead, the bony fish ancestor possessed a primitive lung which was co-opted by teleost fishes for buoyancy control, and in some cases as ear drum for hearing. The teleost fishes rely on gills for breathing underwater. They repurposed the primitive lung, turning its ability to absorb from and release gas into the blood stream into a volume-changing mechanism, thus allowing teleost fishes to move vertically in a water column without the use of fins.
The Mudskipper's Tale shows how animals rediscover long lost faculties and reenact ancient evolutionary events via completely different biological mechanisms. Having forgone air-breathing by repurposing the lung for buoyancy control, some teleost fishes, such as the Siamese fighting fish (Betta splendens), reinvent air-breathing by gulping air and locally oxygenerating water in the gill chamber. The mudskipper Periophthalmus not only takes air into its moist gill chamber, but can also breathe through its skin. Both the gill chamber and the skin must be wet at all times, and this distinguishes the new type of air-breathing from breathing through lungs. Re-equipped with air-breathing apparatus, the mudskipper emerges onto land, replaying ancient lobefin's conquest of the land.
The rapid speciation of haplochromine cichlid fishes endemic to Lake Victoria, Lake Malawi and Lake Tanganyika examplifies adaptive radiation and species flock. The Cichlid's Tale recounts how, by constructing an "unrooted haplotype network" using phylogenetic analysis on mitocondrial DNA of living species from regional rivers and lakes, researchers were able to infer the time and the location of each major speciation event in the evolutionary history of these cichlids. The haplotype network differs from normal phylogenetic tree in that each node represents a haplotype, not a species, and the node size is determined by number of species in which the haplotype is found. By analyzing genetic relationships between haplotypes, relative prevalence of each haplotype, and locations where species currently live, it is possible to trace past waves of adaptive radiation originating from a small founding species, as rivers and lakes rose and fell in level.
The Blind Cave Fish's Tale illustrates how normal organs can degenerate into vestigial organs. Different populations of Mexican tetra (Astyanax mexicanus) have ventured into dark caves separately, and have separately evolved regressive traits such as white skin coloration and regressive or blind eyes. This is partially explained by the opportunity cost theory; resources wasted on building the eye in a pitch-black cave deprives the fish of other traits useful for such environment. But more importantly, without evolutionary pressure to weed out bad mutations on the multitude of genes which together build the eye, any random change is more likely to disrupt the dedicate process of building the eye than to enhance it. There is no need to revert precisely the sets of genes carefully shaped by millions of years of evolution to get back to a blind creature. In other words, there are many more random ways of building a blind fish than of building a sighted one. And this is the essence of the Dollo's Law as Dawkins interprets it - that evolution cannot be precisely and exactly reversed.
The Flounder's Tale is a tale of imperfection. The flounder's contorted head and eyes allow it to lie on its side on the ocean floor, but they betray the lack of an intelligent designer. As expounded in The Blind Watchmaker and Climbing Mount Improbable, evolution does not 'design' every creature anew on a drawing board. Instead, natural selection works without foresight and makes gradual improvements on existing body plans from generation to generation. Because each creature at every step of the process must remain fit for its environment, evolution cannot make sudden and drastic changes to build a better future organism at the expense of current generation.
Sharks and Their Kin
Cartilaginous fishes chondrichthyan including sharks, rays and chimaeras join the pilgrimage in the Middle Ordovician. The newcomers have no bones. Instead, they are supported by a cartilaginous skeleton that never ossifies, in contrast to bony fishes. Their skin is covered in dermal denticles, tiny scale-like protrusions, from which teeth may have evolved. Interestingly, sharks lack a swim bladder for buoyancy, and instead rely on swimming constantly, retaining urea in their blood, and having large livers with plenty of oil to remain afloat. The Carcharocles megalodon from the Miocene is described as a predator more terrifying than the Great White Shark, as it was three times the size. The strange Chimaera has strange gill covers, has no dermal denticles, and swims using their pectoral fins. Dawkins explains that concestor 21 is ancestor to all gnathostomes, animals with lower jaws, a structure which evolved from the gill arches.
Lampreys and Hagfish
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 further develops the gene's eye view of ancestry and pedigree that earlier tales, The Eve's Tale and The Gibbon's Tale, alluded to. In human, four haemoglobin genes are known to be cousin genes of each other. An ancestor globin gene from an ancient vertebrate split into two genes, alpha and beta, which ended up in two different chromosomes and continued to evolve independently. Both alpha and beta further split into more independently evolving genes. All jawed fish show such alpha/beta split as predicated by evolution. However, lampreys and hagfish are ancient enough that they predate this gene split. In fact, jawless fishes, whenever investigated, do not possess split globin genes. As Dawkins explained in the chapter 'All Africa and her progenies' in his book River out of Eden, there are two ways to trace ancestry: via animals and via individual genes. The two mechanisms produce very different results. Ancestry of animals form a family tree (more correctly, a graph because sexually reproducing animals may share female and male parents). On the other hand, Ancestry of an individual gene is always a single chain going back to the first self-replicating RNA, since a gene is either a faithful copy or a mutated form of its single parent gene. The Ancestor's Tale is written from an animal's perspective, following the family tree of human backward in time. But the book could have been written from the gene's point of view. Starting from any gene (e.g. the alpha haemoglobin), each gene gene duplication event could become a rendezvous point where pilgrimage of genes join their cousin genes.
Lancelets are text book examples 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. Dawkins extends 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.
A Sea squirt resemble a sedentary bag of sea water anchored to a rock. It feeds on food particles strained from water. Anatomically, the sea squirt looks very different to 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 larvacea favors Darwin's initial interpretation, that one branch of ancient tadpole-like protochordates evolved a new metamorphosis stage to turn into sedentary sea squirts.
The current pilgrimage, known as deuterostomes, is joined by a much larger group of animals, the protostomes, to meet the ancestor of almost all organisms in the kingdom Animalia, a worm. Just a single class in the joining sub-kingdom of protostomia, the Insecta, represents three quarter of all animal species on Earth. The great divide between protostomes (meaning 'mouth first') and deuterostomes (meaning 'mouth second') was devised by comparative embryologists 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. An extremely large variety of animal phyla constitute protostomia, including annelid worms (e.g. garden earthworms), flatworms (e.g. tapeworms and flukes), molluscs (e.g. snails, oysters, ammonites and octopuses), and arthropods (e.g. insects, crustaceans, spiders and centipedes). Unlike the species, classes and genera of animals from the pilgrimage prior to this rendezvous point, joining animals from different phyla have no obvious relationship to one another based on traditional anatomy. But modern molecular rangefinding has allowed molecular taxonomists to organize all phyla in the pilgrimage into a hierarchy, with the worm as the concestor of all animals in Bilateria who are bilaterally symmetrical, with left and right side, a dorsal and a ventral side, and a head and a tail end.
In what Dawkins calls the "Great Historic Rendezvous", he describes the significantly important event of endosymbiosis, which results in the beginnings of eukaryotic cells. In his estimates, this occurred in two or three steps, roughly two billion years ago. Firstly, bacteria, perhaps related to Rickettsia, entered proto-protozoan cells. For one reason or another, the bacteria were not digested and did not kill the cell. The cell offered protection to the bacteria, and the bacteria provided energy to the cell, resulting in a symbiotic relationship. This is the speculated origin of mitochondria. Subsequently, photosynthetic bacteria (thought to be related to cyanobacteria) entered some, but not all, of these mitochondria-containing cells. These ancient bacteria evolved to become chloroplasts, and the cells became the Plant and Algal lineages. Meanwhile, the cells which this second endosymbiotic relationship did not occur in went on to form the Kingdoms Fungi and Animalia, as well as various Protozoans.
Chloroplasts and mitochondria have their own genomes, and they replicate independent of the cell in which they live. Dawkins acknowledges how the endosymbiotic theory proposed by Lynn Margulis is now virtually universally accepted.