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This chapter develops a microfounded model of institutional changes and uses it to examine the joint production of institutions and economic output. In that model, agents must decide to participate in the political life of the city, participation whose level affects the level of the quality of institutions, as well as the possibilities of long-run economic expansion. It is shown that there exists a critical threshold for the quality of institutions below which agents do not participate to the political life, and above which they do participate. It is also shown that the presence of political participation does not suffice to bring immediate economic take-off: several generations of citizens with positive political participation are needed to achieve economic take-off.
Some snakes are the only vertebrates able to engulf prey with cross-sectional areas several times larger than the area encompassed by the snake’s jaws at peak gape. This ability is conferred by modifying soft tissues ventral to the axial musculoskeletal system for extraordinary extensibility between the mandibles and stomach. Moving large prey into the gut depends on structural decoupling of toothed jaws from the braincase. In all living snakes, kinetic jaws form mobile ratchets. In scolecophidians, transverse maxillary or dentary ratchets have evolved to move small prey into the gut. In alethinophidians, longitudinal palatopterygoid ratchets move the head and body of the snake over the prey. Evidence from extant snakes shows that streptostyly, prokinesis, rhinokinesis and loss of all ventral skeletal elements connected to the axial skeleton were critical to evolution of the upper-jaw ratchet on which macrostomy is based. The existing fossil record gives tantalizing clues that suggest the ancestor of snakes might have been macrostomous. Resolution of this issue will require structural details of the snout, braincase, and toothed ratchets in both ‘basal’ extant snakes and fossils.
The origin and early evolution of snakes has long been studied, but little research has focused on soft-tissue organs such as the brain. I report data from dissections and 3D reconstructions of the endocasts of diverse species, including the Cretaceous stem snake Dinilysia patagonica in order to provide a comparative evolutionary framework for the snake brain. Snakes are a special case among reptiles because the braincase almost entirely encloses the whole brain, so endocasts provide realistic representations of brain size and shape. Diversity of brain gross anatomy among snakes is remarkable, encompassing two major cerebrotypes occurring in surface-dwelling and burrowing species. The repeated acquisition of the burrowing cerebrotype in different and phylogenetically distant snake clades suggests that brain gross anatomy is surprisingly evolutionary labile in snakes. Brain gross anatomy and other features such as body size and the absence of any unequivocal osteological feature related to burrowing is interpreted as evidence that D. patagonica was surface-dwelling, and that at least some of the early history of snakes occurred above ground.
Genomic studies have elucidated some molecular underpinnings for adaptations during the early history of snakes, but studies of dietary adaptations remain sparse. Snakes differ from most other squamates by tending towards diets of vertebrate prey (carnivory), whereas arthropods are common in diets of most other squamates (insectivory). To test whether a shift from insectivory to carnivory occurred early in snake history, I examined chitinase genes (CHIAs) in 19 squamates. Previous studies on mammals found that contraction in the number of CHIAs, which enzymatically digest arthropod chitinous exoskeletons, correlates with transitions from insectivory to carnivory or herbivory. I found evidence that CHIAs have a long history in Squamata, with at least seven paralogs inferred in their last common ancestor. Retention of these CHIAs seems to be commonplace for arthropod-eating squamates, but snakes likely lost six CHIAs between diverging from other toxicoferans and the origin of afrophidian snakes. This genomic signal corresponds with an inferred major shift towards carnivory during the origin and evolution of early snakes, which may have contributed to their successful radiation.
Snakes have distinct body plans that can be traced to the origin of the clade. It remains unresolved whether ancestral snakes were adapted to terrestrial environments as burrowers, or to marine environments as swimmers. Recently, new approaches have been used to infer fossorial and aquatic specialists in the early evolution of snakes, using virtual CT models of the ear of fossils. This chapter reviews variation in the osseous part of the ear of major snake lineages. Vestibules are relatively large in fossorial species and small in aquatic snakes. Using quantitative analyses of bony labyrinth geometry, it has been suggested that putative stem snakes, such as Dinilysia patagonica, were fossorial. Improvements to testing correlations between bony labyrinth morphology and ecology can be made in the refinement of quantitative approaches to capturing and analysing shape variations, as well as better classifications of ecology. Using inner and middle ear morphology to improve the accuracy and precision of inferences of the ecology of the ancestral snake will depend also upon robust, well-resolved phylogenies for extinct and extant taxa, and denser taxonomic and ecomorphological sampling.
Venom, a specialized form of poison, is actively injected by the venomous organism into its target animal to facilitate several quotidian functions. Over a hundred convergent origins of this remarkable functional trait, along with intricate mechanisms of venom delivery, have been documented across animals. Pinpointing the emergence of venom in squamate reptiles has important implications for understanding the evolutionary history of snakes, but it has been challenging. Several competing hypotheses have been put forth to explain the evolutionary origin of squamate venom, including assertions of single, dual and multiple origins. In this chapter, in addition to a summary of this ongoing dialogue, we provide an overview of ecology, composition, delivery mechanisms, and evolutionary models that explain the possible origin and diversification of venom in squamate reptiles.
The distinctiveness of their eyes has played a major role in debates about snake origins and early evolution, having been interpreted as providing evidence for nocturnal and/or fossorial (and to a lesser degree, aquatic) origins. Much of this evidence came from anatomical studies of snake retinas in the 1900s. More recent morphological and molecular studies have provided further evidence for the distinctness of the snake eye that lacks many of the traits present in lizards. Data remain patchy and are particularly sparse for extant lineages (scolecophidians and non-caenophidian alethinophidians) that bear special importance for inferring traits of the ancestral snake. However, evidence is strong for: (1) the ancestral snake having lost multiple anatomical and molecular genetic components present in the eyes of the ancestral squamate and retained by most lizards; (2) the eye of the ancestral snake being adapted for low-light environments and/or activity cycles but being notably less regressed than that of extant scolecophidians; (3) an elaboration and diversification of the eye within endoglyptodont caenophidian snakes.
This chapter considers the close relationship between child rulership and innovative political and administrative adaptation between the eleventh and thirteenth centuries. Cases of child kingship prompted adaptations to some of the tools of governance, but the boy king’s presence and active contribution were often still crucial. The chapter turns first to the documentary evidence and the diversity of administrative experimentation before focusing on the enduring significance of children’s participation in rule. The third and final section examines practical adjustments to and contemporary representations of counsel, a fundamental instrument of royal rule which could be even more crucial when a boy was king. Overall, the chapter presents an alternative narrative of child rulership which stresses aspects of innovation, adaptation and co-operation. Considering shifts in documentary culture, royal government and consilium by the thirteenth century also reveals the extent to which many of the practical solutions adopted during a period of child kingship differed much more profoundly across time than they did geographically.
The next major entrant into the field of postwar paleoanthropology was Louis Leakey, a flamboyant character who had been scouring his East African homeland for hominid fossils since the 1930s. One major site of interest, in what is now Tanzania, was a large erosional gulley called Olduvai Gorge. And in 1959, in the Gorge’s oldest exposed sediments (known as Bed I), Louis’s archaeologist wife, Mary, discovered the cranium of a hyper-robust australopith that had tiny incisors and canines, and huge, flat cheek teeth (Figure 4.1, right). In the same year, they named this amazing specimen Zinjanthropus boisei (it is now regarded as a Paranthropus). The Leakeys had long known that Bed I contained very simple “Oldowan” tools consisting of rock “cores” from which sharp stone flakes had been bashed using hammerstones. The Oldowan material culture (the tools are often referred to as “Mode 1”) clearly preceded anything then known from Europe; “Mode 2” handaxes comparable to those at the bottom of Mortillet’s European sequence only appeared higher in the Olduvai geological section (though the Oldowan is now known in Europe, too).
There is little, if anything, in the hominin fossil record that can be said to closely anticipate the very derived anatomy of our species Homo sapiens. Compared to other members of the genus Homo, modern humans are exceptionally slenderly built, with narrow hips and barrel-shaped rib cages that taper both at the top and the bottom. Our skulls are high, short, and rounded (Figure 8.1); and instead of projecting, our very small faces are retracted beneath the front of our braincases. Our brow ridges vary from modestly protrusive to barely detectable, but they are invariably bipartite, with central and lateral surfaces separated by an oblique furrow. We are also alone among the hominins in having a true chin at the front of the lower jaw. This takes the form of an inverted “T,” with a vertical ridge in the midline of the jaw atop a horizontal bar running between a pronounced tubercle on each side.
The morphological medley of hominin crania known in Africa following the one-million-year mark indicates that the subfamily’s tendency to diversify continued unabated. It is frankly unclear exactly what was happening earlier in this period, but by about 600 kyr ago one hominin species had come to dominate the scene: Homo heidelbergensis. This was the world’s first-documented cosmopolitan hominid, with representatives from France, Italy, and Greece (Figure 7.1, left) in Europe; from South Africa, Zambia (Figure 7.1, center), and Ethiopia in Africa; and from China in Asia (Figure 7.1, right), apparently including the recently ballyhooed “Dragon Man” cranium from Harbin that was dubbed Homo longi. Dating of many of the known specimens is poor, but plausible dates as early as 600 kyr have been claimed for H. heidelbergensis in both Europe and Africa; and, the Dragon Man (>146 kyr) excepted, no proposed date for the species is more recent than about 200 kyr. Most of the fossils that represent H. heidelbergensis are cranial, and present us with a picture of a heavy-boned form with a modestly sized dentition and a reasonably large brain of between about 1,166 and 1,325 ml. Its face is massive, surmounted by very high brow ridges that show a characteristic lateral “twist.” Nothing like a complete skeleton of H. heidelbergensis is known, but the postcranial bones we do have are witness to a robust build, with a moderately wide pelvis and heavily built limbs of basically modern proportions. Allowing for flyaway hair and almost certainly some clothing in northern climes, on the landscape you would likely have had to approach to within a dozen yards of one of these hominids before clearly noticing that he or she looked rather different from you.
When Charles Darwin published On the Origin of Species in 1859, only a tiny handful of human fossils – the material evidence of the ancient human history that his book implied must exist – had been discovered. Some of them had not even yet been properly recognized, although the most significant of them – the partial skeleton from the Neander Valley in Germany that had recently been found alongside the remains of extinct Ice Age animals – was on the brink of becoming the world’s most famous fossil. Discovered accidentally in 1856 by lime miners, and only preserved by great good chance, the Neanderthal skeleton – principally a large skullcap (Figure 3.1, right) and some very robust limb bones – rapidly became the subject of vigorous debate in Germany between those who thought it had belonged to a member of an ancient barbarous tribe, and those who thought it simply the remains of a pathological modern human. In England it caught the attention of the comparative anatomist Thomas Henry Huxley, an expert on dinosaurs who had been Darwin’s most vociferous supporter following the publication of the Origin, but who had also chided him for rejecting the notion that Nature at least occasionally “makes jumps” (i.e., speciates).
Seven million years ago the continent of Africa, actively bulging upward along the north–south line of the volcanically active Great Rift Valley, was also experiencing climatic drying and increased seasonality of rainfall due to a general oceanic cooling. Particularly to the east of the Rift, the formerly ubiquitous forests were giving way to woodlands and bushlands, and even to some early grasslands, stressing the populations of large-bodied, tailless, and mainly fruit-eating apes that the Miocene forests of both Africa and Eurasia had harbored in profusion. But the stress of change also brought with it opportunity, in the form of the very different range of potential food resources offered by the expansion of more open environments. And while modern apes living partially in open environments tend to seek essentially the same resources there as those they exploit when living in closed forest, it appears that some archaic ape lineages were prepared to be a little more flexible, and to explore the new opportunities the expanding mosaic of environments had to offer.
Within the lifetimes of some of today’s older paleontologists, the armamentarium available to those who studied fossils was pretty limited, consisting mainly of hands, brain, and a rock hammer. A fossil, by the way, is any evidence of past life: An ancient footprint or worm burrow is technically a fossil, although as far as mammals like us are concerned the vast majority of fossils are the mineralized remains of bones and teeth. These are the hardest tissues of the body, and thus have the best chance of being preserved in the rock record. For reference, Figure 2.1 shows a human skeleton with the major bones identified.
The message from the population movements briefly recounted in the previous chapter is that Homo sapiens has been an itinerant species from the outset. Even in a world with intensively patrolled political borders there is no reason to believe that this propensity will be contained any time soon: It is an ongoing phenomenon that will have to be managed, hopefully as humanely as possible. More generally, the message deriving from the story told in this book is that we modern human beings have an astonishingly recent origin, and a sudden one. In evolutionary terms, we acquired our extraordinary symbolic reasoning capacities virtually overnight, and we did so exaptively (i.e., not in the context familiar today) rather than adaptively (within that symbolic context). What this most importantly tells us is that we cannot have been fine-tuned by natural selection over the eons to be the kind of creature we are today; there was simply not enough time. Which in turn suggests that, within the limits of circumstance, Nature has (albeit unintentionally) given human beings almost unlimited freedom to become the kinds of creatures they individually choose to be.
The notion that our planet and its inhabitants have not remained exactly as the Creator was supposed to have made them was in the air long before 1859, when the English natural historian Charles Darwin collected and published his evolutionary ideas in his great work On the Origin of Species by Means of Natural Selection. By that time geologists had long known that the 6,000 years allowed by the Bible since the Creation was vastly inadequate for the sculpting of the current landscape by any natural mechanism; and the biologists who were just beginning to study the history of life via the fossil record were not far behind them. Around the turn of the nineteenth century, the French zoologist Jean-Baptiste Lamarck began to argue that fossil molluscan lineages from the Paris Basin had undergone structural change over time, and that the species concerned were consequently not fixed. Importantly, he implicated adaptation to the environment as the cause of change, although the means he suggested – subsequently infamous as “the inheritance of acquired characteristics” – brought later opprobrium.
Once Louis Leakey and his colleagues had described Homo habilis in 1964 the search for his Holy Grail of the “earliest Homo” was on, and the field was wide open for other entrants that had very little in common with Homo sapiens, the species by which our genus is defined. Genera are hopefully monophyletic collections of species descended from the same common ancestor; but there are no formal rules governing how inclusive they can be, so you could in theory create a monophyletic genus by including every primate on the planet. By unwritten convention, however, zoological genera are in effect the largest readily (I almost wrote “intuitively”) recognizable unit, species being basically variations on the theme established by the genus. All species of the genus Felis are identifiably cats, and all Rattus are visibly rats. By this rule-of-thumb, as Bernard Wood and Mark Collard pointed out over two decades ago, the genus Homo should be confined to species that have significant resemblances to Homo sapiens. Just being related to the Homo clade, at some remove, is simply not enough.
Human life, and how we came to be, is one of the greatest scientific and philosophical questions of our time. This compact and accessible book presents a modern view of human evolution. Written by a leading authority, it lucidly and engagingly explains not only the evolutionary process, but the technologies currently used to unravel the evolutionary past and emergence of Homo sapiens. By separating the history of palaeoanthropology from current interpretation of the human fossil record, it lays numerous misconceptions to rest, and demonstrates that human evolution has been far from the linear struggle from primitiveness to perfection that we've been led to believe. It also presents a coherent scenario for how Homo sapiens contrived to cross a formidable cognitive barrier to become an extraordinary and unprecedented thinking creature. Elegantly illustrated, Understanding Human Evolution is for anyone interested in the complex and tangled story of how we came to be.
Our young and originally tropical species Homo sapiens has spread, in an amazingly short period of time, to occupy more areas of our planet than any other animal species has ever contrived to do. Human beings reside on all five continents, and in virtually every environment that those continents have to offer.
Like every one of the many millions of other organisms with which we share our planet, the species Homo sapiens is the product of a long evolutionary history. The first very simple cellular organisms spontaneously arose on Earth close to four billion years ago, and their descendants have since diversified to give us forms as different as streptococci, roses, sponges, anteaters, and ourselves.