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The position of Antarctica at the South Pole and the landlocked Arctic Ocean have been key variables in initiating the current ice age, and another continental configuration that could contribute to initiating an ice age is , which and . A hypothesis is that can accompany supercontinents, so warm water is not pushed to the poles as vigorously. A supercontinent near the equator would not normally have ice sheets, which means that would be enhanced and remove more carbon dioxide than usual. Those conditions could initiate an ice age, beginning at the poles. It would start out as sea ice, floating atop the oceans. Around when Harland first proposed a global ice age, a climate model developed by Russian climatologist concluded that if a Snowball Earth really happened, the runaway positive feedbacks would ensure that the planet would never thaw and become a permanent block of ice. For the next generation, that climate model made a Snowball Earth scenario seem impossible. In 1992, a professor, , that coined the term Snowball Earth. Kirschvink sketched a scenario in which the supercontinent near the equator reflected sunlight, as compared to tropical oceans that absorb it. Once the global temperature decline due to reflected sunlight began to grow polar ice, the ice would reflect even more sunlight and Earth’s surface would become even cooler. This could produce a runaway effect in which the ice sheets grew into the tropics and buried the supercontinent in ice. Kirschvink also proposed that the situation could become unstable. As the sea ice crept toward the equator, it would kill off all photosynthetic life and a buried supercontinent would no longer engage in . Those were two key ways that carbon was removed from the atmosphere in the day's , especially before the rise of land plants. Volcanism would have been the main way that carbon dioxide was introduced to the atmosphere (animal respiration also releases carbon dioxide, but this was before the eon of animals), and with two key dynamics for removing it suppressed by the ice, carbon dioxide would have increased in the atmosphere. The resultant greenhouse effect would have eventually melted the ice and runaway effects would have quickly turned Earth from an icehouse into a greenhouse. Kirschvink proposed the idea that Earth could vacillate between states. Kirschvink noted that reappeared in the geological record during the possible Snowball Earth times, after vanishing about a billion years earlier. Kirschvink noted that iron cannot increase to levels where they would create BIFs if the global ocean was oxygenated. Kirschvink proposed that the sea ice not only killed the photosynthesizers, but it also separated the ocean from the atmosphere so that the global ocean became anoxic. Iron from volcanoes on the ocean floor would build up in solution during the , and during the greenhouse phase the oceans would become oxygenated and the iron would fall out in BIFs. Other geological evidence for the vacillating icehouse and greenhouse conditions was the formation of cap carbonates over the glacial till. It was a global phenomenon; wherever the Snowball Earth till was, cap carbonates were atop them. In geological circles, deposited during the past 100 million years are considered to be of tropical origin, so scientists think that the cap carbonates reflected a tropical environment. The fact of cap carbonates atop glacial till is one of the strongest pieces of evidence for the Snowball Earth hypothesis. Kirschvink finished his paper by noting that the eon of complex life came on the heels of the Snowball Earth, and scouring the oceans of life would have presented virgin oceans for the rapid spread of life in the greenhouse periods, and this could have initiated the evolutionary novelty that led to complex life.Kirschvink is a , was soon pursuing other interests, and left his Snowball Earth musings behind. Canadian geologist had been an ardent Arctic researcher, but a dispute with a bureaucrat saw him exiled from the Arctic. He landed at Harvard and soon picked Precambrian rocks in to study, as it was largely unexplored geological territory. The Namibian strata were 600-700 million years old, instead of the two billion years that Hoffman was familiar with. In the Namibian desert, he soon found evidence of glacial till among what were considered tropical strata when created.Glacial till is composed of “foreign” stones that had been transported there by ice.
Figure 1 is a generalized diagram of the insect gut. The foregut begins at the mouth, includes the cibarium, the pharynx, the esophagus, and the crop (a dilated portion, as in Fig. 2A, or a diverticulum, such as Fig. 2K). The crop is a storage organ in many insects and also serves as a site for digestion in others. The foregut in some insects is reduced to a straight tube (Fig. 2F). The proventriculus is a triturating (grinding into fine particles) organ in some insects, and in most it provides a valve controlling the entry of food into the midgut, which is the main site of digestion and absorption of nutrients.
The midgut includes a simple tube (ventriculus) from which blind sacs (gastric or midgut ceca) may branch, usually from its anterior end (Figs. 1 and 2A). Midgut ceca may also occur along the midgut in rings (Fig. 2F) or not (Fig. 2H) or in the posterior midgut (Fig. 2Q). In most insects, the midgut is lined with a filmlike anatomical structure (peritrophic membrane) that separates the luminal contents into two compartments: the endoperitrophic space and the ectoperi-trophic space. Some insects have a stomach, which is an enlargement of the midgut to store food (Fig. 2R). In the region of the sphincter (pylorus) separating the midgut from the hindgut, Malpighian tubules branch off the gut. Malpighian tubules are excretory organs that may be joined to form a ureter (Fig. 2B); in some species, however, they are absent (Fig. 2O).
The hindgut includes the ileum, colon, and rectum and terminates with the anus (Fig. 1). In some insects it is reduced to a straight tube (Fig. 2G), in others it is modified in a fermentation chamber (Fig. 2F) or paunch (Fig. 2D), with both structures storing ingested food and harboring microorganisms that have a controversial role in assisting cellulose digestion.
The gut epithelium is always simple and rests on a basal lamina that is surrounded by conspicuous circular and a few longitudinal muscles. Wavelike contractions of the circular muscles cause peristalsis, propelling the food bolus along the gut.
Salivary glands are labial or mandibular glands opening in the cibarium. They are usually absent in Coleoptera. The saliva lubricates the mouthparts, may contain an array of compounds associated with blood intake, or may be used as a fixative of the stylets of sap-sucking
FIGURE 1 Generalized diagram of the insect gut.
FIGURE 2 Major insect gut types: Ad, adult; AV, anterior ventricu-lus (midgut); C, crop; Co, colon; E, esophagus; F, fermentation chamber; FC, filter chamber; G, midgut (gastric) ceca; I, ileum; La, larva; M, Malpighian tubules; P, proventriculus; Pa, paunch; PV, posterior ventricu-lus (midgut); R, rectum; V, ventriculus. Not drawn to scale.
FIGURE 3 Diagrammatic representation of typical insect midgut cells: (A) columnar cell with plasma membrane infoldings arranged in long and narrow channels, usually occurring in fluid-absorbing tissues; (B) lepidopteran long-necked goblet cell; (C) columnar cell with highly developed basal plasma membrane infoldings displaying few openings into the underlying space, usually occurring in fluid-absorbing tissue; (D) cyclorrhaphan dipteran oxyntic (cuprophilic) cell; (E) lepidopteran stalked goblet cell; (F) columnar cell with highly developed plasma membrane infoldings with numerous openings into the underlying space, frequently present in fluid-secreting tissue; (G) regenerative cell; (H) hemipteran midgut cell; (I) endocrine cell. Note particles (portasomes) studding the cytoplasmic side of the apical membranes in B, D, and E and of the basal plasma membranes in A. Abbreviations: Bl, basal plasma membrane infoldings; M, micro-villi; Mi, mitochondria; MM, modified microvilli; N, nucleus; P, porta-somes; PMM, perimicrovillar membranes; V, vesicles.
bugs. Saliva usually contains only amylase and maltase or no enzymes at all, although in a few hemipteran predators it may have the whole complement of proteolytic enzymes.
The epithelium of the midgut is composed of a major type of cell usually named columnar cell (Fig. 3A, 3C, and 3F), although it may
FIGURE 4 Midgut cell apexes: (A) Electron micrograph of M. domestica posterior midgut cell. L, lumen; Mv, microvilli; PM, peritrophic membrane. Magnification: 7500 X. (B) Electron micrograph of columnar cell of Erinnyis ello anterior midgut. Detail of microvilli showing glycocalyx (arrows). Magnification: 52,000 X. (Reproduced with permission from Santos, C. D., Ribeiro, A. F., Ferreira, C., Terra, W. R. (1984). The larval midgut of the cassava hornworm (Erinnyis ello). Ultrastructure, fluid fluxes and the secretory activity in relation to the organization of digestion. Cell Tiss. Res. 237, 565-574). (C) Diagrammatic representation of the distribution of enzymes on the midgut cell surface. Glycocalyx: the carbohydrate moiety of intrinsic proteins and glycolipids occurring in the luminal face of microvillar membranes.
have other forms; it also contains regenerative cells (Fig. 3G) that are often collected together in nests at the base of the epithelium, cells (Fig. 3I) believed to have an endocrine function, and also specialized cells (goblet cells, Fig. 3B and 3E; oxyntic cells, Fig. 3D; hemipteran midgut cell, Fig. 3H).
The peritrophic membrane (Fig. 4A) is made up of proteins (per-itrophins) and chitin to which other components (e.g., enzymes and food molecules) may associate. This anatomical structure is sometimes called the peritrophic matrix, but this term is better avoided because it does not convey the idea of a film and suggests that it is the fundamental substance of some structure. The argument that “membrane” means a lipid bilayer does not hold here because the peritrophic membrane is an anatomical structure, not a cell part. Peritrophins have domains similar to mucins (gastrointestinal mucus proteins) and other domains able to bind chitin. This suggests that the peritrophic membrane may have derived from an ancestral mucus. According to this hypothesis, the peritrophins evolved from mucins by acquiring chitin-binding domains. The parallel evolution of chitin secretion by midgut cells led to the formation of the chitin-protein network characteristic of the peritrophic membrane.
The formation of the peritrophic membrane may occur in part of the midgut or in the entire organ (type I), or only at the entrance of the midgut (cardia) (type II). Type I peritrophic membrane occurs in most insects, whereas type II is restricted to larval and adult (except hematophagous) mosquitoes and flies (Diptera) and a few adult Lepidoptera. Although a peritrophic membrane is found in most
insects, it does not occur in Hemiptera and Thysanoptera, which have perimicrovillar membranes in their cells (Fig. 3H). The other insects that do not seem to have a peritrophic membrane are adult Lepidoptera, Phthiraptera, Psocoptera, Zoraptera, Strepsiptera, Raphidioptera, Megaloptera, and Siphonaptera as well as bruchid beetles and some adult ants (Hymenoptera). Most of the pores of the peritrophic membrane are in the range of 7-9 nm, although some may be as large as 36 nm. Thus, the peritrophic membrane hinders the free movement of molecules, dividing the midgut lumen into two compartments (Fig. 1) with different molecules. The functions of this structure include those of the ancestral mucus (protection against food abrasion and microorganism invasion) and several roles associated with the compartmentalization of the midgut. These roles result in improvements in digestive efficiency and assist in decreasing digestive enzyme excretion, and in restricting the production of the final products of digestion close to their transporters, thus facilitating absorption.
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Thus, hindgut fermenters face two functional problems with food comminution in which they differ from ruminants. Not only must they consume more food per day than a ruminant of similar size and diet, but they must also ensure that the cell walls are ruptured on initial food ingestion (while a ruminant can rely on fermentation to break down the cell walls). One would therefore predict that initial food mastication would be more prolonged and intensive in hindgut fermenters than in ruminants. Even though ruminants later regurgitate their food and chew it as cud, at this point the food has been softened by fermentation processes and may present a reduced load on the masticatory system (). Morphological studies do appear to show that hindgut fermenters have deeper jaws, larger areas for the insertion of masticatory muscles and greater cheek tooth occlusal area than ruminants (e.g., ), which would accord with the hypothesis that hindgut fermenters experience a greater load on the masticatory system than ruminants. However, these observations have not been subjected to rigorous biomechanical analysis.
Megalodon was not far behind. The first appeared in the Miocene, and they specialized in eating polar krill. They were the last whales hunted nearly to extinction by humans, after . Rorquals were fast swimmers and until whaling became industrialized.For 10 million years, Earth’s ecosystems readily adapted to the warmer temperatures, but Greenland , and by 14 mya the party was over and a steady that lasted all the way to the beginning of the current ice age, as the Antarctic ice sheets grew like never before. Once again, tropical flora and fauna in high latitudes either . Reefs cannot migrate, so those outside the shrinking tropics died out.The cause of the cooling at 14 mya is the subject of a number of hypotheses, one of which is mountain-building in that created by colliding continents exposed rock that then absorbed carbon dioxide from the atmosphere in . Around the time of the cooling, the Arabian Peninsula finally crashed into Asia and closed off the Tethys Ocean, which by then was more like the Tethys Strait there. The last remnants of the Tethys consisted of an inland sea that includes today’s Caspian, Black, and Aral seas, and the Mediterranean Sea and Persian Gulf. Eurasian mountain building was not the only such Miocene event. The , which I have , began , and so is one of Earth’s younger and more rugged ranges. The of California also formed in the Miocene, and the grew into a formidable climatic barrier. The Rocky Mountains also had renewed uplifting in the Miocene, and the . In the mid-Miocene, the northward movement of Australia toward Asia initiated the plate collision that created the Indonesian archipelago, which blocked tropical flow between the Indian and Pacific Oceans. Grinding tectonic plates have created the , which is Earth’s most seismically active region, and contributed to many Cenozoic mountain-building and volcanic events, but it is only a pale imitation of Mesozoic volcanism. The has steadily declined over the eons, and in about one billion years the plates will cease to move and Earth will become geologically dead, as Mars is today. Life on Earth will then quickly end, if it has not already expired. Complex life will likely be long gone by then.As the cooling event began 14 mya, drying came again, the tropics shrank, rainforests gave way to woodlands, woodlands gave way to grasslands, grasslands gave way to steppes, coniferous forests grew, angiosperm forests shrank, and deserts and tundra grew. In the Miocene, another major new biome appeared: grasslands.
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The African woodlands and plains are extremely dangerous at night, just from roving predators, not to mention being stumbled into by elephants, rhinos, and water buffalos. Today’s African hunter-gatherers sleep around the campfire to keep predators and interlopers at bay; a sentinel keeps watch as everybody sleeps in shifts through the twelve-hour nights. They are safer from predation at night in camp than they are in daytime as they roam.The anatomy of habilines (members of ) spoke volumes about their lives. They had brains of about 640 ccs, with an estimated range of 600 to 700 ccs, nearly 50% larger than their australopithecine ancestors and nearly twice that of chimps, and the artifacts they left behind denoted advanced cognitive abilities. They stood about 1.5 meters tall (five feet), and weighed around 50 kilograms (120 pounds). With the first appearance of habilines about 2.3 mya, Oldowan culture spread widely in East Africa and also radiated to South Africa. Habiline skeletal adaptations to tree climbing meant that they slept there at night, just as their ancestral line did. Their teeth were large, which meant that they heavily chewed their food. Habiline sites have large rock hammers that they pounded food on, to break bones and crack nuts. Those habiline stone hammers may well have also been used to soften meat, roots, and other foods before eating them. Sleeping in trees meant that habilines were preyed on, mostly by big cats. Today, the leopard is the only regular predator of chimpanzees and gorillas, and at times. But if modern studies of chimpanzees are relevant, our ancestors engaged in warfare for the past several million years, and , so simian intra-species mass killings may have tens of millions of years of heritage. Habilines were not only wary of predators, but also of members of their own species.Monkeys, apes, and humans have many traits in common, and one is that members of "out-groups" are fair game. Chimpanzees are the only non-human animals today that form ranked hunting parties, and they are also the only ones that form hunting parties to . Distinct from the killer ape hypothesis, which posits that humans are instinctually violent, the chimpanzee violence hypothesis proposes that chimps only engage in warfare when it makes economic sense: when the benefits of eliminating rivals outweigh the risks/costs. Macaque wars and revolutions appear spontaneously, but chimp wars have calculation behind them, which befits a chimp’s advanced cognitive abilities; they plan murderous raids and carry them out. It is quite probable that the advancing toolset of protohumans was used for coalitionary killing when perceived benefits exceeded assessed risks/costs. Just as with , these traits probably also existed in our last common ancestor. Other animals also engage in intra-species violence, which includes spiders when key resources are scarce and contested, and when ant colonies have power imbalances, they can trigger invasion and extermination by the larger colony. But human and chimpanzee warfare is uniquely organized and calculating.Habilines and australopithecines coexisted, and the went extinct about 2.0 mya. Robust australopiths survived to about 1.2 mya (, ), and habilines , so they overlapped the tenure of a species about which there is no doubt of its genus: , which first appeared about 2.0-1.8 mya, and the first fossils are dated to 1.8 mya. is the first human-line species whose members could pass for humans on a city street, if they dressed up and wore minor prosthetics on their heads and faces. had a protruding nose and was probably relatively hairless, the first of the human line to be that way. That was probably related to shedding heat in new, hot environments, as well as cooling its large brain (molecular data with head and body lice supports arguments that the human line became relatively hairless even before australopiths). There are great controversies about that overlap among those three distinct lines that might all have ancestral relationships. Oldowan culture was a multi-species one. There is plenty of speculation that the rise of and its successors caused the extinction of other hominids, driving them to extinction by competition, predation, warfare, or some combination of them.
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