Before mya the first tetrapods four-limbed vertebrates, usually living on land evolved from fish, perhaps from air-breathing lungfish or coelacanths whose lobed fins work like protolimbs. Subsequent tetrapods, mammals, and birds, as well as amphibians and reptiles, generally have the same skeletal plan. For example, the forelimb our arm, a birds wing has one upper bone closest to the body, two bones below the elbow, several smaller bones in the wrist, and then longer finger bones. Forelimbs have disappeared in snakes but were present in their ancestors. Whales and birds have no external fingers, but finger bones are easily seen in their skeletons.
All tetrapod skeletons, however modified, resemble the same ancestral structure. The original land animals were amphibians tetrapods that lay eggs in water , returning to the water to reproduce, their young hatching as aquatic larvae. Some of their descendants had by mya developed amniote eggs with tough watertight coverings and large yolks for nourishing the embryo.
Immersion in water was no longer necessary for reproduction. These amniotes, the protoreptiles, could live and love fully on land. Systematists attribute great importance to the number of holesone or twoin the cheek of an amniote skull. Though seemingly without functional significance, the number of holes marks an important evolutionary branch point. Early amniotes with two cheek holescalled diapsidsare ancestral to todays reptiles, while amniotes with one cheek holecalled synapsidsare ancestral to mammals.
Fish and amphibian cheeks have no holes. Reptiles, among the few species surviving the great Permian extinction, diversified into niches vacated by those less fortunate. Most spectacular were the dinosaurs, dominating the land from about to 65 mya when the Yucatan asteroid struck. I have in my own modest collection two coprolites fossilized dinosaur fecesabundant in the American West because of the long tenure and huge throughput of these voracious creatures.
There is strong evidence that birds warm-blooded diapsids with feathers evolved from dinosaurs. Numerous fossils dating from mya, including the famous Archaeopteryx, show feathers on creatures otherwise resembling. Paleontologists enjoy saying that dinosaurs are not extinct but live on as birds. The first mammals warm-blooded synapsids with hair and milk glands predate birds, appearing about mya. Small and inconspicuous at first, mammals would become the beneficiaries of the K-T extinction, diversifying to fill niches vacated by the dinosaurs.
Our own order of mammals, the primates, appears by 55 mya. Still the task of reconstructing a phylogeny is like solving a jigsaw puzzle with nearly all its pieces missing. Traditionally, there have been two keys to inferring that one species derived from another. First, the two species must have lived in overlapping times and places. If fossils are greatly separated by period or geography, without recognizable links, it is impossible to connect them in a line of descent.
Second, the two species must look similar but not identical. If their bones are too different, there is no reason to regard one as the ancestor of the other; if they look virtually the same, there is no reason to regard them as separate species. A sequence of fossil species that change gradually in appearance, properly linked in time and place, has long been the most secure foundation upon which to reconstruct an evolutionary line of descent. Classification of species predates our understanding of evolution.
Human languages may have always had labels for important types of animals, and nearly all cultures use broader categories like fish, birds, reptiles, and mammals to encompass similar creatures. The well-known scheme of Carolus Linnaeus is an elaboration of this notion, grouping species on the basis of similarity. Linnaeus thought the orderly arrangement of the living world reflected Gods plan of creation, which could be adequately described using seven levels of nested categories: kingdom, phylum, class, order, family, genus, and species remembered as Kings play chess on fancy glass tables.
He placed similar-looking species, say lions and tigers, in the same genus Panthera , calling lions Panthera leo, and tigers Panthera tigris. At a higher level of abstraction, Linnaeus placed these big cats and domestic cats in the family Felidae cats. Going up another step, he placed Felidae along with dogs, bears, and similar flesh eaters into the order Carnivora carnivores.
Then these were included with all animals that suckle their young in the class Mammalia mammals , and these were lumped with other animals having a notochordan elemental spinal columninto the phylum Chordata chordates. Finally, all animals organisms consuming food to obtain energy were placed into the kingdom Animalia, separating them from the kingdom of plants which obtain energy from the sun via photosynthesis. In all classification schemes, the species is the fundamental unit because it conforms most closely to our notion of a kind of animal.
Species are based on observations of animal bodiesor remnants of bodieswhereas higher categories genus, family, etc. Despite its fundamental importance, a species is difficult to define Hey Similar appearance is not always adequate. Dogs of different breeds, although dissimilar in looks and size, are members of a single. The criterion of breeding fertile offspring cannot be tested on extinct species, so paleontologists revert to visual appearance, sorting fossils into one species or another depending on evaluations of similarity. In the past, degrees of similarity were judged qualitatively; today paleontologists evaluate them statistically, combining numerous measurements on the fossils.
But quantitative methods remain laced with subjectivity, and controversy abounds. There are vehement differences between lumpers who assign most fossils to a relatively few taxa, and splitters who invent a new species every time they uncover another tooth. There is also the problem of birds, bats, and cows. Birds and bats have wings; bats and cows produce milk. Which two are most similar? Linnaeus knew Gods plan contains such puzzles and solved them pretty well, here noting the wings of birds and bats are constructed differently, one using feathers, the other a skin membrane; also the finger bones are differently arranged within bat and bird wings.
On the other side, the milk mechanism of bats and cows is nearly the same.
Linnaeus therefore placed bats and cows together as mammals, a grouping from which birds are excluded. If Linnaeus had understood evolution, he would have reached the same conclusion but for better reason. Today we say the wings of bats and birds are analogous, that is, they serve the same function of flying, but they are not homologous, or inherited from a common ancestor. Bats and birds are very distantly related, their most recently shared ancestor a flightless amniote living perhaps mya. From that progenitor, flightless diapsids evolved into meat-eating dinosaurs whose descendants, around mya, developed feathered wings, thus becoming the first birds.
Along a different line of descent from the same ancient amniote, flightless synapsids evolved into mammals, of which one form insectivores had some descendants that took to the air using wings with skin membranes, and these were the first bats. The wings of bats and birds, having different hereditary origins, are an example of parallel evolution, analogousbut not homologousadaptations giving their possessors the advantage of flight.
This wing example is easy to trace. Often it is difficult to decide if a characteristic shared by two species is analogous or homologous. Darwin had little difficulty using Linnaeuss scheme of nested categories because it resembles a family tree: Lions and tigers belong to the same genus because they have a recent common ancestor; lions, tigers, and domestic cats are in the same family because all three have a common ancestor somewhat further back. But certain features of the Linnaean scheme have become troublesome.
While God might have arranged life on seven levels, evolution did not. To describe the nearly continuous gradations in phylogenies, systematists began inserting additional levels: subphyla e. The cladistic school of systematics rejects this kludging, eschewing all Linnaean levels above genus and species. Traditionalists retain the oft-modified levels but rarely give them deep meaning. Cladists further object to the Linnaean scheme because it classifies animals into groupings that may not reflect lines of descent.
Striving for consistency, cladists argue that any animal descended from, say, dinosaurs is itself a dinosaur. Since birds evolved from dinosaurs, birds should be regarded as dinosaurs, not placed in a separate class Aves , as Linnaeus did. But this viewpoint has problems of its own. By the same logic, amphibians, reptiles, birds, and mammals including humans should all be classified as fish! Imagine what creationists would say about that. Tracing of phylogenies has been revolutionized by molecular technology.
Inferences about descent, traditionally made from fossils of extinct species, are today often based on the DNA of living species. Ideally one would like DNA from extinct animals, but it deteriorates quickly and is rarely available for study, so we depend on comparisons among contemporary species. All living organismsplants, animals, bacteria, fungiare built of cells. All cells contain DNA, which is composed of nucleotides and usually is twisted into double helixes. There are four kinds of nucleotides adenine, thymine, guanine, and cytosine , which can be strung out along the DNA in any order.
A gene is an identifiable section of DNA. The particular ordering of nucleotides on that sectionon that genecodes a particular bit of information. Usually the coding on a gene tells apparatus in the cell how to assemble simple amino acids into a complex protein like hemoglobin or collagen. Many kinds of proteins are assembled to serve as the most important structural and functional elements of the body. Members of the same species have DNA with nearly but not exactly the same coding.
Tiny variations alleles in the genes, along with environmental variations, produce individuals that differ from one another, though all share the common characteristics of their species. Genomes of different species obviously differ more; some genes important to one species are absent from the DNA of another. Yet it is remarkable that species as far removed as humans and fruit flies hold many genes in common, coding many of the same proteins.
This is the most compelling evidence that humans and fruit flies have a common ancestor. The genomes of humans and chimpanzees are more than 98 percent identical Ruvolo Inferring phylogeny from DNA requires the familiar principle of similarity: the more alike the DNA of two species, the closer their relationship, or alternatively, the more recently they had a common ancestor. Determining the coding sequence of a DNA sample is a routine laboratory task. As an example, we might compare DNA from lions, tigers, domestic cats, and dogs.
Dogs, distantly related to the felines, serve as a reference point. We would. DNA of the domestic cat would be less similar to the lion and tiger than the big cats are to each other, but it would be closer to these cats than to the dog. Hence the domestic cat shares a more recent ancestor with the lion and tiger than any of the cats share with the dog.
Molecular methods can be used to estimate the date when the extinct ancestor of todays cats was alive. To understand how this is done, we must appreciate the complexity of proteins. Hemoglobin, for example, is composed of two identical chains, each containing amino acids, and two other identical chains, each with amino acids.
Together these four chains form a nearly spherical structure, a shape that encloses an oxygen molecule to carry it through the bloodstream. All the information needed by a cell to assemble hemoglobin is encoded on a gene common to all vertebrates. If we zoomed in on this section of DNA, we would find a sequence of nucleotides corresponding to the constituent amino acids of the hemoglobin molecule.
It turns out, however, that the coding on the DNA need not be perfect to work. A fair number of errors can be tolerated without ruining the resultant hemoglobin although certain errors produce the defective hemoglobin of sickle-cell anemia or are not viable at all. Perhaps due to solar or cosmic radiation, mutations occasionally occur to the DNA in a sperm or egg cell, altering its coding.
If the mutation causes a tolerable error on the gene for hemoglobin, it is passed on to succeeding generations. Given sufficient time, a fair number of tolerable errors accumulate on the hemoglobin gene.
If we compare the hemoglobin genes of lions and tigers, we find slight differences, the result of mutations that have accumulated in each line since they split from their common ancestor. If we knew the rate at which mutations accumulate, we could estimate the number of years since that split. The mutation rate can be estimated if it is assumed to be constant over timea strong assumption.
By comparing DNA sequences from living species as distantly related as mammals, birds, and insects, one can count the number of mutations that occurred since they split from common ancestors. The lengths of time since these major taxa diverged, in millions of years, are known from the fossil record. This gives sufficient information to estimate the mutation rate. With that in hand, one can calculate the number of years since lions and tigers diverged. Using similar methods, it has been estimated that humans and chimpanzees diverged from a common ancestor as recently as five to eight million years ago, and that all humans alive today are descended from a few peoplemetaphorically, Adam and Evewho lived between fifty thousand and one hundred thousand years ago.
This sounds impressive, but all such estimates must be taken with a grain of salt because molecular methodology requires dubious assumptions such. Phylogenies obtained from DNA do not always agree with those based on fossils, raising the question, which, if either, is correct? DNA comparisons have the advantage of dealing directly with the stuff of heredity. Still, molecular differences between living species may be deceptively large if they experienced very different selection pressures, or deceptively small if they experienced parallel evolution.
Fossils have the advantage of being located in time and place, telling us when and where an extinct animal lived, important information not observable in DNA. Until molecular and fossil methods are reconciled, phylogenies must be regarded skeptically. Soft parts of animals rarely fossilize, and except for occasionally preserved footprints or nests, there is little direct evidence about the behavior of extinct species.
Knowing the DNA of an extinct animal would in principle give us a blueprint of its body and behavior but in actuality would tell little about the creatures looks or actions. How, then, can we reconstruct soft parts and behaviors of species from the distant past? There is an easy solution in a few cases because some ancient species are still with us, more or less. Walking the Delaware shore, I occasionally find a dead or barely living horseshoe crab Limulus , the classic living fossil, scarcely different from specimens dated mya.
Todays scorpions also look nearly the same as ancient fossils. In a fisherman off the coast of South Africa caught a strange fish eventually named Latimeria chalumnae later identified as a lobe-finned coelacanth, thought to have gone extinct 65 mya. Living fossils are similar but not identical to ancient fossils and therefore not truly ancestors of other living species. Indeed, orthodox Darwinians insist no species alive today evolved from another living species. A common error of this kind is to say humans evolved from chimpanzees, or, from the chimps point of view, that chimpanzees evolved from humans.
Neither claim is correct. The lineage of todays chimpanzee is exactly as old as our human lineage. The correct statement is that humans and chimps have a common ancestor, now extinct. If one is willing to deviate a bit from orthodoxy, and nearly everyone is, then living fossils become valuable for reconstructing lifes history. Since hard parts of the modern horseshoe crab look similar to ancestral fossils, it seems likely that their soft parts looked similar too.
On these grounds, hardly anyone objects to using the living animal as a model for its extinct ancestor, Furthermore, living horseshoe crabs can be watched, their habits studied for indications of ancestral behavior. They eat a wide variety of foods, catching worms and mollusks and chopping them into small bits with the bases of their legs, also scavenging bottom debris. They tolerate large fluctuations in salinity and are often the last species driven from an estuary by human pollution. Leaving the water to lay their eggs in nests on the beach, they can survive for days in the air and sunshine.
Altogether they are hardy generalists, able to live in diverse conditions, and probably their ancestors were too, which may explain why they survived with little modification Eldredge Is it safe to assume, just because they look alike, that todays horseshoe crabs behave like their ancestors? More generally, do any two closely related species that look similar probably act similarly? From our experience with living species, this is a reasonable assumption, but before going further it is worthwhile considering some caveats. First, animal behavior is often flexible and modifiable by environmental circumstances.
Canada geese Branta canadensis change their migration routes if the location of winter food changes. Hedge sparrows Prunella modularis studied in a single botanical garden participated in diverse mating combinationsa male and female, a male with two females, two males with one female, or two males with two or more femalesdepending on the size of territories, which in turn depends on the local quality of feeding and density of vegetation Davies Beavers either build stick lodges or burrow their lodges into dirt riverbanks, depending on the availability of suitable sites and materials Wilsson These are examples of normal environmentally contingent actions by individuals of a particular species, which collectively compose the behavioral repertoire of that species.
Any claim about the behavioral similarity of living and fossil species presumes approximately similar environments. Second, physically similar species often act differently in ways that reinforce their reproductive isolation, especially if their geographical regions overlap. Distinctive breeding songs, for example, can be important in identifying the forty-nine species of sparrows in the United States, many looking so much alike that experienced birders lump them together as LBJs little brown jobs.
This was pleasantly impressed upon me during sunrise walks with my daughter through the fields of Saratoga Battlefield National Park in Upstate New York where she was studying grassland birds. She recognized the subtle visual difference between a rare Henslows sparrow Ammodramus henslowii and a common grasshopper sparrow Ammodramus savannarum , which I could not, but I could easily distinguish their songs. She also recognized subtle differences in behavior: Henslows sparrows are especially secretive, more often running low down through the grass; grasshopper sparrows perch higher up on weed stalks.
Nonetheless, these. The classic example among primates of similar-looking species acting differently is the Hamadryas baboon Papio hamadryas. There are four other baboon species, all living in groups containing multiple adult males as well as females. The Hamadryas is unique in having the harem as its basic social unit.
Each adult male that is capable keeps for himself several breeding females and their offspring , which he controls through forceful measures including biting Kummer Otherwise, Hamadryas baboons behave pretty much like other baboons Nowak If physically similar species occupy different geographical regions, behavioral differences are unnecessary to maintain reproductive isolation and may disappear or never develop.
A good example is the beaver, which has two living species, one primarily in North America Castor canadensis , the other in Eurasia Castor fiber. Nearly identical in external features, they differ in chromosome number, in average size, in bones of the nasal opening, and in the character of anal gland secretions. Behaviorally they are virtually the same, constructing similar lodges, dams, and channels, though possibly the Eurasian beaver builds less frequently and impressively for lack of construction materials at many European sites Muller-Schwarze and Sun The gibbon Hylobates , or lesser ape, is another instructive example, having eleven similar-looking species, with the siamang Hylobates syndactylus often distinguished from the others because of its larger size and webbing between its second and third toes.
Living in different geographic areas, all gibbons have a common behavioral pattern including graceful brachiation through the trees, occasional bipedal walking, and monogamous families that are highly territorial Nowak These examples show that physically similar species may behave identically but do not inevitably do so in all ways or in all environments.
As a rule of thumb, we may assume closely related species that look alike have comparable behavior patterns, if we do not push it too far. Some paleontologists do go further, arguing that similar-appearing parts of otherwise dissimilar animals serve the same behavioral functions, for example, horns and hornlike organs crests, frills, etc. Therefore, the current generation of paleontologists is probably quite accurate in ascribing social functions to horns and related features present in dinosaurs Sampson , We need not go so far to presume todays horseshoe crabs in toto have soft anatomies and behaviors more or less like their fossil ancestors.
Most mammals evolved rapidly, leaving few species alive today that closely resemble ancient forms. Therefore we must go farther out on a limb, modeling the behavior of long-gone animals after that of somewhat dissimilar living relatives. We claim, for example, that since all living mammals nurse their young, all have strong mother-infant bonds, and all their juveniles play, then probably these behaviors were present in the most recent common ancestor of living mammals even if that ancestor did not look much like any extant form. This assumptionin essence that behavior and soft parts are conservative within lineageswhile not directly testable, is nonetheless common in theorizing about extinct animals.
It has been formalized by paleontologists in a methodology called the Extant Phylogenetic Bracket, where an extinct species is tentatively assigned behaviors and soft parts found in its living descendants, unless there is contrary evidence Witmer ; Bryant and Russell The assumption occurs implicitly in most discussions of longgone lifestyles, and we will use it here. However, its theoretical underpinnings have lost some of their early credibility.
This is partly due to the famous but now discredited biogenetic law of nineteenthcentury anatomist Ernst Haeckel, that ontogeny [development of an individual] recapitulates phylogeny. Comparing the development of embryos of various living species, Haeckel claimed each embryo resembles in turn its evolutionary ancestors. The human embryo, for example, looks first like a fish, then an amphibian, and so on. There is a grain of truth herehuman embryos do have gill slits at one pointbut Haeckel exaggerated these stages with the unintended result that many scientists became skeptical of the contribution of comparative anatomy to our understanding of evolution.
Another target of skepticism was the belief of Haeckel and other evolutionists of his time that species evolve in a progressive manner, with new forms carrying forward the design of earlier forms. Haeckel sought naturalistic reasons for this apparently directionality or orthogenesis in the fossil record, but others gave it teleological or spiritual meaning, seeing the preordained working-out of Gods plan for the development of humankind. Today natural scientists agree that teleology and predestination have no role in evolution through natural selection.
Each species adapts to its own ecological niche in its own way, constrained by its own evolutionary history. Some theorists carry the argument further, insisting one cannot say humans are a higher species than, say, worms. Perhaps to entomologists, ornitholo-. Personally, I cannot regard ant colonies on a par with human societies, nor can I believe there is no progressionin retrospectfrom fish through monkeys to human beings.
My perspective is frankly anthropocentricI am a sociologist so I seek in evolutionary history those changes whereby creatures moved in a particular direction, from amphibian life toward humanity without attributing any religious or mystical significance to that development. Not all evolution was in this direction, but I am not interested here in other paths. If trout were my focal animal, I would arrange this discussion differently. To understand our evolutionary development since the Cambrian explosion, from invertebrates to fish to amphibians to reptilians to mammals to humans, it makes sense to compare modern representatives of this sequence.
The comparative anatomy of living species has always complemented interpretation of extinct fossils and is the grandparent of todays DNA comparisons. It bears repeating that modern species do not form a true evolutionary sequence; they are cousins, related through common ancestors now extinct. Obviously, one must be wary of inferences from a constructed quasi-evolutionary sequence, but there is no good reason to reject them altogether.
In the following section I consider the evolution of the brain, that soft part of the body most intimately connected to human behavior. The method is comparative anatomy, using living species to imagine brain development from its origins to what we now have in our heads. This approach inevitably suggests, however faintly, that the evolutionary process is somehow directional, goal oriented, teleological, continuous, free of dead ends, and the like.
I intend no such meaning, nor that past trends can be projected into the future. My intent, rather, is to look backward from the present, to trace how we evolved to the point where we are now. It communicates with the rest of the body through electrical signals passed along peripheral nerves, and also via chemical hormones manufactured and injected into the bloodstream by endocrine glands the thyroid, adrenals, testes, ovaries, etc.
Nerve and hormone signals are coordinated through two small structures, the hypothalamus and pituitary, located at the base of all vertebrate brains. Given the close relationship of chimpanzees and humans, we expect their brains to share important features, but it may be surprising that both have homologues of the amphibian brain too. Brain evolution is conservative; once a structure appears in the central nervous system, it often remains there. Newer structures may be added and old ones modified considerably, but they rarely disappear.
Amphioxus Branchiostoma , a chordate without a backbone, looking something like a small eel, spends most of its life buried in the sandy bottom of marine bays, filter feeding on microorganisms in the water. The species is primitive and must have appeared long ago. It has a fibrous rod a notochord running the length of its body, the defining feature of a chordate. Vertebrates, a subphylum of chordates, have embryonic notochords that develop into backbones.
Above the notochord runs a neural cord with a rudimentary brain at its front, the precursor of a vertebrates central nervous system. As in all nervous systems, electric signals are transmitted from one neuron nerve cell to another. There are gaps synapses between neurons. The signal crosses these gaps when chemical neurotransmitters, released from one neuron, flow across the synapse and lock into receptors on the other neuron.
Serotonin, a neurotransmitter produced in some amphioxus neurons, is found in all nervous systems that have evolved from primitive chordates, functioning to stabilize the organisms neural circuits. Serotoninproducing neurons occupy virtually the same location in every vertebrate brain that they do in amphioxus Allman In humans these neurons are numerous and project to all parts of the brain and spinal cord, having a profound effect on our mood. High serotonin levels are associated in humans with a sense of well-being and freedom from anxiety. Prozac and other antidepressive drugs work by increasing the concentration of serotonin in the gaps between neurons.
When some chordate evolved into a fish, fishlike behaviors were enabled by new structures added to the spinal cord and brain, including olfactory bulbs, optic lobes, a cerebellum, hypothalamus and pituitary, and small cerebral hemispheres. As fish evolved into amphibians, then amphibians evolved into reptilelike amniotes, and then amniotes into birds and mammals, new forms of behavior were associated with new or modified brain parts.
Rather than replacing old structures, new elements were figuratively stacked on top, forming an agglomerate brain. We may imagine the development from fish to human looking something like the series of brains pictured in Figure 4. Ignoring considerable variation within taxa, these are representative brains from modern fish, amphibians, reptiles, mammals, and humans. When mammals evolved from reptile-like amniotes, the cerebral hemispheres expanded considerably, becoming the most prominent features of the mammalian brain. The surface cortex of these hemispheres is covered by gray matter a few millimeters thick called the cerebral cortex.
This can be divided into neocortex, which is the most recently evolved gray matter composing most of the cortex of higher mammals, and allocortex, the most primitive cortex. Neocortex has a unique structure with six neuron layers laminated onto the surface of the brain; allocortex has a simpler structure, usually. Figure 4. Brains of a fish cod , amphibian frog , reptile alligator , mammal cat , and human; not to scale adapted from Truax and Carpenter , The little cerebral cortex found in reptiles is all allocortex.
Neocortex occurs in all mammals and only in mammals. It is involved in perception, volition, memory and movement, reasoning and, in humans, language. The corpus callosum, found only in placental mammals, is a white band of nerve fibers connecting the neocortex of the two hemispheres and. The hemispheres are also connected through the anterior commissure, which is present in reptiles and birds as well as mammals. The neocortex is especially large relative to both body and brain mass in primates. During embryonic development of apes and humans, the cerebral hemispheres fold as they grow, cramming a lot of neocortex into the skull, producing the convoluted walnut-like appearance of our brains.
This voluminous neocortexonly about a third of it shows on the surfaceengulfs our allocortex, which lies buried near the middle of the brains hemispheres, partly encircling the corpus callosum and brainstem. This submerged allocortex receives nerve input from several other elements within or below the hemispheres. These submerged structures are often designated the limbic system. The term is much criticized, first because different authors disagree on the structures to be included, and second because system infers a functional unity that has never been demonstrated.
Nonetheless neurologists commonly speak of the limbic system, or limbic structures, to refer to those parts of the mammalian brain implicated in the emotions. Limbic structures are present in at least rudimentary form in the reptilian brain; however, they are more developed in mammals MacLean A major limbic element is the amygdala, which increases in size and development from the lower to the higher mammals and is particularly well developed in humans.
Biosociology of Dominance and Deference by Allan Mazur | Waterstones
A central task of the amygdala is the establishment of links between stimuli and their emotional value, for example, whether something or somebody is good or bad. Bilateral destruction of the amygdala in monkeys causes reduced expressions of emotion, difficulties in social interaction leading to isolation, and loss of aggressive and defensive reactions. In contrast to normal monkeys, those with destroyed amygdalae show no signs of fear, as when confronted with a snake.
In humans, the amygdala has been stimulated in conjunction with brain surgery under local anesthesia. A wide spectrum of autonomic and emotional reactions has been produced, but most pronounced is a feeling of anxiety Brodal , The reptilian brain necessarily fits the basic pattern of reptilian behavior, which is asocial, cold-blooded and therefore lethargic in cool temperatures, and stereotypic MacLean From a human perspective, reptilian behavior seems very simple.
In some species an adult males mates occupy his territory for long periods, but interaction between the sexes is usually brief and. Caretaking of young after hatching or birth is usually absentalthough alligators are relatively attentive mothers. Reptiles engage in dominance contests for desirable sites, mates, or food, using stereotyped displays of challenge or submission. Males often defend core sites from intruding neighbors, effectively spacing themselves across the ground.
If circumstances such as a concentration of food induce lizards to remain in close proximity, males form a dominance hierarchy wherein the alpha male can trespass others males sites, but they avoid his Zug The reptiles central nervous system necessarily enables all of these behaviors as well as handling the basic drives of hunger, thirst, sex, sleep, and pain avoidance, but it lacks the capacity for much more.
Birds evolved from reptiles via dinosaurs but are warm-blooded and therefore more energetic. Brains of warm-blooded vertebrates tend to be larger than brains of cold-blooded vertebrates of the same body weight, accommodating new mechanisms for maintaining a constant body temperature. Birds are far more social than reptiles, usually acting cooperatively to raise their young. The typical avian behavioral pattern, with exceptions, is for the male to stake out a territory in the spring, defending it from nearby males with stereotypic auditory songs, calls and visual displays of dominance.
These displays also attract a female to the territory, who builds a nest of a certain architecture, perhaps aided by the male. Copulation is followed by egg laying, hatching, and then prolonged feeding of the young until they fledge, much of this a cooperative undertaking between the parents. The young may stay with one or both parents after leaving the nest.
Families often join flocks in the fall for migration to an area with a better winter food supply. Similarities between reptilian and avian ways of life, including the common male propensity for territorial spacing through dominance displays, may be embedded in brain structures shared by birds and reptiles, and these probably were present in their most recent common ancestor. Some invertebrates display analogous territorial and dominance behaviors, but lacking anything like a vertebrate brain, their actions must be governed by other mechanisms with differently evolved roots.
Differences between bird and reptile behavior, such as presence or absence of parental cooperation in rearing young, must reside proximately in structures differentiating birds from reptiles. With some exceptions, birds are generally more dependent on sight than smell, the reverse of reptiles. This is mirrored in the relative size of their olfactory and optic structures. The cerebellum, which controls coordinated movement, is far larger and more developed in birds.
Brains of some birds contain microscopic grains of iron oxide, magnetite, which may aid navigation by sensing the direction of magnetic north. The hyperstriatum, a structure unique to the avian brain, is implicated in the imprinting of chicks to their mothers Horn, Nicol, and Brown The mammalian nervous system has a unique set of reflexes inducing a newborn to seek its mothers nipple with its mouth and, once the nipple is grasped, to suckle.
Biosociology of Dominance and Deference
Production of mothers milk is controlled by the hormone oxytocin, which is made in the hypothalamus. Oxytocin also stimulates maternal care in mammals. These distinctly mammalian behaviors, also including juvenile play, must involve the neocortex, which is the only brain structure unique to mammals, and the limbic system, which is considerably modified in mammals.
In classic experiments of the nineteenth century, the entire neocortex was surgically removed from animals, depriving them of will, intelligence, memory, perception, yet they would live for months cited in Allman , At about the same time it was noted that certain language abilities are degraded in patients with particular injuries to the neocortex. Lesions near the front of the left hemisphere produce loss of speaking ability. Lesions to the left temporal lobe i.
These specialized language areas named for their discoverers, Broca and Wernicke have since been verified, although today we know about 5 percent of people locate these functions on the right rather than left side of the brain. In most people the right hemisphere controls prosody, the emotional or affective content given a string of spoken words by the speakers melody, pauses, intonations, stresses, and accents.
A sentence such as I went to the store takes different meaning and coloration depending on the tone, timing, and inflections of the utterance. Lesions to the right hemisphere have produced difficulties in producing or comprehending prosody in patients who can still speak and understand the words. Some people with left-hemisphere lesions who have lost speech can still sing lyrics, apparently enabled by the right hemispheres capacity for prosody Filley The limbic structures are more concerned with emotions, motivation, and affective behavior Brodal Hormones are especially active in the emotions, producing bodily responses that are the hallmark of emotional response, for example, the flush of anger, embarrassment, or sexual excitement; the pallor, sweat, and trembling of fear.
Mammals and some other animals show emotions with recognizable human counterparts, notably rage analogous to human anger , fear, and pleasure or contentment roughly analogous to happiness. A small number of basic human emotionshappiness, sadness, anger, fear, and disgustare associated with characteristic facial expressions recognizable across cultures. Ekman and Friesen Any human youngster, without coaching, can produce appropriate faces for each of these emotions Figure 4.
Not all emotions have associated facial expressions. The essence of an emotion is that we feel it viscerally, it has neurohormonal aspects, and it produces some distinct affect that motivates or inhibits our behavior Weisfeld At least in humans and probably more broadly, maternal care is motivated by vicariously feeling comfort or discomfort expressed by the infant. The tendency to nurture ones vulnerable children makes good evolutionary sense, for otherwise parents would leave few survivors in succeeding generations.
There is an emotional mechanism at work here, acting through the sympathetic attachment of mother and child, a bond that intensifies the more they interact. If an infant is content, then usually its mother is content; if the infant suffers, then its mother suffers vicariously. Infants reflect their mothers moods too.
That is why parents take special pains to ensure the well-being of their offspring. Mammals and some other animals vicariously attach not only to offspring but to close associates and to members of their group, reflecting one anothers fear, contentment, or agitation. Humans, especially, attach vicariously to anyone with whom we interact positively. Perhaps this is a generalization of our evolved tendency to favor our children, a spillover to all people we like, or to those we perceive to be like us. We share their joy and feel their pain, whether or not they are genetically related.
Humans also attach negatively to people whom we dislike, or perceive as different from ourselves, or to whom we are hostile; then we are unhappy if they succeed, glad if they fail. Coalitions based on vicarious attachment are common among social mammals. If conflict breaks out between two animals in the same group, each ones closest associates provide support. If hostilities break out between two troops or two packs, each animal reliably supports its own side.
In human terms, this explains why my friend and I will set aside our own argument to coalesce against a neighbor who threatens one of us, and why we and the neighbor will set aside that dispute to join forces against foreigners. See the appendix on stress-induced coalitions. Any conceptualization of the brain as an accretion of new structures atop ancient ones necessarily raises a question of how well these elements work together. Does the human brain operate as a fully integrated entity, or does it perform different functions in specialized modules that are not completely coordinated, pitting our emotions against our cognitions?
That separate parts of the brain are capable of independenteven conflictingthoughts and actions is illustrated in patients undergoing split-brain surgery for the control of epilepsy. In this operation the corpus collosum is severed, thereby preventing the spread of abnormal nerve discharges from one hemisphere to the other, but at the same time halting normal communication between the hemispheres. Contrived laboratory tasks demonstrate independent operation of the two disconnected sides of the brain. For example, a patient can speak the name of an object held in the right hand, because the sensations of touch by the right hand go to the left hemisphere, where the speaking function is located; but the patient cannot say the name of an object held in the left hand, since those sensations go to the right hemisphere, now severed from the speaking function on the left.
These patients manage well in everyday life because sights and sounds from the environment usually reach both hemispheres at the same time, but occasionally peculiar behaviors result from the disconnection. Neuroscientist Joseph LeDoux tells of seeing a patient several days after surgery who was. Anyone who has resolved to diet and then gorged on cookies and ice cream, or tried to quite smoking and failed, or has had sex in a manner thator with a person whoan hour earlier seemed a foolish choice, knows the appetites and emotions generated in one part of the brain may sharply contradict the good sense residing elsewhere.
The joke that God gave man two gifts, a penis and a brain, but forbade him the use of both at the same time, carries enough truth to make the point but shows as well that it can be overstated.
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When split-brain research appeared in the s, it triggered a wave of extreme claims, never well supported, that each hemisphere specializes in certain kinds of mental processingthe left devoted to analytic, linguistic, rational, scientific, linear thought; the right to intuitive, nonlinguistic, mystical, artistic, and holistic thought. With opposing halves vying like yin and yang, our personality was said to be determined by the dominant hemisphere.
This view is exaggerated, and it is worthwhile emphasizing the obvious but often overlooked point that split brains are anomalies.
While they demonstrate the possibility of independent actions in different parts of our heads, they do not indicate that this is normal operation. The pendulum of commentary is today swinging back, emphasizing that the emotional and cognitive parts of the brain, while in some ways separate, are on the whole highly connected and intensively interdependent.
Brain scans made during diverse mental tasks typically show both hemispheres participating, if not equally. Anatomically, there are dense connections among and between limbic elements and the neocortex. Emotional and cognition content are intertwined, as when depression comes with pessimistic thoughts, elation with optimistic plans Dolan Information content in the form of bad news can make us feel down; good news cheers us up. The deep integration of cognitions and emotions is illustrated by any escalating conflict between ethnic, religious, or nationalistic groups.
Nearly every involved persons intellectual view of the conflicthow it started, whose fault it is, what solutions are just and unjustis predictable from his emotional identification with one side or the other, whether Israeli against Palestinian, Pakistani against Indian, or Russian against Chechen. This alignment is so predictable that sociologists speak of the Rashomon effect after a classic Japanese movie when two people give inconsistent verbal accounts of the same conflict, and each account suits the tellers interests and the position he or she wants to defend Mazur It explains why the same suicide bomber or airplane hijacker is perceived as a terrorist by his enemies and as a freedom fighter by his allies.
Freudians call this rationalization: the devising of a self-satisfying and ostensibly cogent explanation for our beliefs and actions, when in fact their causes lie elsewhere. The minds attitudes, beliefs, and logical deductions are consistent with its feelings; there is an alignment of the neocortex and the limbic system. The deepest intellectual arguments of philosophy and science rarely contradict the thinkers gut emotions or sympathetic identities. While human actions of dominance and deference are based at least partly on the same motivations as in other primates, we nonetheless rationalize them in purely human terms, constructing linguistic justifications for advantageous rank as being legitimate, or well deserved, or divinely mandated.
With historical hindsight, we get a better perspective on aristocracy and slavery, for example, than was available to contemporaries who preached that these social arrangements were natural, inevitable, and ordained by God. There is no better historical example than the United States in the early nineteenth century, which, despite its slavery, was an intensely liberal democracy, dedicated to individual freedom of thought and action. That these freedoms were limited to whites and men meant that America was backwardly racist and, like Europe, sexist.
While these seem contradictory positions today, Americans at the time could defend the institutions of democracy and slavery in the same breath. With proper attention to potential pitfalls, comparative anatomy helps us imagine the evolution of soft parts and behavior. By arranging living species on the basis of physical characteristics into a quasi-evolutionary series, we approximate the true evolutionary path from a distant ancestor to a living descendant.
Behaviors that emerge in a smooth fashion along the quasi-evolutionary series may have followed a similar historical path. As in all evolutionary reconstructions, it is important to exclude anomalous characteristics that likely represent unique adaptations. Species-specific behaviors must have physical underpinnings, either in the central nervous system or in some peripheral hormonal mechanism.
Confirmation that a particular behavior is an evolved trait would require that the physical mechanism underlying that behavior be identified. In humans, as in other animals, there is a linkage between brain structure and behavior. No longer can we explain human thoughts and actions as if they were independent of our bodies. The mind-body distinction that has been salient in Western philosophy for the past three centuries cannot be sustained.
Rationality and language do not occur on a detached mental platform, completely separated from visceral emotions. We think and we feel in tandem. Traditional sociology has given little attention to primitive emotional processes in explaining the status hierarchies of small groups. Its theories speak in Cartesian terms of language, cognition, and symbolism. As a graduate student in sociology, I was taught that a leader emerges in a small group because he is internally rewarded by the knowledge that he is right, that both the leader and the members identify with a symbol system, and the leader is identified by the members as the true spokesman and interpreter of the symbol system Bales , Another popular theorist of the period, noting that girls prefer association with near-peers, explained that nearpeers are apt to hold similar values, and so they are apt to reward and like one another.
Homans , But monkeys also associate mostly with near-peers, and for monkeys an explanation in terms of similar values is meaningless. Other influential theorists thought a groups status ranking derived from chains of logical deduction by its members, as if such rankings are found only among rationally acting human beings Berger, Cohen, and Zelditch It did not occur to most sociologists of that time that human behavior followed a broader primate pattern. Evolution dictates that we understand the actions of Homo sapiens as primate behavior, unique in some ways but in other ways typical of all primates and especially of apes.
So let us now focus on these cousins of ours. In his great work, Systema Naturae , Linnaeus listed the major categories of mammals, for example, the Carnivora, the Rodentia, and the Cetacea whales and dolphins. It was obvious to naturalists of the time that the category containing humans must be classed above the rest, so Linnaeus called that order the Primates, from Latin for first in rank. Linnaeus named our species Homo sapienswise man.
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Despite our position atop the pinnacle of wisdom, humans are genetically so similar to the African apes chimpanzees, bonobos, and gorillas that we must have diverged from common ancestors less than ten million years ago. For crossspecies comparisons to seriously aid our understanding of human affairs, we must focus on the primates. There is no simple definition of our order because primate bodies are less specialized than the bodies of other mammals and therefore lack the unique features that make other orders easier to describe.
Primatologist R. Martin proposes a definition running nearly two pages , , far too complex for nonspecialists. The most important thing that primates have in common is that they are or were tree-dwellers, with features useful for that habitat. Five digits at the end of each limb, including opposable thumbs and usually opposable big toes, are ideal for grasping branches. His book is about the social biology of face-to-face dominance interactions and it explores the evolution of behavior through connections among biology, language, culture, and socialization.
Topics include comparative primate behavior, physiological and brain mechanisms underlying status processes, and the relevance of the body surface face, physique, gestures to status allocation. Soft Parts and Behavior. Allocating Ranks. StressInduced Coalitions. About the Author. And People.