(Ir a la versión en español La evolución biológica)
Departament of Genetics and Microbiology.
|Darwinian Revolution||The Study of Evolution||Human Evolution|
|Nothing makes sense in biology except in the light of evolution
Few ideas have changed our vision of nature as profoundly as the very idea of change underlying the evolution of living beings. Biological organisms are grouped into natural units of reproduction that we call species. The species that now populate Earth come from other species that existed in the past, through a process of descent with modification. Biological evolution is the historical process of transformation of a species into other descendant species, and its reverse is the extinction of the vast majority of species that have ever existed. One of the most romantic ideas contained in the evolution of life is that any two living organisms, no matter how different they are, shared a common ancestor at some time in the past. We and any current chimpanzee share an ancestor something like 5 million years ago. We also have a common ancestor with any of the existing bacteria, although in this case the time to the ancestor goes back to more than 3000 million years.
Evolution is the great unifying principle of Biology, we need it to understand the distinctive properties of organisms, their adaptations; as well as the relationships of greater or lesser proximity that exist between the different species. Evolutionary theory is related to the rest of biology in a way analogous to how the study of history is related to the social sciences. The famous phrase of the geneticist Theodosius Dobzhansky that opens this topic is just a particular statement of the more general principle that declares that the present of a temporal process can not be understood without a historical perspective.
Although the idea of evolution had precedents, it was not until 1859, with the publication of the work The origin of the species of the British naturalist Charles Darwin, that biological evolution was definitively established as explanation of diversity of life. Darwin compiled and interpreted a large number of observations and experiments from very diverse research disciplines and presented them as an irrefutable argument in favor of the fact of evolution. But Darwin also provided a mechanism to explain the complex and characteristic adaptations of living beings: Natural Selection. What did the theory of evolution and natural selection mean in the context of nineteenth-century biology? In 1802 the theologian W. Paley publishes the work Natural Theology, where he argues that the functional design of the organisms evidenced the existence of an omniscient creator. According to him, the human eye, with its delicate design, was a conclusive proof of the existence of God. For naturalists who wanted to explain biological phenomena by natural processes, explain the adaptation, the wonderful fitting of organisms to their environment, was the fundamental problem.
Paley's design argument had a great influence on nineteenth-century naturalists, despite the fact that this interventionist vision flagrantly violated the concept of nature that had been established with the development of physics in the sixteenth and seventeenth centuries. The phenomena of the Universe, according to this new conception, were explicable by natural processes. Nature, per se, was the proper object to ask and answer scientifically. With the Origin of Darwin this revolution break through Biology. The truly revolutionary thing in Darwin was to propose a natural mechanism to explain the genesis, diversity and adaptation of organisms.
The British naturalist Charles Darwin (1809-1882) introduced in his book The Origin of Species (1859) two revolutionary ideas: Biological Evolution and Natural Selection
There is grandeur in this view of life... that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
To make his theory of evolution and natural selection intuitive, Darwin had to introduce a new way of understanding variation in nature: Population Thinking. In Darwin's time species were considered fixed and immutable entities; they represented the Platonic types, the perfect idea that the Creator has in his mind of each species. The differences in the form, in the behavior, or in the physiology of the organisms of a given species were not more than imperfections, errors in the materialization of the idea of the species. In contrast to this dominant essentialist view, individual variation, far from being trivial, is for Darwin the cornerstone of evolution. Variation between individuals within species or populations is the real stuff, the raw material of evolution, from which all biological diversity is going to be created. There are the differences between organisms of a species that, when magnified in space and time, will produce new populations, new species, and by extension, all biological diversity.
The Study of Evolution
Studies and statements about evolution generally refer to one of two different aspects: (1) the investigations about the fact of evolution and (2) those that refer to the mechanism of evolution. The first cover the biological disciplines, such as paleontology, taxonomy, comparative biology, population biology... which unequivocally show the fact of evolution. The second ones, the affirmations about the mechanism of evolution, are the main subjects of Population Genetics, and they inform us of the factors, forces or processes responsible of evolutionary change, that is, the natural mechanisms that cause descent with modification. An everyday analogy that illustrates this distinction is that of weather. Precipitations, Winds, Cold drops, Typhoons, are all evidences that make weather a fact of nature. Now, if we want to explain why the different meteorological phenomena occur, then we have to introduce affirmations on processes, about the meteorological mechanisms causing the weather states. Then we must propose those factors, such as temperature differences between different air masses, underlying meteorological phenomena.
The evolution that occurs on a reduced scale, within a species and in the interval of a few generations, is called microevolution. Macroevolution is the evolution on a large scale, spanning considerable periods of time and large transformation processes; in the most extreme case it would comprise the whole evolution of life. Small-scale experiments and/or observations of current populations or species can be carried out to obtain direct evidence of evolution. There are many examples in which evolution is detected in action, like the classic case of industrial melanism that will be discussed later. Artificial selection made by man in dogs or horses are also clear examples showing the potential for modification within species. Due to its own temporal dimension, we can not demonstrate macroevolution directly, except in cases of creation of new plant species through the crossing of different species by man. Although the evolutionary evidence provided by the fossil record, comparative biology, and molecular biology is indirect, it is no less conclusive for demonstrating the community of origin of all organisms.
The sediments that have accumulated on the earth's crust during its geological history leave an inestimable trace, generally in the form of bones or hard petrified skeletons, of dead organisms in the past: these remains are the fossils. The fossil record is a wonderful window into the history of life. If it did not exist, we could not invent it. Without it, the gap about our knowledge of evolution of life on earth could not be bridged. We could speculate, theorize infinitely, but who could have imagined that the Earth was dominated for 150 million years by immense and fantastic reptiles, the dinosaurs, which disappeared in a relative instant of time, if the dinosaur fossils did not tell us? The disintegration of the radioactive chemical elements in the rocks has made it possible to estimate that the Earth originated around 4600 million years ago. The Earth, which was a hot sphere, gradually cools, initiating a period of chemical evolution that will culminate with the formation of the first cells. In Australia and Africa sediments have been found retained and fixed by bacteria of 3600 million years ago, which makes this date a minimum estimate of the age of onset of biological evolution. The magnitude of time in which evolution has elapsed is completely beyond our comprehension, we can not even imagine, limited to the tiny scale of our vital time, the transformation potential represented by 3600 million years of evolution.
The geological time has been divided into a series of hierarchical stages, the ages, the periods, and the epochs, which do not follow a linear chronology, but rather chronicle the key moments of the history of life. The transitions between the four eras, the Precambrian, the Paleozoic, the Mesozoic and the Cenozoic represent great changes in the fauna and flora of the whole Earth. In the first period of the Paleozoic era, the Cambrian, 570 million years ago, pluricellular animals that have hard parts, such as shells, and exoskeletons, suddenly appear in the fossil record... The end of the Paleozoic coincides with the greater extinction on Earth, in which 96% of the species disappeared. At the end of the Mesozoic, in the transition between the Cretaceous and Tertiary periods, there is the known extinction of the dinosaurs, together with 70% of the existing species..
What does the fossil record teach us about the history of life on Earth? This is a list of the most important events
If the whole history of the Earth compressed it in one hour, at 20 minutes the bacteria would appear, at 55 the dinosaurs, the anthropoids appear 40 seconds before the end, and the humans at the end of the hour.
There is a coherence between the fossil record and the evolutionary origin of the species and their corresponding groups. If life was due to the spontaneous and independent creation of species, we would not have to wait for man to emerge with mammals, we could have arisen at any time, when the fish appeared, or the dinosaurs, or before the primates emerged.
The history of life is a story of extinction and death... with a few survivors. 99.9% of the species that have ever existed are now extinct. Whole groups of organisms, such as dinosaurs (excepting birds), trilobites, ammonoids, have been extinct forever without leaving any descendants.
As noted by paleontologist S. Gould, the fossil record is not a conventional story that leads to different lineages to more excellence, more complexity, more diversity. The history of life does not show a defined course, it has no direction or meaning. Evolution is a story of massive elimination followed by differentiation within a few survivors. It is a priori impossible to determine the direction of evolution because of the importance of concrete, contingent events, such as the extinction or not of a group of organisms in the case of a mass extinction, or the possession or not of an adequate adaptive variant when this is required, they are the true agents of history.
There are two extinction regimes: normal extinction, which affects species that fail to follow their environment in their daily struggle to adapt, and mass extinction, which are faster and more devastating in its magnitude. At least five mass extinctions have occurred, and they have left many ecological holes that have been occupied by the descendants of the surviving species. This occupation of the available biospace is often accompanied by a rapid and extensive morphological diversification called adaptive radiation. The cause of mass extinctions is not known with certainty, although physical causes such as the impact of asteroids or climate changes seem more likely than biological causes. According to a recent hypothesis, there are cycles of mass extinction approximately every 26 million years, and the periodic impact of cometary showers on Earth could explain this cycle.
If we had the opportunity to return to a moment in the past, the period before the Cambrian would undoubtedly be one of the most attractive. In the Cambrian, 570 M ago, the fossil record undergoes the large explosion of the first multicellular animals with hard parts. Darwin wondered why these first animals were already anatomically complex and without apparent precursors. The answer lies in the Burgess Shale site, located in the Canadian Rockies. Due to very special conservation conditions, here it is found the only soft-body fauna (without hard structures) that exists from a time immediately before the Cambrian explosion.
The great surprise of Burguess Shale is that the few species analysed contain a disparity of anatomical designs that far exceed the modern range that exists throughout the world. In no other fauna is the anatomical richness of this site repeated. Of the 120 genera analysed, 20 are unique arthropod designs, and in addition to representing the four groups of animals that exist today, there are eigth designs that do not fit into any known animal group. The era of Burguess Shale seems to have been an amazing time of experimentation, an era of great evolutionary flexibility, which was followed by a great extinction.
Opabinia was the fossil of Burguess Shale that revealed the box of surprises that there was in this deposit. Its unique design does not belong to any known animal group. It has 5 eyes!, flexible tube formed by a cylindrical striated tube, a U-shaped digestive tube, a trunk of 15 segments, with thin lateral lobes. The last 3 segments form a tail with three pairs of thin, lobed leaves facing outwards.
During the Triassic period, dinosaurs from a line of reptiles emerge and initiate a dynasty over the terrestrial environment that would last 150 M. More than 350 species of dinosaurs have been identified, and it is believed that this is a much lower estimate than the number of species that really existed. Among the dinosaurs are the largest animals that have ever lived on Earth. The Seismosaurus, the largest known herbivore, was about 40 meters long. Tyrannosaurus rex, one of the largest carnivores, was 12 m long. Dinosaurs are divided into two large orders, those of lizard pelvis (Saurischians), which include both carnivorous and herbivorous species, and those of bird pelvis (Ornistiquios), whose species were all herbivores. The dinosaurs became extinct almost entirely, except for a minor line that gave rise to the birds, 65 M ago. Along with them, 75% of the existing species disappeared in a short period of time. It has been proposed that the impact of an asteroid on the earth's surface could be the cause of this mass extinction.
Who could have imagined that the Earth was dominated for 150 million years by immense and fantastic reptiles, the dinosaurs, that disappeared in a relative moment of time, if there had not been fossils of dinosaurs that told us about it?
Comparative Biology: Homology and Analogy
When one observes similarities between species, one can distinguish between two types of similarities, analogy and homology. The wing of a bird and that of a fly form a flat extension and have a similar flapping motion; fish, dolphins, or penguins have a flattened cross section that allows them to move through water. These similarities, called analogies, are rather superficial and are due to the fact that these organisms are subject to the same functional or adaptive restrictions, and are not due to their having a recent common ancestor.
In contrast to the analogy, a homology is the similarity between characters of different species because they have a common origin, and not the direct action of a functional pressure. For example, all tetrapods (terrestrial vertebrate animals with four limbs) have the extremity of five fingers, and this is found both in the wings of birds and bats and in the hand of the human being, although these extremities represent very different functional roles. The reason for this common structure is that all tetrapods preserve the same basic structure of the original ancestral species.
The classification is based on the comparison of the characters of the species, and the homologous characters are the key elements to establish an evolutionary classification. If the species come from other species by evolution, and also do not vary so quickly as to lose all their historical heritage, it would be expected that the different living beings shared a series of homologous characters. The analysis of the different phenotypic characters, such as morphology, behavior, chromosomes, external and internal anatomy, embryonic development, metabolism, genetic and protein variation show that the species have homologous similarities at all levels of the phenotype. The closer the species is, the greater the degree of resemblance, and the opposite is also true, the farther away they are the fewer similarities we will find. Thus, the differences that we see today between the species are due to the new variations that they have acquired since their separation from the common ancestor. The similarities we attribute to homologies could not be explained if the species originated independently of one another.
An especially significant case of homology is that of vestigial or residual organs. The pelvis of the tetrapods is a bony structure whose function is to articulate the hind limbs. The whales are tetrapods whose mammal ancestors left the earth to inhabit the sea. In this new environment the whales lost their lower extremities, but still retain as an accusatory trace of their tetrapod past, the complete series of pelvic bones. Similarly, snakes have vestiges of the hind limb carried by their ancestors.
Molecular biology has provided the most universal evidence of homology. All living organisms share the same hereditary material, DNA, a helical molecule whose information is encoded in 4 different letters or nucleotides. Likewise, the genetic code is practically Universal, all organisms share the same dictionary that gives the meaning to the DNA sequence. Both examples are very robust proofs of the intimate relationship that exists between the living beings.
If the history of life is change and branching by descent, then its representation would be that of a tree or phylogeny, in which the trunk and internal branches would correspond to the ancestors of the current species and the ends of the outer branches would be the current species. How is a phylogeny established? Sorting the current species according to the morphological and/or genetic similarity of their homologous characters.
The Swedish botanist Carolus Linnaeus (1707-78) devised the classification system that is used, with some modifications, nowadays. There are seven inclusive levels of classification, which are, from least to greatest, the species, the genus, the family, the order, the class, the type or Phylum and the kingdom. The scientific name of each species has two parts, the lion, for example, is called Panthera leo. The first part refers to the genre and the second to the species. Consider an example of how the current species are grouped in the different linnean categories. The lion, the panther, the tiger, belong to the genus Panthera, which along with the genus Felix (the domestic cat) and others are grouped in the family of felines. The felines, with the canids and bears, constitute the order of the carnivores. Primates, rodents, carnivores, ... meet in the mammal class. These organisms share characteristics such as suckling their young with milk, gestate them in the uterus through a complex organ, the placenta. Their skin is protected by fur or hair. Mammals, birds, reptiles, amphibians, and fish gather in a single type or phylum, because they all have a backbone, a maximum of four members, and red blood with hemoglobin, are chordates. Insects, spiders, crustaceans, and centipedes are classified in another type, the arthropods. Clams, snails and squid are grouped in mollusks, and so on. The chordate type, arthropod, mollusk and others form the Animal kingdom. The tree of life is ordered following divisions that go from general characteristics to more specific aspects.
Why is biological diversity ordered at different hierarchical levels? The features of the most general divisions correspond to basic or main adaptations that arose in the initial moments of the evolution of the progenitor species of these groups. For example, there are five great kingdoms, Moneras, Protists, Fungi, Plants and Animals, which correspond to the five main differentiations of life on Earth. The Phylum or type is the next basic unit of differentiation between kingdoms, and could be understood as basic fundamental projects of anatomy. In the Animal kingdom, these fundamental planes would be Sponges, Annelids (worms), Arthropods (insects, crustaceans...), Chordates... From these main adaptations, new variations as sub-adaptations arise subsequently, that because they have a shorter evolution time, they have a lower classificatory range.
The fact that the diversity of life is hierarchized is a strong argument in favor of evolution. The anatomical structures and basic adaptations shared among species can be easily explained if we assume that current species shared ancestors, but we would not expect such a pattern if the species had been created independently.
The universality of the carrier molecule of genetic information makes DNA a very appropriate character for the comparative and phylogenetic study of the species. Morphologically it is not possible to compare a bacterium with a man, however it is possible to establish a comparison with DNA molecules of both organisms, since they are formed by the same nucleotide base language. With sequence data we can compare any group of organisms, however distant they may be. The molecular data have other additional properties that all together make them the ideal character of phylogenetic studies. Many works obtain and analyse the sequences of genes and proteins of different species to solve questions still doubtful about the relations among organisms. The molecular data has shown that our species is much closer to the chimpanzee and the gorilla than we thought.
The molecular analysis of sequences has also taught us that there is a division in the very root of the tree of life that is more fundamental than the division of 5 kingdoms that is taught normally. In place of the two canonical cell types, prokaryotes and eukaryotes, there are three major cell types, the archaebacteria, the eubacteria and the eukaryotes. This new tree is called the universal phylogenetic tree.
The Sixth Mass Extinction
The natural destiny of each species is its extinction. But life continues because many species leave descendant species before dying. Biological diversity is a dynamic process resulting from the balance between extinction and the production of species. Neither of them can predominate for long. An extinction that was superior to the production of species for many generations would lead to the loss of life on Earth, while a reverse situation would lead to the depletion of resources and therefore to extinction. Currently one species disappears every 15 minutes. The disproportionate growth of the human species has increased by a thousand times the normal rate of extinction, creating a situation that is analogous to that of a great catastrophe. If we do not act responsibly, we confront the sixth, and perhaps definitive, great extinction of the Earth.
Despite the initial resistance to the Darwinian vision, at the beginning of the 20th century the idea of the evolution of life was already accepted by most Christian confessions. However, in the South and Midwest of the US, numerous and very active groups of evangelical Christians have repeatedly tried to prohibit the teaching of evolution in schools or share it in time and scientific recognition with the Biblical account of Genesis. In recent times, creationists have been renewed and organized through the intelligent design movement, gaining adherents of other confessions. Fundamentalists give supreme value to a text written more than 2,500 years ago in a historical and cultural context that has nothing to do with the current one, and that still consider it more credible than the whole body of knowledge, empirical data and theoretical structures that thousands of endowed minds of our species have developed working together for the last 150 years. The universalization of the practice of science has not translated into the adoption of a common worldview of the Universe and life. The persistence, a century and a half after the Origin of the species, of openly antiscientific beliefs is a great paradox of our time. The attainment of a universal Darwinian-based creed would lead to a higher plane of consciousness of the human species and would undoubtedly be Darwin's greatest legacy. (See the article Darwinism - Creationism written by the author)
The morphological, biochemical, and genetic similarities place the human being in the order of primates of the mammalian class. Within primates, they are the chimpanzee, the gorilla and the orangutan, their closest relatives. The sequence comparison data show that there is a similarity of 98.5% between human and chimpanzee DNA. This resemblance is greater than that between the chimpanzee and the gorilla or the gorilla and us, so the chimpanzee and humans share a more recent common ancestor than both with the gorillas. This closeness, which has been estimated around 6M years, is much greater than what was inferred only with morphological data, and shows the ability of DNA data to reveal kinship relationships. In human evolution there are two major acquisitions, the bipedal gait, and the extraordinary development of the brain. The fossil record shows us that the upright posture preceded brain development and that Africa is the cradle of humanity. Australopithecus, 1.5-5M years old, is the first anthropoid with bipedal gait. His cranial capacity was similar to that of the current chimpanzee and gorilla. Homo habilis and Homo erectus are the lines that follow chronologically until the arrival of our species, Homo sapiens, 100,000 years ago.
In 1997, the genome sequence of mitochondria DNA from the first fossil found in Neanderthal man, an extinct subspecies of the human species, was published. It was the first time that the molecular sequence of a hominid fossil was obtained. When the sequence was compared with homologous sequences of current human DNA it was inferred that the common ancestor of us and Neanderthal man lived 500,000 years ago, from which it was tentatively concluded that the Neanderthal man became extinct without mixing with the current man.
The publication of the complete sequence of the Neanderthal nuclear genome in May 2010 revealed, however, that there was some genetic flow after an initial contact between Neanderthal and Humans in Asia Minor, so that between 1-4% of the current human genome (except sub-Saharan Africans) comes from the Neanderthal man.
Bellaterra, Universitat Autònoma Barcelona
Citación: Barbadilla, A (1999, 2010) La evolución biológica (http://bioinformatica.uab.es/divulgacio/evol.html)
Citation: Barbadilla, A (1999, 2010) Biological Evolution (http://bioinformatica.uab.es/divulgacio/biological_evolution.html)
Otros artículos relacionados del mismo autor: