Andreas Vesalius Theories Of The Human Body

HISTORY OF THEORY

OF EVOLUTION

In 1543, a young Flemish anatomist Andreas Vesalius challenged Galen’s theories of the Human Body. This discovery had an impact on scientists. Vesalius’ discovery of the important differences between species also helped usher in the science of comparative anatomy, in which researchers studied animals to find their similarities and differences. In the process, they gradually began to recognize humans as being one species among many, with a few unique traits but many others shared in common with other animals. Some 300 years after Vesalius first shook off the blind obedience to Galen, Darwin used that vast stock of anatomical knowledge to build his theory of evolution.

In 1666, Nicholas Steno dissected
a shark; he was struck by how much the shark teeth resembled “tongue stones,” triangular pieces of rock that had been known since ancient times. Steno made the leap and declared that the tongue stones indeed came from the mouths of once-living sharks. He showed how precisely similar the stones and the teeth were. But he still had to account for how they could have turned to stone and become lodged in rock. Steno said that the fossils were snapshots of life at different moments in Earth’s history and that rock layers formed slowly over time. It was these two facts that served as the pillars of paleontology and geology in future centuries. And fossils ultimately became some of the key evidence for how life evolved on Earth over the past four billion years.

In the 1800’s, Theology is the understanding of and providing reasoned discourse of religion, spirituality and the Gods. Natural Theology dominated English thinking for two centuries. Natural theology was important scientifically because it guided researchers to the fundamental question of how life works. Even today, when scientists discover a new kind of organ or protein, they try to figure out its function.
William Smith was surprised to find that the fossils in the layers often were arranged in the same distinctive order from the bottom to the top of the rocks. And as he traveled across England, he discovered the same sequences of fossils in rock layers. Each type of animal, he realized, had a widespread existence for a particular span of time, a span that partially overlapped with that of other animals. That made it possible for Smith to recognize the order in which rocks had been formed throughout much of England. In 1831, new generations of geologists appreciated Smith's contribution. This theory impacted geologists everywhere. Geologists used his methods to discover even older geological formations whose outcrops were scattered across England. Meanwhile on the continent, Georges Cuvier and his student Alexandre Brongniart used much the same method to decipher the rocks of the Alps. It became inescapably clear to geologists that Earth and its life were far older than a few thousand years.

"Catastrophism," as this school of thought came to be known, was attacked in 1830 by a British lawyer-turned-geologist named Charles Lyell.
For inspiration, Lyell turned to the fifty-year-old ideas of a Scottish farmer named James Hutton. In the 1790s, Hutton had argued that the Earth was transformed not by unimaginable catastrophes but by imperceptibly slow changes, many of which we can see around us today. Rain erodes mountains, while molten rock pushes up to create new ones. The eroded sediments form into layers of rock, which can later be lifted above sea level, tilted by the force of the uprising rock, and eroded away again. These changes are tiny, but with enough time they could produce vast changes. Hutton therefore argued that the Earth was vastly old — a sort of perpetual-motion machine passing through regular cycles of destruction and rebuilding that made the planet suitable for mankind. Lyell had an equally profound effect on our understanding of life's history. He influenced Darwin so deeply that Darwin envisioned evolution as a sort of biological uniformitarianism. Evolution took place from one generation to the next before our very eyes, he argued, but it worked too slowly for us to
perceive.

In 1865, Gregor Mendel revealed that distinct traits were inherited in a well defined and in predictable manner. When Mendel’s work was rediscovered in 1900, disagreements over the rate of evolution predicted by early geneticists and biometricians led to a rift between the Mendelian and Darwinian models of evolution. This contradiction was reconciled by Ronald Fisher. The end result was a combination of Darwinian natural selection with Mendelian inheritance, the modern evolutionary synthesis or Neo-Darwinism.
In 1859, Darwin turned biology upside down in 1859 with the publication of Origin of Species.
Natural selection is the process by which favorable traits that are heritable become more common in successive generations of a population of reproducing organisms, and unfavorable traits that are heritable become less common. Natural selection acts on the phenotype, or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and reproduce than those with less favorable phenotypes. If these phenotypes have a genetic basis, then the genotype associated with the favorable phenotype will increase in frequency in the next generation. Over time, this process can result in adaptations that specialize organisms for particular ecological niches and may eventually result in the emergence of new species. The book was not only a best seller but also one of the most influential scientific books of all time. Yet it took time for its full argument to take hold. Within a few decades, most scientists accepted that evolution and the descent of species from common ancestors were real. But natural selection had a harder time finding acceptance. In the late 1800s many scientists who called themselves Darwinists actually preferred a Lamarckian explanation for the way life changed over time. It would take the discovery of genes and mutations in the twentieth century to make natural selection not just attractive as an explanation, but
unavoidable.

Thomas Morgan and several other scientists carried out breeding experiments in the late 1890s and rediscovered Mendel’s three-to-one ratio. But this new generation could offer a clearer interpretation of what was happening in their experiments. We each carry two copies of the same gene, one from each parent, but in many cases only one copy produces a trait while the action of the other is masked. Here was the secret behind Mendel's three-to-one ratio of smooth and wrinkled peas. The work of scientists such as Morgan established a new science: genetics. Genetics influenced many scientists and it led up to the discovery of molecular structure of genes (DNA).

Wallace had already accepted evolution when he began his travels in 1848 through the Amazon and Southeast Asia. On his journeys, he sought to demonstrate that evolution did indeed take place, by showing how geography affected the ranges of species. He studied hundreds of thousands of animals and plants, carefully noting exactly where he had found them. The patterns he found were compelling evidence for evolution. He was struck, for example, by how rivers and mountain ranges marked the boundaries of many species' ranges. The conventional explanation that species had been created with adaptations to their particular climate made no sense since he could find similar climatic regions with very different animals in them. In 1876, Wallace published his book, The Geographic Distribution of Animals, has plates depicting the animal life of the biogeographic regions he identified. Biogeographers now recognize that as continents collide, their species can mingle, and when the continents separate, they take their new species with them. Africa, South America, Australia, and New Zealand, for example, were all once joined into a supercontinent called Gondwanaland. The continents split off one by one, first Africa, then New Zealand, and then finally Australia and South America. The evolutionary tree of some groups of species such as tiny insects known as midges — show the same pattern. South American and Australian midges, for example, are more closely related to one another than they are to New Zealand species, and the midges of all three land masses are more closely related to one another than they are to African species. In other words, an insect that may live only a few weeks can tell biogeographers about the wanderings of continents tens of millions of years ago.

In 1937, Dobzhansky published, Genetics and the Origin of Species. In it, he sketched out an explanation for how species actually came into existence. Mutations crop up naturally all the time. Some mutations are harmful in certain circumstances, but a surprising number have no effect one way or the other. These neutral changes appear in different populations and linger, creating variability that is far greater than anyone had previously imagined. Dobzhansky's ability to combine genetics and natural history attracted many other biologists to join him in the effort to find a unified explanation of how evolution happens. Their combined work, known as "The Modern Synthesis," brought together genetics, paleontology, systematics, and many other sciences into one powerful explanation of evolution, showing how mutations and natural selection could produce large-scale evolutionary change. The Modern Synthesis certainly did not bring the study of evolution to an end, but it became the foundation for future research.

A population of birds, or any organism, can speciate if isolated from its neighbors. In 1942, Ernst Mayr published the book, Systematics and the Origin of Species, Mayr argued that the most significant way to cut off a population is by geographical isolation. For example, a glacier may thrust down a valley, creating two separate populations, one on either side of the glacier. A rising ocean may turn a peninsula into a chain of islands, stranding the beetles on each of them. This sort of isolation doesn't have to last forever; it needs only form a barrier long enough to let the isolated population become genetically incompatible with the rest of its species. Once the glacier melts, or the ocean drops and turns the islands back into a peninsula, the animals will be unable to interbreed. They will live side by side, but follow separate evolutionary fates. Today, scientists studying the origin of species can compare not just the bodies of species, but their genes as well. Geographic isolation remains a crucial element in forming new species, but a number of biologists now argue that the formation of species can take several different paths. It may be possible, for example, for a population to continue breeding with other members of its species — and trading genes — while still diverging into a distinct group. All that may be required is that a few of its genes diverge, thanks to strong natural selection. If the conditions are right, this genetically distinct population may then become a new species.

In 1953, Watson, Crick, and other researchers figured out the basics of how DNA works. Each gene, they realized, consists of a stretch of base pairs. A single-stranded copy of the gene was created (known as messenger RNA) and transported to protein-building factories in the cell called ribosomes. There, the sequence of the bases guided the assembly of a string of amino acids that became a new protein. When a cell divides, the double helix is unzipped and the DNA is replicated. Evolutionary biology was revolutionized by the discovery of DNA. Thanks to the discovery of DNA, it is now possible for scientists to identify not just the genes, but the individual bases. Before the discovery of DNA, scientists could only uncover the evolutionary tree of life by comparing the bodies and cells of different species. Now they can compare their genetic codes, working their way down to the deepest branches of life dating b