History of Theory of Evolution

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 вЂ" 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