Biological evolution refers to the changes in inherited traits in a specific population -- humans, for example -- from generation to generation. Some of these genetic changes introduce variations into the species, while others remove them. According to the theory of evolution, these changes, known as natural selection, result in organisms more adapted to their environments and more suited for survival.
Although the ancients had some inkling of evolution -- Aristotle classified all living creatures, from plants to humans, on the Great Chain of Being -- evolutionary theory did not receive widespread acceptance until the 1859 publication of Charles Darwin's The Origin of Species [sources: Internet Encyclopedia of Philosophy, American Museum of Natural History]. Darwin declared that all creatures are descended from a common ancestor. The book created a firestorm of public outcry -- indeed, Darwin sat on his findings for nearly 20 years, fearing just such a reaction. Even now, 150 years later, some people still don't accept the theory, saying it goes against the Biblical story of creation.
In this article, we'll take a look at some of the top minds in advancing evolutionary theory. Some of these men laid the groundwork for Darwin, while others came later and have expanded the usefulness of his theory. We'll learn why, more often than not, the Darwin Awards are given posthumously, and we'll visit a museum in Kentucky where men and dinosaurs coexist.
For most, Charles Darwin is the first name that comes to mind when they think about evolution.
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10: Al-Jahiz
Ninth-century scholar Al-Jahiz, born in what is now Iraq, is generally credited with being the first to present a theory of biological evolution. His work had great influence on the evolutionists who followed him in the 18th and 19th centuries. While Aristotle and other Greek thinkers on evolution classified living things from most simple to most advanced, Al-Jahiz expanded on their work. What he called the "struggle for survival" existed not only between creatures of different species, such as predator and prey, but between creatures of the same species. A devout Muslim, Al-Jahiz said the will of Allah could transform one species into another, more adapted, species. Al-Jahiz went on to explain that these more successful creatures lived to breed and pass on their genes (although he didn't know that word) to their offspring. He also credited the environment with causing changes, saying that he had noticed that people from Morocco looked different from other people. He attributed those characteristics to differences in dust and water [source: Bayrakdar].
9: Andreas Vesalius
A young anatomist had the courage to buck several thousand years of medical knowledge to bring comparative anatomy to Europe. In the 1500s, Andreas Vesalius, like all Western scientists of that time, relied on the work of Galen, a physician of the Roman Empire, for his anatomical knowledge. But as Vesalius became more familiar with the human body, he realized Galen was incorrect on many points. The reason for Galen's inaccuracies: Roman law forbade the dissection of human corpses. Galen had to base his work on animal dissections. Vesalius encouraged his students to compare the differences and similarities between species. They realized that humans are merely one species of thousands, and that we share far more similarities with other species than we have differences. Darwin and his followers used this knowledge to develop the theory of evolution.
8: Jean-Baptiste Lamarck
Jean-Baptiste Lamarck, the first scientist to propose a cohesive theory of evolutionary change, eventually lost his career, his fortune and his reputation because of his beliefs. But less than 50 years after his death, Charles Darwin called him a "justly celebrated naturalist" [source: Berkeley]. Through his study of living animals and fossils, Lamarck, a founding professor at the Museum National d'Histoire Naturelle in France, began to believe that animals changed to adapt to their environments. For example, the giraffe stretches its neck to reach leaves. Over time, the neck of the giraffe becomes elongated. Lamarck had no idea about genetics and inherited traits. He thought the changes were simply physical manifestations of behaviors. More controversial were Lamarck's beliefs that life occurred through natural processes, not divine intervention.
7: Charles Darwin
Charles Darwin was destined for a career as a clergyman before a fortuitous circumstance put him aboard the HMS Beagle on a voyage to the Galapagos Islands in 1831. The 22-year-old embarked as a confused theological student, but returned a naturalist, famous throughout England for the strange plants and animals he had sent back from halfway around the world [source: American Museum of Natural History].
But ideas were the most valuable souvenirs of Darwin's trip. He began to wonder if the remarkable creatures he had observed had evolved through a process of natural selection -- survival of the fittest, so to speak -- offering an explanation of why life on Earth is so diverse. Darwin pondered these thoughts for more than 25 years, even writing a paper on the subject but never publishing it. Then, another naturalist, Alfred Wallace, proposed a similar theory. Darwin knew he had to act if he was to get credit for his work, so he hurried to publish Origin of the Species by Means of Natural Selection. He later produced The Descent of Man, and Selection in Relation to Sex. The books caused a sensation, to put it mildly. Pastors railed against the theory from the pulpit, saying it went against the Biblical story of creation. Some scientists also denounced Darwin's work. The law of natural selection, said astronomer John Herschel, was "the law of higgledy-piggledy" [source: American Museum of Natural History]. However, within a few years, the theory was generally accepted, at least in the scientific community. Advances in biology, medicine, paleontology and many other fields are based on Darwin's theories.
6: Gregor Mendel
One of the aspects of natural selection that puzzled Darwin and his colleagues was the laws of heredity. After all, who inherits what traits and why is the central question of the evolutionary process. They had no way of knowing that hundreds of miles away, in a monastery in what is now the Czech Republic, a monk named Gregor Mendel was beginning to answer those questions.
Mendel, a trained mathematician who was always interested in nature, began to wonder about traits in the peas grown in the monastery garden. He discovered that when he hybridized smooth and wrinkled peas, the peas in the first generation were all smooth. But in the following generation, about a quarter of the peas were wrinkled. Mendel had discovered recessive and dominant genes, although that concept wouldn't be fully explained for about 100 years. Mendel was unable to interest botanists in his work, primarily because they weren't used to statistics being used in natural history [source: University of California Berkeley]. It wasn't until 1899, 15 years after Mendel's death, that scientists discovered and verified his work.
Gregor Mendel first began his work in evolution when experimenting in the garden at his monastery.
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5: Ernst Haeckel
While he was a proponent of Darwin's theory, Ernst Haeckel, a German scientist working in the late 1800s, also subscribed to the earlier notion that each successive form of life encapsulated all the forms that came before it. He used his studies of the human embryo to "prove" this theory, saying that the embryo was first a single-celled, amoeba-like organism, then a fish with gills and on up the hierarchy. Haeckel wasn't above tampering with his results if they didn't show the outcome he desired. The discovery of genes in the early 1900s rendered Haeckel's theory invalid because genes can mutate and change the growth of embryos. Some historians have attempted to link Haeckel's "law of recapitulation" with Nazi anti-Semitism and beliefs about racial purity, but no firm links have been found. Still, Haeckel did believe the different races were the result of natural selection [source: Richards].
4: Edward Drinker Cope
With more than 1,200 published papers, Edward Drinker Cope holds the record as the most prolific scientist ever [source: Jaffe]. His contribution to evolution, Cope's Law, says that species tend to become larger over time -- a theory influenced by his work with dinosaur fossils. This law held up for more than a century, but has been called into question recently by scientists using computer models. These models show that, over the millennia, some marine organisms did get larger, but others stayed the same, and some actually grew smaller [source: Trefil]. A brilliant paleontologist with no formal scientific training, Cope is now primarily known for his decades-long feud with fellow American paleontologist O.C. Marsh, a dispute that led to the misnaming of the brontosaurus.
3: August Friedrich Leopold Weismann
August Friedrich Leopold Weismann, a professor at the University of Freiburg in Germany, was responsible for one of the most important biological discoveries of the 19th century: that sex cells divide through meiosis, a type of division necessary for sexual reproduction. The division results in the cells ending up with half of a chromosomal set, which will determine development in future generations.
In his 1893 work The Germ-Plasm, Weismann proposed that the union of two parents creates a substance called germ plasm, which is responsible for the variations seen in their offspring. He used a variety of experiments to prove that changes caused by environmental conditions cannot be passed on from parent to child. For example, in one experiment, he cut the tails off of a group of mice. When the mice reproduced, their offspring and future generations were all born with complete tails. Weismann was also a supporter of Darwin's theories, and he wrote several works on the subject of evolution in addition to The Germ-Plasm.
2: Thomas Henry Huxley
Thomas Henry Huxley, a proponent of evolution so fierce he was called "Darwin's bulldog," did not hesitate to point out problems with Darwin's work. He was especially critical of Darwin's contention that evolution was a slow, methodical process. Huxley supported the view that evolution might make dramatic leaps.
Huxley is best known for debating Archbishop Samuel Wilberforce at Oxford. At one point, the story goes, Wilberforce taunted Huxley by asking him which side of his family the ape from which he was descended was from. Huxley allegedly replied that he'd rather be the offspring of two apes than be a man afraid to face the truth [source: University of California Berkeley]. Huxley was bolder than Darwin in applying the theory of evolution to human evolution. His book, Evidence as to Man's Place in Nature, explores the new science of human paleontology.
1: Francis Crick and James Watson
Although Gregor Mendel had uncovered some of the principles of heredity in the mid-1800s, his work was lost for many years. When it was rediscovered, scientists began to understand that evolution is a process of genes and mutations. But just how were those genes relayed to offspring? Francis Crick and James Watson, working together at Cambridge University in 1953, uncovered the secret to DNA: the breakthrough that DNA is like a twisted ladder of nucleic acids [source: University of California Berkeley].
The discovery of DNA revolutionized the study of evolution, finally offering an explanation of how mutations occur. Mutations that give an individual an advantage will become more common through successive generations and will eventually drive the original unmutated genes out of existence. The discovery of DNA gives scientists the ability to track evolution by comparing the genetic codes of species both modern and far, far in the past.
To learn more about evolution and genetics, check out the links on the next page.
James Watson, shown here in his lab, worked closely with Francis Crick to unlock the mysteries of human DNA.
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Pierce, J. Kingston. "Scopes Trial." HistoryNet. June 12, 2006. (Nov. 24, 2010) http://www.historynet.com/scopes-trial.htm
Richards, Robert J. "Myth 19: That Darwin and Haeckel were Complicit in Nazi Biology." (Nov. 28, 2010) http://home.uchicago.edu/~rjr6/articles/Myth.pdf
Trefil, James S. "The Nature of Science." Houghton Mifflin Harcourt. (Nov. 28, 2010) http://books.google.com/books?id=JVj9SylSuB4C&pg=PA95&lpg=PA95&dq=cope's+law&source=bl&ots=ytuu0QnOdC&sig=D5cwjEVI6KMFDlnGr9YDaOvqibQ&hl=en&ei=1fbyTK3NJoTGlQein9X1DA&sa=X&oi=book_result&ct=result&resnum=5&ved=0CC0Q6AEwBA#v=onepage&q=cope's%20law&f=false
University of California Berkeley. "Comparative Anatomy: Andreas Vesalius." Understanding Evolution. (Nov. 22, 2010) http://evolution.berkeley.edu/evolibrary/article/0_0_0/history_02
University of California Berkeley. "Discrete Genes Are Inherited: Gregor Mendel." Understanding Evolution. (Nov. 22, 2010) http://evolution.berkeley.edu/evolibrary/article/0_0_0/history_13
University of California Berkeley. "DNA, the Language of Evolution: Francis Crick & James Watson." Understanding Evolution. (Nov. 22, 2010) http://evolution.berkeley.edu/evolibrary/article/0_0_0/history_22
University of California Berkeley. "Early Concepts of Evolution: Jean Baptiste Lamarck." Understanding Evolution. (Nov. 22, 2010) http://evolution.berkeley.edu/evolibrary/article/history_09
University of California Berkeley. "Early Evolution and Development: Ernst Haeckel." Understanding Evolution. (Nov. 22, 2010) http://evolution.berkeley.edu/evolibrary/article/0_0_0/history_15
University of California Berkeley. "Fossil Hominids, Human Evolution: Thomas Huxley & Eugene Dubois." Understanding Evolution. (Nov. 22, 2010) http://evolution.berkeley.edu/evolibrary/article/0_0_0/history_17
University of California Museum of Paleontology. "Ernst Haeckel." (Nov. 22, 2010) http://www.ucmp.berkeley.edu/history/haeckel.html
University of California Museum of Paleontology. "Evolution: Theory & Science." (Nov. 28, 2010) http://www.ucmp.berkeley.edu/history/evotheory.html
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