Aboard the Beagle, Darwin realized that some of his own observations of fossils and living plants and animals cast doubt on some accepted scientific theories. He noted, for example, that certain fossils of supposedly extinct species closely resembled living species in the same geographical area. He also observed that in the Galapagos Islands, each island supported its own form of tortoise, mockingbird and finch - their forms were closely related but they differed in structure and eating habits on each island. Both observations raised the question of possible links between individual but similar species, and upon his return in 1836 he began recording his ideas about changeability of species in his Notebooks on the Transmutation of Species. He was influenced by other naturalists" essays, and his theory was announced in 1858 in a paper presented at the same time as one by Alfred Russel Wallace, another young naturalist that had come to the theory of natural selection independently. Subsequently, Darwin published his complete theory in a book called On the Origin of Species in 1859; the book was sold out on the first day of publication and was referred to as "the book that shook the world".
The challenge facing scientists nowadays in this area is a lack of evidence, so that they have to rely on information they gather from present-day species. The most common method of gathering information is molecular anthropology, which is where DNA strands of the two species to be compared are heated to 86?C. At this temperature the DNA comes apart, and then scientists can match one half of the DNA of one species to half the DNA of the other. They then look at the percentage of parts that fit together. For instance, if two species are 75% the same, then this is because 75% of their DNA fits together exactly. The scientists then look for patterns as to how similar different species are to each other.