At one time the knowledge of animals was limited to the bare necessities of surviving in a natural world. Paleolithic hunters were acutely aware of the dangers posed by animals in the wild, while Neolithic farmers had a better degree of understanding of the domesticated animals in their yards. However, zoology as a proper and scientific study of animals must be said to begin with the ancient Greeks in the classical age, and more specifically with Aristotle.
A pre-scientific, but nevertheless systematic, approach to the study of animals and plants was made in ancient India by the practitioners of ayurveda medicine. The Sushruta Samhita from the sixth century B.C. collates thousands of natural substances, 57 of which are from animal sources.
Before Aristotle, Hippocrates, in the fourth century B.C. cataloged some observations of animals. But when Aristotle (384-322 B.C.) set to the task, he brought with him a approach that we are able to identify proximately as scientific. To Aristotle, everything in the natural world was dictated by its "formal cause" - an intrinsic purpose to exist. In the case of plants and animals, he believed that each one could be placed at some point in a graded scale of perfection, the Great Chain of Being, and whose apex describes the human being. In this way he classified 540 animal species, each placed at some point along his "scala naturae."
Aristotle both advanced the study of zoology a long way and hindered it at the same time. The breadth and depth of his vision, covering the entire scope of learning, was such that his immediate posterity took him to be the revered authority on all matters. This discouraged further observations, and zoology, along with all the other sciences, lay practically dormant in Europe for nearly two thousand years. The Roman physician Claudius Galen, in the second century A.D., vivisected animals and sought to explain physiology in terms of the "four humors," a mistaken notion that persisted long into the age of science.
The Muslims in the golden age of Islamic science not only preserved the knowledge of Aristotle and the ancient Greeks, but also made great strides on their own. The ninth century Arabian scholar al-Jahiz studied animals in the context of environment and the food chain. Some of these ideas possess uncanny resemblance to those of Darwin later, including the "struggle for existence." The Arabian physician Ibn al-Nafis discovered the pulmonary circulation of blood 400 years before Harvey. He also described the concept of metabolism.
The zoological quest was resumed in Europe through the renaissance Italian scholar Albertus Magnus (1206-80) whose De animalibus is a substantial compendium of animal observations. But true advance continued to be hampered by the lack of physiological understanding. A turning point was reached in 1543 when Andreas Vesalius published an in-depth study of human anatomy. Very soon after Swiss scientist Conrad Gessner published this Historiae animalium (1551-58), a four volume zoological study based on a much improved understanding of anatomy. This is normally taken to be the starting point of modern zoology.
Yet zoology today incorporates far more than mere anatomical observation, and it required the development on various fronts before the truly modern synthesis materialized. The crucial springboard was supplied by the institutions of science established in various European centers in the 17th century, such as the Royal Society of London and the Academy of Sciences of Paris. It soon became customary to carry naturalists on state-sponsored voyages of exploration, thus greatly expanding the scope of investigation. A seminal contribution was made by the German explorer naturalist Alexander von Humboldt (1769-1859) who studied animals in the context of geography and habitat.
The microscopic view was just as important as the macroscopic one. The compound microscopes of the middle of the 17th century opened up to view an entirely new world of microorganisms. In 1665 Robert Hooke was the first to observe the cell, which was eventually accepted to be the basic unit of living organisms. The similarity between the cell and independent microorganisms became more and more apparent, until the "cell theory" was formally proposed by Germans Matthias Schleiden and Theodor Schwann in 1838.
The first attempt towards a grand synthesis was by the Swedish naturalist Carl Linnaeus in 1735, who proposed a taxonomy to classify all living organisms. This is still in use today. Though Linnaeus himself saw the species as fixed and determined, others took in the vista of the animal kingdom and imagined otherwise. According to Georges-Louis Leclerc, the hierarchy of the animal kingdom was suggestive of a family tree, and he was even bold enough to suggest that species were malleable and that similar species were related by common descent. The French naturalist Jean-Baptiste Lamarck was the first to suggest a mechanism for such an "evolution of species." But his explanation of how individual adaptations are inherited did not convince the scientific community. The theory of evolution was only established ones the British naturalist Charles Darwin proposed the mechanism of "natural selection," whereby the fittest of the species survive and pass on their favorable characteristics to the next generation.
Darwin's theory represented an incredible synthesis of the various strands of zoological understanding up until 1859, which included geology, paleontology , ecology, anatomy, physiology and cell theory. It was also supported by the developing field of genetics, first established by Gregor Mendel even while Darwin was publishing his seminal The Origin of Species, though discovered much later. The theory of evolution both explained the hierarchy of the animal kingdom and opened the way towards better classification.
Genetics also proved to be the key towards a better understanding of physiology and reproduction. In the 20th century great advances led to specialization. Disciplines such as cytology, bacteriology, morphology, embryology and ecology established themselves as separate fields while zoology itself concentrated on the classification and description of animals (taking into account, of course, the perspectives from its independent sub-fields). The most remarkable advances have been in the field of molecular biology, and more specifically in the mapping of the DNA molecule, the fundamental determinant of all life.