Zoology was recognized as a science after Aristotle wrote "History of animals" around 350 BC. He was the first person (the first documented anyway) to classify animals. He studied 500 animals and placed them into two groups. Animals with blood and animals without blood.
Although others added to Aristotle's findings the next major turning point in zoology came with Pliny the Elder around 25-80 AD. He was a biologist and wrote a fairly comprehensive book, "Historia Naturalis", a book of natural history. Pliny was a Roman and his book was the authoritative work of the time.
Although an interesting book was written around the time of the rise of Christianity, the author is unknown. It is called "Physiologus" and listed 49 animals, some of which didn't even exist.
Leonardo de Vinci, in the 1400's was an influential and well known zoologist. His experiments with, and knowledge of, the human anatomy contributed much to the science.
Not long after de Vinci, the microscope was invented and in 1555 Conrad Gesner wrote "Historia Animalum" which was first of a series of books. The microscope assisted greatly with zoology studies and led to works on specific animals such as fish and birds.
In the 1700's the process of classification involving the use of two Latin names was devised. This system is still used today and we know the two Latin titles as scientific names.
In the early 1800's, scientists finally began to understand fossils when George Cuvier discovered comparative anatomy. This led to a greater understanding of the anatomy of animals and how organs related to the anatomy. This was the beginning of Paleontology.
The amount of knowledge increased dramatically and, in the 1860's, perhaps the most famous of all zoologists, Charles Darwin, developed his theory of evolution. This theory was the turning point in modern biology.
There have been many other zoologists over the centuries, but the above mentioned and their works have marked major turning points in our understanding of the natural world.