What aspects of the novel can be perceived as it evolved into a major literary form?
One important feature of novels as they evolved was the sheer number of characters and subsequent sub-plots that could be explored. Some early novels, such as Pamela by Richardson, started with the idea of having only a few characters reveal the events and conflicts of their lives through a series of letters written back and forth between other characters. These are the first "first person narrators." As the novel form evolved, authors used third person narration, but usually still had the focus of the story be on a single protagonist and his or her interactions with any number of minor characters. From the early part of the 1700's to 1800 when Jane Austen wrote Pride and Prejudice for example we can see that transistion. Austen still used the convention of letters to reveal plot pieces and character's thoughts, but there is a narrator telling us the story of the Bennet sisters, primarily through Elizabeth's point of view. Austen has a huge cast of characters all interacting in various ways to showcase the themes of marriage and society's influence on the individual. By have several minor characters, she can use them as foils to the main characters of Darcy and Elizabeth. The complexity the characters and the plotting is one way to evaluate the evolution of the novel into the literary form we see today.
Below are two links that will provide a lot of credible information about the earliest novel forms in English.
A novel must involve a story that includes characters, themes and conflicts. There is a always a plot that follows a main character (or characters) through events that require the character(s) to learn something and change or, conversely, not change and endure the consequences.
For me, one of the most distinguishing features of a novel is the author's use of voice. How is the story narrated? A novel must always be a story told by some narrative voice. Even in the third person -- an omniscient, objective voice -- there is still a perspective on the action.
When the voice is first person, the narrator is also the main character, and this provides one of the unique storytelling perspectives and author can take. Think of Huckleberry Finn or David Copperfield or Catcher In the Rye. Distinct voices telling stories in first-person narrative really give a novel a powerful point of view in telling a story, and serve as an important aspect of the novel as a major literary form.