Final answer:
Humans did not evolve from present-day monkeys; they share a common ancestor. Evolutionary biologists understand evolution as a diverging process through natural selection. Primates have evolved into a wide variety of niches, resulting in the diversity of monkeys and apes we see today.
Step-by-step explanation:
An evolutionary biologist would explain that humans did not evolve from the monkeys that are living today, but rather, humans and monkeys share a common ancestor that lived millions of years ago. This common ancestor diverged into different lineages, one leading to modern monkeys and another leading to apes and eventually humans. The process of evolution works through natural selection, where organisms that are better adapted to their environments tend to survive and reproduce more successfully, passing on their traits to the next generation.
As for the question regarding fossils, the overwhelming majority of species that have ever lived on Earth are now extinct, and only a tiny fraction of these have been rediscovered alive. While it's possible that some presumed-extinct species might still exist in unexplored regions, it is highly implausible for the vast majority.
Primates have evolved over time by adapting to various ecological niches, leading to the diversity we see today, such as the difference between New World and Old World monkeys. New World monkeys evolved in isolation in South America and have several unique features, such as prehensile tails for arboreal living. In contrast, Old World monkeys evolved in Africa and Asia and display a larger variety of living habits, both arboreal and ground-dwelling.