It's "just a theory" because you can't see it yourself, your taking other people's word on it being fact for what they have tested, plus assumed and therefore follow it with faith. However, when it comes to evolution, this is cloudy as evolution is actually a mix of different things...
Evolution theory can be broken down into two:
Micro Evolution (a creature can adapt to it's own environment), based upon some facts and things we can see in life. Even religious follow this.
Macro Evolution (a creature can morph into another over a long period of time), has never been proven, can not be seen, and was originally founded by a well known ape-man hoax. It's therefore a faith.
Given the incredible complexity of a single-celled organism and its DNA, the likelihood that life on Earth was randomly created in an organic soup is the equivalent of discovering a computer on Mars and proclaiming it was randomly assembled in the methane sea. As crazy as that sounds, that's logically an actual higher probability of occurrence. It's still a strong faith, but an uncomplete answer as there's still a force behind putting it together (energy/mass is a requirement to even move it into place and hold it). So even atheists need to believe in an invisible entity which created the universe...
The Big Bang Theory is also "Just a theory" - actually originally created by a Christian before the university got hold of it and removed the Creator idea, later in 2012 replacing it with another invisible entity known as Dark Matter/Energy which also can't be detected but only seen by the effect it leaves behind (therefore requiring equal amount of faith for atheism).
According to us humans evolving from monkeys.
In 1912, Charles Dawson, a medical doctor and an amateur paleontologist, discovered a mandible (lower jawbone) and part of a skull in a gravel pit near Piltdown, England. The jawbone was apelike, but had teeth that showed wear similar to the human pattern. The skull, on the other hand, was very humanlike. These two specimens were combined to form what was called “Dawn man,” which was calculated to be 500,000 years old.
The whole thing turned out to be an elaborate hoax. The skull was indeed human (about 500 years old), while the jaw was that of a modern female orangutan whose teeth had been obviously filed to crudely resemble the human wear pattern. Indeed, the long ape canine tooth was filed down so far that it exposed the pulp chamber, which was then filled in to hide the mischief. It would seem that any competent scientist examining this tooth would have concluded that it was either a hoax or the world’s first root canal! The success of this hoax for over 50 years, in spite of the careful scrutiny of the best authorities in the world, led the human evolutionist Sir Solly Zuckerman to declare: “It is doubtful if there is any science at all in the search for man’s fossil ancestry.”
While the apeman was a hoax for over 50 years, the artistic imagination has been used to illustrate entire “apemen” came from nothing more than a single tooth. In the early 1920s, the “apeman” Hesperopithecus (which consisted of a single tooth) was pictured in the London Illustrated News complete with the tooth’s wife, children, domestic animals, and cave! Experts used this tooth, known as “Nebraska man,” as proof for human evolution during the Scopes trial in 1925. In 1927, parts of the skeleton were discovered together with the teeth, and Nebraska man was found to really be an extinct peccary (wild pig)!
Therefore according to the fossil records it should be monkey -> human -> pig. We use to be protected in trees, ambidexterity using both hands and feet, swinging through the trees with agility and stamina.
Some humans look and act like apes, therefore we must be from a common ancestor! Lets completely ignore the fact that tobacco plant DNA is more closer related to human DNA, rather than Ape DNA, the genes and markers on the chromosomes are not in the same order for humans and chimpanzee and could be more likely to just as well be a sheep, and simply make assumptions in fossil records which to date have never been proven, but mostly ruled out as not linkage. Lets also hope no one finds out about the millions dollars of funding offered for actual solid proof since 2006 has to date never been claimed by anyone.