Up until the mid-sixties the majority of geochemists considered the current atmosphere as the final product of the degassing of the chemical elements forming the Earth's crust. They were convinced that reactions through non-biological processes had determined its current state. Oxygen, for instance, was merely supposed to be the product of the breakdown of water vapor molecules coupled with the release of hydrogen in the environment, resulting in an excess of free oxygen. It was thought that living beings simply “borrowed” gases from the atmosphere and returned them unmodified.
However, the atmosphere’s composition deviates considerably from the chemical equilibrium of the steady state. Almost everything seems to violate the rules of chemical equilibrium. In fact, the atmosphere’s composition is such a curious and incompatible mixture that it could not have formed or persisted by chance.
"millions of years" ago. Aliens terrafromed earth. Humans being the aliens were comprised of 5 or so races and landed in different parts of the earth. Religion is a concept to answer questions without truly answering questions as is evolution. The life on earth is constructed from the same DNA we are constructed from. Our bodies being the hardware for the software of information gathering. We are the same because we are all created from the same science. Aliens may take 10 seconds to get to earth but the changes and whatnot are perceived by humans on a different time table of what seems like a million years. The elite are aware of this ofcourse and seek to protect the planet from the people.
Intelligent alien life with equal/greater intelligence than humans and had learned the evolution of life. Open your mind to the idea that life can be created in a petrie dish and another planet is the dish. I would send my DNA on the back of a comet aimed at another planet. No doubt the result in 4 billion years is a 6 foot dark haired blue eyed biped would walk the planet but some evolution of my DNA would carry on. Sign me up and let's terraform something!
In order for us to have been "placed" here for that purpose, there would need to be a godlike entity doing the "placing", a phenomenon which there is currently no evidence for. However, what there is evidence for is the gradual evolution of the human race from apes, which does not include any "placing". So my answer would have to be, nope.
This idea requires an entity deciding to terraform the earth. If you can make a case for the existence of some being powerful enough to "place" beings one earth maybe i would entertain the idea. Until then you might as well ask if the easter bunny put people on earth....
The fossil record shows a very gradual development of life on Earth over 3+ billion years, from very simple up to multicellular animals like ourselves. Any alien wanting to terraform Earth would dump a mix of fast-growing highly-engineered bacteria on the planet, leave, and come back as soon as terraforming goals were reached (oxygen atmosphere developed, climate stabilized, etc). It would make no sense to leave humans here, as we only seem to degrade the environment. And the long history of the fossil records clearly shows our roots here on Earth. We evolved here, we weren't just dropped here recently.
According to scientists modern humans have a history stretching ~150,000 years. By comparison bacterias have a history of ~4 billion years and multicellular organisms have a history of ~500 million years. What sort of alien civilization would plant bacterias to terraform Earth into having good living conditions and not reap it's harvest? And by the time humans got on Earth conditions were prime for any sort of alien colonization without any additional changes that humans would bring to the table.