Not everyone accepts this theory. The most adamant are the inhabitants of Z-Tel? who maintain that they were the original humans. They point to the Ark Royal series of ships which archaeologists have accurately dated to pre-Terran times. They maintain it was the missing Ark Royal II which was responsible for bringing humans to Earth. They claim that antique records stored on Z-Tel? in time dilation fields prove that they were the first spacefaring humans. Unfortunately they have refused to let any historians see these records and until they do their claims will always have an aura of doubt around them.
Perhaps the most interesting theory or recent years has come from Benthic Space expert Vladamir Mbeki. He claims that because of the trouble in crossing the Grey Zone it is unlikely that any other non-Terran civilisation would have any reason to come anywhere near Earth and drop colonists on it, when there are easier planets to get to. He also cites the lack of any prehistoric mechanical artefacts being found on Earth, let alone a crashed spaceship, and the evidence for human evolution on Earth. His explanation sounds crazy, but it does answer these questions. He claims the prehistoric spacefaring humans actually came from Earth and it was they who colonised Z-Tel? and the ancient East Rim. To Mbeki there is far more reason for others to leave the Grey Zone than there is for people outside to enter it. As for the lack of evidence for machinery on prehistoric Earth he claims that this ancient civilisation used biotechnology to create organic machines, including the bio-tech [Thorn Vessels]?, which meant that most of the evidence of them on Earth has biodegraded. His theory is very controversial, but has proved very popular on Earth itself.