The Back to the Future franchise remains at the peak of its popularity for a reason, having captivated audiences for decades. The films are often praised for their logical script structure and how simply and accessibly the plot explains seemingly complex concepts. This primarily pertains to the very possibility of traveling from the present to the future, but in this case, the test subjects that clearly demonstrate the principles of time travel are not the main characters at all — the true pioneer is somehow overlooked by everyone.
Einstein (not the one you might be thinking of) became the first time traveler.
The test subject of the DeLorean time machine was not Doc and not even Marty, but rather Doc's pet — a dog named Einstein. In the first part of the franchise, Doc does not provide any detailed information about how the time machine works; he simply places the dog in the cabin and activates the device — this leads Marty to the horrifying thought that the dog might just dissolve into thin air. But everything ends well — having taken precautions to ensure that Einstein travels just one minute into the future, Doc confirms the success of the experiment by noting that the dog's clock shows exactly one minute later than the one that remained in the present.
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Einstein repeatedly helped Marty and Doc — and ultimately got a happy ending.
Although after that, Doc no longer put his pet at risk and didn't take him on adventures into the future, it doesn't mean that the pioneering dog was forgotten. For instance, Einstein assisted his owner and Marty in finding Jennifer when she mysteriously disappeared, and when Doc plans to travel to 1855, he asks Marty to look after Einstein during his absence — which Marty, of course, does.