If you could travel back in time, the earliest period you could survive without the technology of today is the Pleistocene epoch, which began about 2.6 million years ago. During this time, the Earth's climate was similar to today's, with alternating periods of glaciation and warming. Humans evolved during this time, and they were able to survive by hunting and gathering, and by using simple tools and clothing. However, if you went back any further in time, you would not be able to survive because the Earth's climate was much different. The atmosphere was thinner and contained less oxygen, and the temperature was much colder. Plants and animals were also very different from what they are today, and there would be no food that you could eat.
