"Take a hundred modern humans, give them stone-age tools, and drop them into a random point in Earth's 4.6 billion-year history. The question is not when they would die, it is when they would thrive. The answer turns out to be surprisingly recent. For most of Earth's existence, the atmosphere was either too thin in oxygen, too rich in carbon dioxide, or too unstable to keep a community alive for more than a few weeks. The honest floor is roughly 298 million years ago. Anything earlier, and the air itself would kill them."
cosmosunveiled.com

"What is the earliest, geologically, that humans could have survived on Earth?
Fancy a trip to the Jurassic or Cambrian era? In order to survive the environments of the past people would need a number of things...

"If we used a time machine to travel back to a prehistoric period, the earliest we could survive would be the Cambrian (around 541 million years ago). Any earlier than that and there wouldn't have been enough oxygen in the air to breathe.

"At the beginning of the Cambrian, the air at sea level would have felt like base camp at Mount Everest, but the climate was milder and more uniform than today. A bigger problem would be finding something to eat because there were no land plants or animals. You'd need to find a way of catching trilobites and other strange-looking shellfish, without wood to make a spear or plant fibre to make a fishing net. And you'd have to eat them raw, unless you could find a way to extract oil from these animals, or burn dry seaweed."
www.sciencefocus.com