When I invent a time machine, what is the earliest period in Earth’s history in which I could comfortably survive?
Reymond Aguinaldo
Ilagan, The Philippines
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The answer depends on what you mean by “comfortably survive”.
If you mean breathing the air, drinking the water and avoiding extreme temperatures, then you might be able to go back as far as the Ediacaran Period, roughly 640-540 million years ago (mya), when oxygen levels were closer to our own and there was abundant simple, multicellular life.
However, if you mean finding food and avoiding predators, then you might have a hard time in any period before the Holocene Epoch, starting around 11,700 years ago, when modern humans began to domesticate plants and animals.
Of course, you would also need to consider the risks of altering the course of history, creating paradoxes or encountering hostile civilisations. So maybe the best time to visit is the present.
Thomas Barker
Cambridge, UK
Earth’s surface is thought to have solidified roughly 4 billion years ago, so any attempt to travel back earlier than that is certainly off the cards. Even after the mantle solidified, “comfortable” would be a stretch – almost all of the planet’s surface was volcanically active, and there was next to no oxygen, so you would need breathing apparatus and a protective suit.
Earth’s atmosphere remained almost oxygen-free until near the end of the “great oxygenation event” (around 2 billion years ago). It is thought that this raised Earth’s oxygen concentration to 10 per cent of its present level, which is just about survivable for a few seconds.
Those are the absolute earliest points at which it is safe to try the time machine out. If you are planning to stick around in the past for a bit, though, you also need to worry about finding something to eat. Multicellular life doesn’t enter the equation until the mid-Ediacaran Period (around 600 mya), and you will probably have to wait until the Cambrian explosion (around 540 mya) before any organisms are big enough to catch.
I would suggest the Devonian Period (starting at approximately 420 mya) as a good earliest point for long-term survival. Fish have evolved, oxygen levels are closer to today’s and, in North America, the first forests are starting to grow. Wait a few more million years for the Carboniferous Period (roughly 360 mya), and you will even get to meet some of your earliest land ancestors, in the form of primitive tetrapods.
Bad luck if you are a vegetarian, though. Fruit won’t evolve until the early Cretaceous Period (145 mya), so you are going to get pretty sick of eating ferns.
Graham Smith
Melbourne, Australia
If you were able to go back to the late Proterozoic Eon (say 600 million years), you would have barely enough oxygen to breathe for a short time, and you would need to survive on an unappetising soup of cyanobacteria.
Better to wait until after the Cambrian explosion so you could supplement your diet with shellfish. Better still, wait another 300 million years so that there will be plenty of land plants and animals to eat.
But don’t wait too much longer, or you may become dinosaur food.
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