Could there have been a civilisation on Earth long before humans existed?

It’s the sort of far-out idea discussed with breathless excitement on UFO sites – could there have been an advanced civilisation on Earth before humans existed?

Surprisingly, scientists have actually given the idea serious thought – and looked closely at a ‘warming spike’ 56 million years ago.

Could the event – the Paleocene-Eocene Thermal Maximum (PETM) – have been caused by climate change from an ancient civilisation?

Professor Adam Frank of the University of Rochester investigated the idea, he writes in The Atlantic – and in the process, gained insight into our impact on our planet, and possibly on how life might impact other distant planets, too.

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Frank and his colleagues concluded that there was no evidence of a previous civilisation on Earth – but that such evidence might be hard to find.

Frank wrote, ‘There is a conundrum here. If an earlier species’s industrial activity is short-lived, we might not be able to easily see it.

‘The PETM’s spikes mostly show us the Earth’s timescales for responding to whatever caused it, not necessarily the timescale of the cause.

‘So it might take both dedicated and novel detection methods to find evidence of a truly short-lived event in ancient sediments. In other words, if you’re not explicitly looking for it, you might not see it.’

The researchers thought about what signs our civilisation might leave behind – plastics in the ocean, evidence of fertiliser use, and (most of all) evidence of fossil fuel use.

Frank says that the research opened up the idea that – on other worlds – civilisation might come in cycles.

Frank says, ‘Our work also opened up the speculative possibility that some planets might have fossil-fuel-driven cycles of civilisation building and collapse. If a civilisation uses fossil fuels, the climate change they trigger can lead to a large decrease in ocean oxygen levels.

‘These low oxygen levels (called ocean anoxia) help trigger the conditions needed for making fossil fuels like oil and coal in the first place. In this way, a civilisation and its demise might sow the seed for new civilisations in the future.’