In the 1660s, Galileo confidently declared that an entirely uninhabited or unpopulated world is “naturally impossible” on account of it being “morally unjustifiable”. Gottfried Leibniz later pronounced that there simply cannot be anything entirely “fallow, sterile, or dead in the universe”.

Along the same lines, the trailblazing scientist Edmond Halley (after whom the famous comet is named) reasoned in 1753 that the interior of our planet must likewise be “inhabited”. It would be “unjust” for any part of nature to be left “unoccupied” by moral beings, he argued.

Around the same time Halley provided the first theory on a “mass extinction event”. He speculated that comets had previously wiped out entire “worlds” of species. Nonetheless, he also maintained that, after each previous cataclysm “human civilisation had reliably re-emerged”. And it would do so again. Only this, he said could make such an event morally justifiable.

Later, in the 1760s, the philosopher Denis Diderot was attending a dinner party when he was asked whether humans would go extinct. He answered “yes”, but immediately qualified this by saying that after several millions of years the “biped animal who carries the name man” would inevitably re-evolve.

This is what the contemporary planetary scientist Charles Lineweaver identifies as the “Planet of the Apes Hypothesis”. This refers to the misguided presumption that “human-like intelligence” is a recurrent feature of cosmic evolution: that alien biospheres will reliably produce beings like us. This is what is behind the wrong-headed assumption that, should we be wiped out today, something like us will inevitably return tomorrow.

Back in Diderot’s time, this assumption was pretty much the only game in town. It was why one British astronomer wrote, in 1750, that the destruction of our planet would matter as little as “Birth-Days or Mortalities” do down on Earth.

This was typical thinking at the time. Within the prevailing worldview of eternally returning humanoids throughout an infinitely populated universe, there was simply no pressure or need to care for the future. Human extinction simply couldn’t matter. It was trivialised to the point of being unthinkable.

For the same reasons, the idea of the “future” was also missing. People simply didn’t care about it in the way we do now. Without the urgency of a future riddled with risk, there was no motivation to be interested in it, let alone attempt to predict and preempt it.

It was the dismantling of such dogmas, beginning in the 1700s and ramping up in the 1800s, that set the stage for the enunciation of Fermi’s Paradox in the 1900s and leads to our growing appreciation for our cosmic precariousness today.

But then we realised the skies are silent

In order to truly care about our mutable position down here, we first had to notice that the cosmic skies above us are crushingly silent. Slowly at first, though soon after gaining momentum, this realisation began to take hold around the same time that Diderot had his dinner party.

One of the first examples of a different mode of thinking I’ve found is from 1750, when the French polymath Claude-Nicholas Le Cat wrote a history of the earth. Like Halley, he posited the now familiar cycles of “ruin and renovation”. Unlike Halley, he was conspicuously unclear as to whether humans would return after the next cataclysm. A shocked reviewer picked up on this, demanding to know whether “Earth shall be re-peopled with new inhabitants”. In reply, the author facetiously asserted that our fossil remains would “gratify the curiosity of the new inhabitants of the new world, if there be any”. The cycle of eternally returning humanoids was unwinding.

In line with this, the French encyclopaedist Baron d’Holbach ridiculed the “conjecture that other planets, like our own, are inhabited by beings resembling ourselves”. He noted that precisely this dogma – and the related belief that the cosmos is inherently full of moral value – had long obstructed appreciation that the human species could permanently “disappear” from existence. By 1830, the German philosopher F W J Schelling declared it utterly naive to go on presuming “that humanoid beings are found everywhere and are the ultimate end”.

And so, where Galileo had once spurned the idea of a dead world, the German astronomer Wilhelm Olbers proposed in 1802 that the Mars-Jupiter asteroid belt in fact constitutes the ruins of a shattered planet. Troubled by this, Godwin noted that this would mean that the creator had allowed part of “his creation” to become irremediably “unoccupied”. But scientists were soon computing the precise explosive force needed to crack a planet – assigning cold numbers where moral intuitions once prevailed. Olbers calculated a precise timeframe within which to expect such an event befalling Earth. Poets began writing of “bursten worlds”.

The cosmic fragility of life was becoming undeniable. If Earth happened to drift away from the sun, one 1780s Parisian diarist imagined that interstellar coldness would “annihilate the human race, and the earth rambling in the void space, would exhibit a barren, depopulated aspect”. Soon after, the Italian pessimist Giacomo Leopardi envisioned the same scenario. He said that, shorn of the sun’s radiance, humanity would “all die in the dark, frozen like pieces of rock crystal”.

Galileo’s inorganic world was now a chilling possibility. Life, finally, had become cosmically delicate. Ironically, this appreciation came not from scouring the skies above but from probing the ground below. Early geologists, during the later 1700s, realised that Earth has its own history and that organic life has not always been part of it. Biology hasn’t even been a permanent fixture down here on Earth – why should it be one elsewhere? Coupled with growing scientific proof that many species had previously become extinct, this slowly transformed our view of the cosmological position of life as the 19th century dawned.

Seeing death in the stars

And so, where people like Diderot looked up into the cosmos in the 1750s and saw a teeming petri dish of humanoids, writers such as Thomas de Quincey were, by 1854, gazing upon the Orion nebula and reporting that they saw only a gigantic inorganic “skull” and its lightyear-long rictus grin.

The astronomer William Herschel had, already in 1814, realised that looking out into the galaxy one is looking into a “kind of chronometer”. Fermi would spell it out a century after de Quincey, but people were already intuiting the basic notion: looking out into dead space, we may just be looking into our own future.

People were becoming aware that the appearance of intelligent activity on Earth should not be taken for granted. They began to see that it is something distinct – something that stands out against the silent depths of space. Only through realising that what we consider valuable is not the cosmological baseline did we come to grasp that such values are not necessarily part of the natural world. Realising this was also realising that they are entirely our own responsibility. And this, in turn, summoned us to the modern projects of prediction, preemption and strategising. It is how we came to care about our future.

As soon as people first started discussing human extinction, possible preventative measures were suggested. Bostrom now refers to this as “macrostrategy”. However, as early as the 1720s, the French diplomat Benoît de Maillet was suggesting gigantic feats of geoengineering that could be leveraged to buffer against climate collapse. The notion of humanity as a geological force has been around ever since we started thinking about the long-term – it is only recently that scientists have accepted this and given it a name: “Anthropocene”.

Will technology save us?

It wasn’t long before authors began conjuring up highly technologically advanced futures aimed at protecting against existential threat. The eccentric Russian futurologist Vladimir Odoevskii, writing in the 1830s and 1840s, imagined humanity engineering the global climate and installing gigantic machines to “repulse” comets and other threats, for example. Yet Odoevskii was also keenly aware that with self-responsibility comes risk: the risk of abortive failure. Accordingly, he was also the very first author to propose the possibility that humanity might destroy itself with its own technology.

Acknowledgement of this plausibility, however, is not necessarily an invitation to despair. And it remains so. It simply demonstrates appreciation of the fact that, ever since we realised that the universe is not teeming with humans, we have come to appreciate that the fate of humanity lies in our hands. We may yet prove unfit for this task, but – then as now – we cannot rest assured believing that humans, or something like us, will inevitably reappear – here or elsewhere.

Beginning in the late 1700s, appreciation of this has snowballed into our ongoing tendency to be swept up by concern for the deep future. Current initiatives, such as Bostrom’s Future of Humanity Institute, can be seen as emerging from this broad and edifying historical sweep. From ongoing demands for climate justice to dreams of space colonisation, all are continuations and offshoots of a tenacious task that we first began to set for ourselves two centuries ago during the Enlightenment when we first realised that, in an otherwise silent universe, we are responsible for the entire fate of human value.