In 1960 the American astronomer Frank Drake initiated Project Ozma, a pioneering attempt to detect radio signals from the vicinity of nearby stars. Several months of observation failed to detect any aliens and doomed the project. Drake’s curiosity, however, was undimmed, so, in the absence of telescope time, he decided to estimate the number of civilizations likely to reside in the Milky Way. The resulting Drake Equation has been a lodestone for space exploration ever since, not because Drake could quantify the odds of encountering intelligent aliens – it remains impossible to put numbers on any of the equation’s variables – but because it provides a thoughtful listing of what we need to know to interpret the question: are we alone in the universe?

Is our planet a cosmic singularity, unique among the billions of known galaxies, each with billions of stars? Or is the universe pulsing with life, some of it bacterial, some of it intelligent, perhaps benevolent, perhaps menacing? The issue has been pondered at least since Galileo recognized stars that wander across the night sky as planets, much like the Earth and perhaps equally endowed with life. Twenty-first-century astrobiologists, armed with spacecraft and telescopes, commonly focus on “how” we might detect extraterrestrial life, but other creative minds ask “what if?”. What might alien life be like, and what would happen should we meet it? Over the years writers and film-makers have imagined a universe of possibilities, from the brutal invaders of The War of the Worlds to ET. In The Possibility of Life Jaime Green skilfully interweaves fact and fiction, interviews and close reading, as she explores who or what might populate the heavens.

Earth is the only place in the universe known to harbour life, so our own story strongly informs human imaginings about extraterrestrial beings. With this in mind Green structures her narrative around key events in the journey from primordial planet to technological mankind, beginning with life’s origin and proceeding through the emergence of complex multicellular organisms to humans and technology – all key parameters in the Drake Equation. Life as we know it began on a rocky planet with lots of water and no oxygen. Simple molecules containing carbon and a few other elements came together and, with energy supplied by heat, lightning, solar radiation or radioactive decay, reacted to form amino acids, lipids and nucleotides – the building blocks of life. Chemical reactions of increasing complexity ensued, eventually leading to entities that could grow, reproduce and evolve. Is any part of this scenario likely to be universal? Rocky planets or moons with liquid water are a good start. The chemistry thought to underpin life’s origins doesn’t work in the presence of oxygen gas, so an anoxic atmosphere and oceans are also likely. Moreover, as Green points out, carbon has properties that make it particularly bio-friendly, so life elsewhere will probably be carbon-based, as it is on Earth. And evolution seems necessary to put developmental distance between the physical and biological worlds.

From here, however, the list of possibilities expands exponentially. Life is hard to imagine without a capacity for storing and using information, interacting with the environment and heredity; but will alien life include DNA and proteins, chromosomes and membranes? We don’t know, and the potential evolutionary pathways continue to spiral outward as we contemplate complex organisms such as animals, with trillions of cells all working in remarkable co-ordination. Will multicellular aliens resemble us, or Earth-bound animals in general? Green cites the late Stephen Jay Gould, who argued that, if we could play Earth’s evolutionary tape again, the results would look different, with technological intelligence a vanishingly small probability.

Extinction prunes the tree of life, and mass extinctions prune it in ways that have little to do with adaptations over preceding epochs. Yet there is a clear vector to life’s history, and it runs straight through extinction episodes. Moreover, similar forms have evolved repeatedly, suggesting a measure of predictability in evolution. But, while the convergent shapes of fish, whales and plesiosaurs reflect common adaptations for movement in water, they are also constrained by descent from a common ancestor. Half a century ago the French biologist Jacques Monod wrote that evolution is like a tinkerer, fashioning novelty from materials on hand. For the descendants of any given ancestor, there may be only so many viable evolutionary pathways, limiting the universal application of our specific evolutionary history.

For most of our planet’s history, visiting aliens would have found only microbes in the biosphere. Animals emerged in just the most recent 15 per cent of life’s story, and only in the latest blink of a cosmic eye would such visitors have encountered anything capable of agriculture, automobiles or computers. Humans are evolutionary newcomers, characterized by distinctive genes, morphology and physiology, but also consciousness, philosophy and morality – “reason and reflection”, in the words of John Locke. A key aspect of modern humanity is technology, which has developed at an astonishing pace relative to the biological evolution that gave rise to us in the first place. Given this technological trajectory, what might terrestrial life look like 50,000 years from now? Green explores this question carefully and insightfully. Will humanity eventually give way to intelligent machines, with potential parallels in other solar systems? Both writers and scientists have entertained the idea, however unlikely it may seem. The astronomer Steven J. Dick has even proposed an order of succession: on long timescales, he argues, biology overtakes physics as the primary force shaping the cosmos, before invention overtakes biology.

The possibilities of life turn out to be nearly limitless, although our ability to imagine them is tethered to terrestrial experience – and, in practice, the search for life beyond the Earth will need to focus on what life does, not what it is. Astrobiologists will scour the heavens for observable patterns not likely to be generated by physical processes alone: molecules of chlorophyll-like complexity in Martian sediments, co-existing oxygen and methane in the atmospheres of nearby exoplanets, or radio waves and Dyson spheres (imagined constructions capable of capturing a galaxy’s energy) in deep space. Maybe, too, we won’t find them – they’ll find us.

Contact has long motivated science fiction. Humans have community, and we attempt to extend that community to other terrestrial intelligences such as dolphins and octopuses. There are already difficulties with that extension, and still more when we contemplate community with intelligences from space. Problems of communication run both ways. If aliens intercept the Pioneer spacecraft, with their inscribed aluminium plaques, will they grasp the nature of its senders or simply scratch their heads (if they have them)? Will aliens want to help us, or not? Or, as Mary Doria Russell imagines in The Sparrow (1996), a novel inspired by the history of European exploration, might it be human explorers who bring catastrophe to the worlds they discover?

An observation halfway through Jaime Green’s book neatly sums up the attempt to conjure a sense of alien life: “Even if we can’t imagine truly strange, truly different life, we push against the inherent xenophobia of our imaginations when we try, while what we know pulls us back like gravity”. The Possibility of Life is powered by just this combination of curiosity and resistance. It is an entertaining and instructive rumination on both earthbound existence and the prospect of extraterrestrial encounter.

Andrew H. Knoll is the Fisher Professor of Natural History at Harvard University. His most recent book is A Brief History of Earth: Four billion years in eight chapters, 2021