What Do Aliens Look Like? Here's How Earth's Extreme Terrestrials Offer Astrobiology Clues

When astrobiologists are asked what aliens might look like, many now point not to distant galaxies but to some of the strangest corners of Earth, where researchers on several continents are using extreme microbes, toxic lakes and even sperm whales as templates for how non-human life, and perhaps alien intelligence, could appear and communicate.

The question 'What do aliens look like?' has long been shaped by science fiction, from the tentacled Martians of H. G. Wells' The War of the Worlds to the humanoid Superman and the hand-like heptapods of Arrival. But a growing group of scientists told National Geographic that the most realistic guide we have is the only biosphere we can actually touch. By pushing Earth life to its limits in laboratories and in the field, they are trying to sketch what might survive on Mars, in the oceans of distant moons or in the skies of exoplanets, and how we might one day communicate with something that evolved under very different rules.

What Aliens Might Look Like

André Antunes, a microbiologist and dean at the Institute of Science and Environment at the University of Saint Joseph in Macao, is blunt about the cultural baggage around aliens. Popular imagination, he says, is still clogged with 'little green men' and near-humans in rubber foreheads, and that bias towards humanoid forms is exactly what he thinks must be shaken off.

Antunes and his colleagues instead study extremophiles, microbes that survive in places that seem hostile to life, including hydrothermal vents, sub-glacial lakes, the stratosphere and hyper-salty basins such as the Dead Sea and the Great Salt Lake.

His view is simple: simple life is probably far more common than complex intelligence, so the most honest answer to what aliens might look like is microbes.

To make that idea more testable, researchers such as Alexandre Rosado, a bioscience professor at King Abdullah University of Science and Technology, recreate alien-like environments in the lab. He studies extremophiles that tolerate conditions similar to Mars or the icy ocean worlds thought to lie beneath the crust of moons such as Europa and Enceladus.

Rosado says those simulations show how hardy microbes might behave off world. Under severe stress, they tend to slow their metabolism or go dormant, which changes what scientists should be looking for. Instead of colourful alien rainforests, early life elsewhere might show up as faint biosignatures, strange gases, unusual minerals or traces of organic material.

Studying Earth's toughest microbes, he argues, helps move astrobiology from speculation towards a science grounded in real, testable expectations of what life beyond Earth might actually look like.

Antunes points to halophiles, which thrive in extreme salt and can survive for tens of thousands of years inside microscopic water pockets in salt crystals. Given that fossilised microbes have been found in Australian salt deposits nearly a billion years old, he says dormant or extinct life could plausibly be preserved in Martian salt or buried oceans on outer solar system moons.

Lisa Kaltenegger, founding director of the Carl Sagan Institute at Cornell University, takes the same question into deep space. Her team studies Earth's harshest habitats and how they interact with light, building an online library of roughly 250 kinds of life with different colours.

Her warning is practical. Future telescopes will read exoplanets in slivers of colour, so assuming life must look Earth-green could make scientists miss it entirely. 'I don't want us to miss signs of life looking for the wrong thing,' she said. 'Don't just look for green'.

How Alien Minds Might Speak

For many people, the more intriguing question is not what aliens look like, but how they might behave and communicate. A growing group of zoologists and linguists is trying to answer that by studying non-human communication on Earth.

Arik Kershenbaum, a zoologist at the University of Cambridge, works in xenolinguistics, the study of hypothetical extraterrestrial languages. In his 2020 book The Zoologist's Guide to the Galaxy: What Animals on Earth Reveal About Aliens and Ourselves, he argues that natural selection should create similar pressures across the cosmos. Any intelligent species will need to find food, avoid danger and cooperate, and those demands may produce complex communication and social structures again and again.

'I think that many of the animal behaviours we see on Earth are fundamentally universal, such as living in groups,' he said. But he notes that sociality long predates humans here, which raises the possibility that alien 'speech' might sound more like the whistles of dolphins or the mimicry of parrots than anything recognisably human.

To broaden that thinking, researchers have looked to animals as far from us on the family tree as possible while still having large, intelligent brains. Sperm whales, which last shared a common ancestor with humans roughly 90 million years ago, are one such test case.

David Gruber, a National Geographic Explorer and founder of Project CETI, the Cetacean Translation Initiative, is leading an interdisciplinary effort to decode sperm whale communication. The name deliberately echoes SETI, the search for extraterrestrial intelligence. Gruber said his team is developing tools that could be applied to any non-human system, including an alien one, if intelligent life is ever found elsewhere.

Sperm whales live in tightly knit matriarchal societies and exchange complex sequences of clicks known as codas to coordinate behaviour and pass knowledge down generations. Project CETI has already found that these vocalisations contain elements comparable to human vowels and that whales use them to support one another during births, a midwife-like role never previously documented outside primates.

For Gruber, sperm whales are not a literal rehearsal for aliens. They are a practical test of how to move from raw audio data to something like translation. He notes that first contact is often imagined as a single cinematic moment, but in reality it would involve years of pattern-finding, hypothesis and error.

Kershenbaum, meanwhile, suspects that if we ever receive a message, it will come from a species at least as curious and systematic as we are. He points to Carl Sagan's novel Contact, later turned into a film, where an alien signal is designed so scientists can recognise the logic in it. Whoever sent such a message, he suggests, would be counting on minds that value patterns, mathematics and evidence.

What aliens look like may matter less than what they care about. But if Earth's strangest microbes and most eloquent whales are any guide, humanity should prepare for something both familiar and utterly alien.