When an astrobiologist says “life is hard,” they may be speaking on a cosmic level. They could mean that life itself is a puzzle we are nowhere near solving. 

“We’re like second graders when it comes to understanding life,” says Betül Kaçar, PhD, professor of bacteriology at the University of Wisconsin-Madison. “We don’t know much.” 

It might sound strange in an era of genetic engineering and antibiotics, but to those exploring the possibility of life beyond Earth, our ideas about biology appear far from complete. We don’t have a full picture of what life can be made of, where it can thrive, or what it might look like throughout the Universe. To search for alien life, scientists make do with what we know. That is almost entirely based on life on Earth.

Yet there are hints that creatures beyond our planet, if they exist, could be very different. By exploring these alternatives in the lab — and even resurrecting extinct forms of life — researchers are now challenging some of our most basic assumptions about the search for life. Their work may not only change how we think about aliens but what we know about ourselves.

Living atoms

As the only planet proven to be habitable, Earth has a special place in the search for life. It stands as an ideal, one we compare worlds to in hopes of finding somewhere as lush. 

If scientists only cared about finding Earth’s twin, though, they might miss out on unfamiliar forms of life. The most common kind of creatures in the Universe might thrive in environments we consider toxic. They could be based on a completely different chemistry, and they might look nothing like us. 

“It’s very possible — unfamiliar planets and their unfamiliar molecules creating life,” says Kaçar. 

Kaçar’s work is dedicated to exploring these alternatives. To her, life as we know it is a starting point: something to learn from but also to push past. 

“If we are to discover life as we do not know it, we must challenge our deepest assumptions. We need to consider the very atoms that make life possible,” Kaçar adds. 

On Earth, these atoms mostly come down to just six chemical elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur — or CHNOPS for short. Add in trace amounts of other elements, like potassium and iron, and you can make all the chemicals that are crucial to our lives. That includes molecules like DNA and RNA for storing information, proteins to jump-start chemical reactions, carbohydrates for energy, and membranes to keep things separate and organized. All are brought together by water, which allows other chemicals to mix and interact. 

What we don’t know is how inevitable any of this is. Different chemistries might be able to create life in different environments. Even on Earth, we’re not sure whether it is the only way life could have ended up.

Main ingredients

Scientists have thought of many possible alternatives to the way life works on our planet. Each has its own basic building block like a main ingredient in a recipe. 

On Earth, this is carbon. Carbon underpins the chemistry of all known organisms, and it is particularly good at building long, complex structures like proteins and DNA. Like any good main ingredient, carbon is also readily available. It is the fourth-most-abundant element in the entire Universe. 

But experiments suggest other building blocks might work too. Sulfur is relatively common on other worlds, and sulfur-based chemistry can provide ways for life to gain energy and store information. Today, clusters of iron and sulfur are a widespread and important part of Earth biology. This might mean they were once essential to the origin of life. 

Boron-based lifeforms could be another possibility. Like sulfur, boron is used by life today and may have played a role in the beginning of life as we know it. But unlike sulfur, boron tends to lock itself together with oxygen in ways that aren’t useful for building life. Boron is also relatively rare in the Cosmos, though it might still work as a building block on planets without much oxygen. 

Then there’s silicon, the most sci-fi-famous alternative to carbon-based life. Unfortunately, research shows that silicon is unlikely to form life even in very alien environments. Almost anywhere silicon-based life has a chance of blooming, carbon-based life would stand a better chance because its life-fostering reactions tend to cost less energy. At best, silicon might work alongside carbon as a sort of joint building block.

Primordial soup stock

Whatever life’s ingredients are, they need a way to mix and react in order to thrive. This is the other defining factor for any chemistry of life: its medium. Scientists almost always imagine a liquid playing this role. Because water is excellent at dissolving things and encouraging them to interact, it is by far the astrobiologist’s favorite. 

Yet some researchers think life on our own planet didn’t begin in water. Instead, a different liquid called formamide may have acted as both a crucial ingredient and a medium for life’s early chemistry. Though this is far from proven, formamide’s potential to have supported life on Earth makes it a good candidate for sustaining similar organisms on other worlds. 

Liquid methane, which fills entire lakes on the surface of Saturn’s moon Titan, offers a more alien alternative. Hydrocarbons like methane don’t dissolve the same substances that water does, so lakes of them could foster lifeforms that are very unlike those found on Earth. Instead of using DNA, creatures on Titan might store information using the number of electrons attached to molecules or by patterning pairs of atoms into binary code. They could have cell membranes made of chemicals that would be carcinogens and industrial solvents on Earth. 

Other alternatives to water include sulfuric acid, which clouds up the skies of Venus, and ammonia, which might puddle in the icy crusts of worlds like Saturn’s moon Enceladus. 

Both chemicals could only support exotic kinds of life. Ammonia rips apart some molecules crucial for life as we know it, and sulfuric acid is even more destructive. But a mix of ammonia and water might be the most habitable place for creatures on worlds like Enceladus. On Venus, life built jointly on carbon and silicon could potentially thrive in sulfuric acid.

Old recipes

While scientists often test these chemistries by whipping them up in the lab, Kaçar explores what alien life might look like a little differently. Rather than just trying out ingredients, she focuses on the recipe. 

For life, that means genomes. By looking at the genes of organisms today and reconstructing what their ancestors were like in the past, researchers can see how life’s chemistry changed over time. They can pick out which parts of the recipe have been flexible and which parts have always been essential. 

This process, called ancestral sequence reconstruction (ASR), uses computer models to predict the genomes of extinct species from their living descendants. Researchers can then study those genes to learn what long-dead organisms were once like. Scientists have learned when mammals evolved to be nocturnal, for instance, by tracing genes related to how sensitive we are to light.

The worlds of the past

Kaçar’s team is among a handful who take this one step further. Instead of just predicting the genes of the past, they bring them back to life. They make real, living organisms express parts of their ancestors’ genetic code. 

So far, Kaçar’s group has mainly used ASR to track how microbes “ate” carbon and nitrogen billions of years ago. Back then, our planet was very different, with almost no oxygen in its atmosphere and fewer ingredients for life available. ASR allows Kaçar to access this version of Earth as a kind of alien world in itself — one that we know hosts life. 

“We are able to imagine different atmospheres, different temperatures, different oceans,” Kaçar says. 

Kaçar’s research has found instances where life’s core chemistry seemed to change in response to its environment, but she has also been surprised to discover how much about life seems to be set early on. Kaçar says it’s like life was “put on railroad tracks” after a certain point. That would make understanding Earth’s ancient conditions valuable in a whole new way.

Expanding the search

All of these exotic possibilities, including those contained within our past, are changing how we search for life beyond Earth. 

Already, treating the ancient Earth like another planet has taught us about other worlds that might be out there. In a recent study, Kaçar and her collaborators reconstructed aspects of the chemistry that ancient marine life once used to perform photosynthesis, then predicted how their signs of life would have looked from space. With this information, astronomers could decide not only if a planet looks like Earth today but if it looks like the kind of world that Earth was billions of years ago. 

All of this is building to a more complete universal definition of habitability. By exploring different ingredients and recipes, scientists like Kaçar are slowly defining the edges of a broader search for life. They are preparing us to interpret new signs of life, whether from a planet just like our own or from strange, undiscovered worlds.

And if we don’t find aliens? Life on Earth would be all the more special, and understanding how life formed here would be even more important. 

“Ultimately, it will tell us more about ourselves,” says Kaçar. “What better way to honor our own ancestors?”