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?”