Nearly every depiction of extraterrestrial life are of green figures with large heads, and bug eyes, that look, well, alien. But new research suggests that if advanced beings from far away worlds exist, they may look a lot like us.

Scientists at the University of Oxford in the United Kingdom, published a paper asserting that evolutionary theory can help describe aliens and their behavior in the International Journal of Astrobiology. They said aliens are potentially impacted by natural selection, just like us, making them evolve to be stronger and more adaptable.

Sam Levin, a researcher in Oxford's Department of Zoology, said in a statement that predicting exactly what aliens look or act like is difficult, as we simply haven’t seen them. “We only have one example of life—life on Earth—to extrapolate from,” he said.

Past astrobiologists have used chemistry, geology and physics that explain our planet to try and predict what life might be like on other planets, Levin explains. His team’s approach, however, is different. Evolutionary theory can be used to make predictions that aren’t biased by our own facts about Earth.

Within evolutionary theory is the idea that organisms grow into a higher status through a series of major transitions. "Major transitions are important because the new higher-level organisms that they produce can lead to a great jump in complexity," the authors wrote in the paper.

For example, single cells evolve into multicellular organisms. We would not be as we are now, with eyes, kidneys and brains if these major transitions didn't occur. Evolutionary theory and data suggest that extreme conditions occur before these transitions take place, according to Levin. The same is likely true for aliens, too.

A photo showing liquid water on planet Mars. Extraterrestrial life has not been found on other planets, but scientists speculate they do exist. NASA/JPL-Caltech/Univ. of Arizona via Getty Images

"We still can't say whether aliens will walk on two legs or have big green eyes,” said Levin. But, using evolutionary theory, they were able to assume that their forms adapted to be similar to us.

“Like humans, we predict that they are made up of a hierarchy of entities, which all cooperate to produce an alien,” he said. “At each level of the organism there will be mechanisms in place to eliminate conflict, maintain cooperation and keep the organism functioning.”

Of course, this is all theoretical, and there are hundreds of thousands of planets in our galaxy, so there’s no saying whether they exist—or where they might live.

But one idea emerged last week from scientist Alan Stern, who worked on NASA’s New Horizons mission. At a meeting of the American Astronomical Society, Stern said that intelligent life could have built civilizations in subsurface oceans. He believes these buried oceans would provide a better environment to help life evolve more complexly.