In the latest breakthrough, scientists have revealed that aliens are potentially shaped by the same processes and mechanisms that shaped humans, such as natural selection.
The study published in the International Journal of Astrobiology shows for the first time how evolutionary theory can be used to support alien predictions and better understand their behaviour.
The theory supports the argument that foreign life forms undergo natural selection, and are like us, evolving to be fitter and stronger over time.
"A fundamental task for astrobiologists (those who study life in the cosmos) is thinking about what extra-terrestrial life might be like," said Sam Levin, a researcher in Oxford's Department of Zoology.
"But making predictions about aliens is hard. We only have one example of life - life on Earth to extrapolate from.
"Past approaches in the field of astrobiology have been largely mechanistic, taking what we see on Earth, and what we know about chemistry, geology, and physics to make predictions about aliens," said Levin.
In the new study, researchers offer an alternative approach, which is to use evolutionary theory to make predictions that are independent of Earth's details.
"This is a useful approach because theoretical predictions will apply to aliens that are silicon-based, do not have DNA, and breathe nitrogen, for example," Levin said.
Using this idea of alien natural selection as a framework, the team addressed extra-terrestrial evolution, and how complexity will arise in space.
Species complexity has increased on the Earth as a result of a handful of events, known as major transitions.
These transitions occur when a group of separate organisms evolves into a higher-level organism - when cells become multi-cellular organisms, for example.
Both theory and empirical data suggest that extreme conditions are required for major transitions to occur.
The study also makes specific predictions about the biological make-up of complex aliens and offers a degree of insight as to what they might look like.
"We still cannot say whether aliens will walk on two legs or have big green eyes. But we believe evolutionary theory offers a unique additional tool for trying to understand what aliens will be like, and we have shown some examples of the kinds of strong predictions we can make with it," Levin said.