Close up of giant radio telescope antenna array of CSIRO - part of NASA space communication network in Tidbinbilla of ACT, Canberra, Australia.

Natural selection and survival of the fittest varieties of life is a reality on a sometimes-unforgiving planet Earth. The same probably holds true for alien organisms, according to a new paper by University of Oxford scientists.

Darwin’s theories are quite literally universal, they write in the International Journal of Astrobiology. And though they don’t have a detailed predictive picture, life forms from other undiscovered planets are likely to resemble humans in some key respects, even if they don’t breathe oxygen and are not carbon-based organisms, they add.

“We still can’t say whether aliens will walk on two legs or have big green eyes,” said Samuel R. Levin, one of the authors, of Oxford’s Department of Zoology. “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 strong predictions we can make with it.”

Most of the conventional predictions about what aliens would be like are based on what happened on Earth. For instance, eye-like organs made evolutionary leaps dozens of times over the course of natural history. Carbon’s abundance in the universe means it was an ideal foundation for the rise of life on our planet, as well.

But this is all guesswork, says the new study.

“There is not theoretical reason why aliens could not be silicon-based and eyeless,” they write.

The new theory instead incorporates a more conceptual basis of the life forms that may exist out there.

Any lifeform that has variety is bound to give some of its members an advantage, they write. That advantage will mean those individuals are likely to survive longer and better, and pass on their diversity.

Complexity, too, is a natural outcome, they add. On Earth, major evolutionary transitions resulted in increasingly complex and specialized groupings that were refined over epochs into species that either died out, or thrived and eventually died out, or persisted, they write.

The living cells are parts working together, in concert, and in the interest of genetic relatedness – such as the wings and head of a bird complementing each others’ existence. Likewise, heart cells and brain cells play a role for the overall goal of survival of the species – and don’t try to compete with sperm and egg cells to reproduce.

“Consequently, complex aliens will be composed of a nested hierarchy of entities, with the conditions required to eliminate conflict at each of those levels,” they write. 

The new theory of overall evolutionary dynamics doesn’t provide a specific picture of an alien – but it could complement the mechanistic predictions based on how life arose on our Earth, they add.

“Mechanistic understanding is a good way to extrapolate from what we see on Earth. The (new) theory is a good way to make predictions that are independent of the details of the Earth,” they write. “Combining both approaches is the best way to make predictions about the many hundreds, thousands or millions of hypothetical aliens.

“Now we just need to find them,” they add.