If conditions on a distant planet allowed life to flourish, would it look anything like life here on Earth?
It's a question that's seen a Darwinian rise of contradictory theories over the years.
Now, in an interview with the BBC's Science Focus magazine, Simon Conway Morris, an evolutionary palaeobiologist at the University of Cambridge, says "with reasonable confidence" that human-like evolution has occurred in other parts of the universe.
Applying Darwin's theory to the entire universe
The idea is part of a wider school of thought called "convergent evolution." It states that the random mutations driving evolution average out in any given environment, meaning that similar organisms are likely to evolve independently of each other.
This has, in fact, been observed on Earth, where birds, bats, insects, and pterosaurs all evolved to fly independently. Eyes may also have evolved independently as many as 40 times on Earth.
"Convergence is one of the best arguments for Darwinian adaptation, but its sheer ubiquity has not been appreciated," explains Prof. Morris. "One can say with reasonable confidence that the likelihood of something analogous to a human evolving is really pretty high.
And given the number of potential planets that we now have good reason to think exist, even if the dice only come up the right way every 1 in 100 throws, that still leads to a very large number of intelligences scattered around, that are likely to be similar to us."
Polar bear-sized humans?
Not all scientists are in agreement, though. The late evolutionary biologist Stephen J. Gould, for example, argued that if you could reset evolution on Earth, the probability of humans coming into existence once again would be almost negligible.
Another scientist, cosmologist Fergus Simpson, who was at the University of Barcelona, highlights the role of planet size in determining the nature of any potential intelligent life we could discover.
Using Bayesian statistics, Simpson posited that aliens are most likely to be as large as a polar bear due to the estimated average size of planets throughout the universe. As the Earth has a larger than average size, we are likely to be smaller than any aliens we ever encounter — the lower a planet's gravity, the larger a lifeform is likely to grow.
Whether its inhabitants are polar bear-sized or not, it's certainly interesting to imagine a human-like civilization millions of light-years away from Earth.
Could such a civilization also be wondering about life elsewhere in the universe? With NASA having recently launched its ambitious James Webb Telescope, which will help to search for signs of alien life, we may soon have a better idea of what's out there.
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