We see extraterrestrials in movies all the time. They may look odd, but they are usually around the size and shape of a human, and most of the time they speak English. Science can't tell us (yet) what real aliens might look like, so how would we even know if something we find on an exoplanet someday might be living, or even evidence that it once lived?
One thing that sets life apart from nonlife is its apparent design. Living things, from the simplest bacteria to the great redwoods, have vast numbers of intricate parts working together to make the organism function. Think of your hands, heart, spleen, mitochondria, cilia, neurons, toenails – all collaborating in synchrony to help you navigate, eat, think and survive. The most beautiful natural rock formations lack even a tiny fraction of the myriad parts of a single bacterial cell that coordinate to help it divide and reproduce.
This design comes from natural selection, which would not necessarily depend on DNA as it does on Earth. But life must adapt to changing conditions one way or another, or it won't last long enough to develop a design. Read the requirements of life and how to recognize them at Aeon.
(Image credit: Helen Cooper/Cambridge University)