Is there any evidence of alien life?
Short answer: Not a lot.
Long answer: In terms of useful scientific evidence, we don't have much to indicate that aliens exist. There is ample evidence that conditions suitable for life are common throughout the Universe, so aliens could exist, but there's not enough direct evidence to convince us they they do exist.
Here are some of the most interesting examples of scientific investigation...
The First Alien Investigation
Percival Lowell's incorrect drawing of Mars canals
The first known serious attempt to identify alien life came from astronomer Percival Lowell in the late 19th Century. Using a telescope, he claimed to see lines on Mars that he believed were canals made by a Martian civilization. Largely due to this hypothesis, it became common for people to think that Mars was populated. Eventually, though, Lowell’s canals were shown to be imaginary. In the 1960s the first close-up photos of Mars showed no signs of alien civilizations. So the first attempt to prove the existence of alien life was a failure.
The Viking Results
In the 1970s, two Viking spacecraft landed on Mars carrying equipment designed to detect signs of tiny microbial life. Unfortunately the results were ambiguous, meaning that scientists weren't able to say for sure what the results meant.
Some scientists insist the Viking results show solid evidence of microbial life but most scientists are unconvinced. NASA’s official position is that it’s not worth repeating these experiments—a decision that has been heavily criticized by supporters of the results. It is possible that privately-funded missions may try again in the future.
The Wow Signal
In 1975, a telescope at Ohio State University picked up a radio signal that seemed to be artificial and coming from the constellation Sagittarius. The duty scientist saw the data and wrote “Wow!” next to the signal. It became known as the “Wow! Signal”. Although it looks very much like an alien transmission it doesn’t actually contain any data that could be a message. Astronomers have spent years searching for a similar signal but have never heard it repeated. It remains a mystery.
The Mars Meteorite (ALH84001)
Close-up of meteorite ALH84001
In 1984 a meteorite was found in Antarctica that is believed to have come from Mars. It would have been blown into space by a large impact and eventually landed on Earth. It contains microscopic structures that look like fossilized nanobacteria. This was touted as evidence of life on Mars.
However, biologists have pointed out that this structure can also be made by non-biological processes. In 2016 NASA astrobiologist Professor Malcolm Walter visited Te Awamutu Space Centre and was questioned about this — his response was that ALH84001 is unlikely to be biological.
Conditions for Life
Although not direct evidence of life, we have found that the Universe is full of the basic chemical “building blocks” of life; for example, organic compounds including amino acids. We also find water everywhere and plenty of environments that could support life. Even in our own Solar System we have underwater oceans and other possible life abodes. We believe there are at least 100 billion planets in our galaxy and at least 100 billion similar galaxies in the Universe. Assuming that the building blocks of life are spread throughout these galaxies, it seems overwhelmingly likely that the Universe has plenty of alien life.