For as long as we can remember, humans have wondered if we are alone in the universe.In fact, in September 2013, a team of British scientists claimed a cell fragment found by a balloon flight in the upper atmosphere may be <a href="http://www.space.com/22875-alien-life-claim-space-microbes.html">proof of life from space</a>. While individuals and conspiracy theorists often come forward with new "proof" of alien visitations, even scientists have claimed to find evidence for extraterrestrial life. Here are our top five scientific claims for aliens.
<p>FIRST STOP: Microbes in Meteorites
NASA scientist Richard Hoover published a paper March 4, 2011, claiming to have found <a href="http://www.space.com/11049-alien-life-meteorites-skepticism.html">fossil evidence for cyanobacteria</a> in carbonaceous meteorites from outer space. Hoover observed slices of meteorites through scanning electron microscopes, and identified filaments and structures that he said resemble the tiny single-celled algae.
Reaction from some scientists was skeptical, in part because the study was published in the questionable Journal of Cosmology. Other researchers said the study was conducted thoroughly, but it was too soon to say for sure whether the claim would hold up.
<p>NEXT: Viking Lander Results
In 1976 NASA's two Viking landers touched down on the surface of Mars. The probes conducted a host of biological experiments, including collecting samples of Martian soil to test for organic compounds – the building blocks of life – and biosignatures that could indicate the presence of microorganisms.
The <a href="http://www.space.com/3038-martian-life-evaded-detection-viking-landers.html">landers found little</a> evidence for organics, but the onboard Labeled Release experiment found a reactive agent in the surface material of Mars that produced increased carbon dioxide. Gilbert Levin, an engineer who designed Labeled Release, concluded that this activity was triggered by living microorganisms lurking in the Martian soil. However, that interpretation has not been widely accepted by the scientific community.
More recent research has also called into question Viking's negative results in searching for organic compounds. A study published in December 2010 in the Journal of Geophysical Research suggested that these <a href="http://www.space.com/10418-life-building-blocks-mars-research-finds.html">compounds were present on Mars</a>, but they were just destroyed by other chemicals before Viking could detect them.
<p>NEXT: Arthur C. Clarke's Bushes on Mars
Science fiction author <a href="http://www.space.com/5140-arthur-clarke-luminaries-pay-tribute.html">Sir Arthur C. Clarke</a>, famous for penning the novel "<a href="http://www.space.com/7755-arthur-clarkes-2010-reality.html">2001: A Space Odyssey</a>," made headlines in the year 2001 when he claimed that recently returned photos from NASA's Mars Global Surveyor showed evidence of trees and bushes on Mars.
Most scientists scoffed at the claim, but the writer stood by his belief.
"I'm quite serious when I say have a really good look at these new Mars images," Clarke said at the time. "Something is actually moving and changing with the seasons that suggests, at least, vegetation."
Clarke died in 2008 in Sri Lanka.
<p>NEXT: Mars Canals
The idea that Mars was traversed by a complex <a href="http://www.space.com/1583-mars-history-false-impressions.html">network of canals</a> was first put forward in 1877 by Italian astronomer Giovanni Schiaparelli, and later popularized by astronomer Percival Lowell. Lowell made intricate drawings of what he took to be canals based on observations he made at his observatory in Flagstaff, Ariz.
The idea gained relatively wide acceptance until the early 20th century, when astronomical observations improved and higher-resolution imaging revealed the "canals" to be optical illusions and geologic formations.
<p>NEXT: Allan Hills Meteorite
When scientists announced in 1996 that they'd discovered evidence of fossilized microbial life in a meteorite from Mars, it was a huge event. President Bill Clinton make a national address about the discovery, which seemed to herald the news that we are not alone.
Subsequent analysis of the research, on a <a href="http://www.space.com/8229-oldest-mars-meteorite-younger-thought.html">meteorite called Allan Hills 84001</a> (ALH 84001), produced controversy, with many experts asserting that the fossils could have been created by non-living processes.
The evidence is still debated, and the space rock remains a topic of ongoing research.
<ul><li><a href="http://www.space.com/17135-life-on-mars.html">The Search for Life on Mars</a></li>
<li><a href="http://www.space.com/16287-poll-alien-life-extraterrestrials.html">Poll: Do You Believe in Aliens?</a></li>
<li><a href="http://cms.space.com/cms/countdowns/15716-alien-life-search-solar-system">6 Most Likely Places for Alien Life in the Solar System</a></li></ul>