Throughout history alien life has been claimed to be on Earth, yet most of those claims have had no scientific basis.
Explorers have asserted that they made discoveries of alien life in the darkest corners of the world, but were able to provide no proof. Others throughout the centuries have claimed contact with beings or entities not of this Earth, however, those claims too fell far short of any scientific validity.
Today, the Internet is rife with "proof" that Earth is being visited—seemingly on a daily basis—by advanced, weird, or outright bizarre beings intent on studying us, breeding with us or simply playing with our minds.
All those claims, unfortunately fall flat when subjected to the scientific method.
Despite the failure to prove alien life does exist on Earth—or even sister planets such as Mars—some scientists have stuck their proverbial necks out and offered evidence that has some scientific backing that alien life not only exists on Earth, but may well have been on this planet for many thousands of years.
As far back as 1743 Benoît de Maillet, a French nobleman and natural historian mused that all life on Earth may have been seeded by germs from space falling into the world's waterways. This was the first known break with the more widely held theory of abiogenesis,
The idea was debated, but found difficult to prove or disprove.
By the 19th Century the cause for panspermia was championed by a triad of brilliant scientists: Jöns Jacob Berzelius (1779–1848), Lord Kelvin (William Thomson) (1824–1907) and Hermann von Helmholtz (1821–1894). Intense debate raged throughout Europe on the validity—or lack therof—of the daring hypothesis that panspermia was the root cause of all life on Earth.
"[W]e must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space. If at the present instance no life existed upon this Earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with vegetation," Lord Kelvin asserted during a written response to critics during 1871.
A theory that entails alien microbes seeding Earth, Panspermia was first hypothesized by the Swede Svante Arrhenius a Nobel laureate born in 1859.
Panspermia assumes that life can develop on other planets and alien microbes reach our planet via a variety of means including as meteors from Mars or other life-bearing planets, comets, or even interstellar dust.
Then, during the 1970s a new canine disease—canine parovovirus—suddenly erupted around the world. Although accounts now claim it took two years to travel about the globe, veterinary doctors at the time were stunned because a totally new virus suddenly appeared worldwide within a matter of days.
Some microbiologists and exo-biologists at the time claimed the outbreak was strong proof of panspermia.
2. Vikings on Mars
In recent years the original Viking lander findings have been revisited. During 1976 the two Mars landers tested the Martian soil for life utilizing three tests. Two of the tests came back positive, the third result was ambiguous. Because of that ambiguity NASA refused to officially go on record to proclaim that life had been discovered on the red planet. That position has remained the same until today.
Engineer Gilbert Levin has argued strenuously during the decades following the Viking test results that the two positive readings were obtained from bacteria living in the soil.
Levin may be vindicated in the end, however, as a 2010 study published in the Journal of Geophysical Research lends credence to the engineer's argument for life.
3. The Allan Hills 84001 meteorite
Meteorite hunters discovered a space rock on the Antarctic ice and the find sat on a shelf for some years before anyone really took a close look at it. When scientists finally analyzed it they got the shock of the century—and the news was big enough to motivate then President Clinton to announce to a surprised world that alien life had been discovered on Earth.
The story generated headlines around the globe.
Although the Martian meteor Allan Hills 84001 seems to have the remains of fossilized life embedded within it, some exo-bilogists argue that the "fossils" might have been formed by an exotic geological process. Or abiotically through chemical means.
4. The Hoover meteorites
More meteorites were analyzed by scientist Richard Hoover of NASA. He contends, in a 2011 non-peer reviewed publication, the Journal of Cosmology, that cyanobacteria fossils are present in several specimens of carbonaceous meteorites. He discovered the evidence for life after subjecting thin slivers of the rocks to electron microscope scans.
At the time of this writing the scientific community is investigating Hoover's claim; the final verdict is still forthcoming.
5. Airborne aliens
Not all alien life necessarily makes it to Earth's surface. Several researchers have claimed discovering alien life in the atmosphere.
An Indian and British team of researchers led by Chandra Wickramasinghe reported on 2001 that air samples over Hyderabad, India, gathered from the stratosphere by the Indian Space Research Organization, contained clumps of living cells. Wickramasinghe calls this "unambiguous evidence for the presence of clumps of living cells in air samples from as high as 41 km, above which no air from lower down would normally be transported". Two bacterial and one fungal species were later independently isolated from these filters which were identified as Bacillus simplex, Staphylococcus pasteuri and Engyodontium album respectively. The experimental procedure suggested that these were not the result of laboratory contamination, although similar isolation experiments at separate laboratories were unsuccessful. [Wikipedia]
Pushkar Ganesh Vaidya of the Indian Astrobiology Research Center stated in his 2009 report on the Wickramasinghe discovery: "The three microorganisms captured during the balloon experiment do not exhibit any distinct adaptations expected to be seen in microorganisms occupying a cometary niche."
Scientists at NASA's Ames Research Lab, however, expressed doubt. Maxwell Bernstein, a respected space scientist at Ames, stressed Wickramasinghe's results were best approached with caution. He said, "It would strain one's credulity less to believe that terrestrial organisms had somehow been transported upwards than to assume that extraterrestrial organisms are falling inward."
Are aliens already on Earth with us? Although it remains to be seen the possibility exists that alien life has been visiting and affecting Earth for a very long time.