In summer 2011, a few Moroccan night owls saw Martians in the Sahara Desert. The otherworldly visitors didn't arrive in a flying saucer, but on a 2.4-pound (1.1-kilogram) fragment of rock that glowed red-hot in Earth's atmosphere before landing near the village of Tissint.
OK, we're taking a few liberties with the story. The rock -- a meteorite -- did indeed come to our planet from Mars. Whether we can slap a "Life On Board" sign to its black, dimpled surface remains to be seen. Other Martian meteorites, however, have provided clues about a truly tantalizing possibility: that primitive bacteria first formed on our red neighbor and then traveled through space to Earth, where they became the seeds of our spectacular biological diversity. If that idea were proved true, we'd all be Martians rather than Earthlings.
It's not a new proposal. In the 19th century, British physicist William Thomson Kelvin, known to most students as the father of the absolute temperature scale (measured in kelvins), also had a few ideas about geologic history and the evolution of life on Earth. One was that seed-bearing meteorites zipped through outer space.
"If at the present instant no life existed upon this earth, one such stone falling upon it might ... lead to its becoming covered with vegetation," Kelvin said while addressing the British Association for the Advancement of Science in 1871.
Swedish chemist and Nobel laureate Svante Arrhenius pursued a similar concept in "Worlds in the Making," which was published in 1906. In the book, Arrhenius introduced the term panspermia to describe a process by which bacterial spores could drift through the solar system on undulating currents of electromagnetic energy.
For a while, these notions seemed like the musings of hardcore scientists turned a little batty. Then, in the 20th century, evidence for panspermia became more abundant and more compelling. One of the real turning points came when NASA sent two Viking probes to Mars in 1975. The Viking 1 lander touched down in Chryse Planitia, Viking 2 in Utopia Planitia. Both snapped photos of the Martian landscape and then measured various properties of the atmosphere and soil. The Viking data didn't conclusively prove the existence of life on Mars, but it did reveal the red planet had a unique ratio of noble gas isotopes in its atmosphere.
In the 1980s, scientists discovered a similar chemical signature in a group of space rocks known as the SNC meteorites (named after three representative members of the group: Shergotty, Nakhla, Chassigny). The gases found trapped in these meteorites matched the gases detected by the Viking landers when they tested the Martian atmosphere in the 1970s.
They also study previously collected specimens to see if any should be reclassified. Of the 53,000 meteorites that we've officially cataloged on Earth, 104 have been labeled Martian [source: Marlow]. Eyewitnesses have only seen five of these rare rocks arrive on our planet. The rest made a quiet entry and were found after their impact, often in Antarctica or North Africa because they're easy to spot on ice or sand.