In the next decades, paleontologists in Africa found 3 billion- year-old fossil traces of microscopic bacteria that had lived in massive marine reefs. Bacteria can also form what are called biofilms, colonies that grow in thin layers over surfaces such as rocks and the ocean floor, and scientists have found solid evidence for biofilms dating back 3.2 billion years.
But at the time of the NASA press conference, the oldest fossil claim belonged to UCLA’s William Schopf, the man who spoke skeptically about NASA’s finds at the same conference. During the 1960s, ’70s and ’80s, Schopf had become a leading expert on early life-forms, discovering fossils around the world, including 3 billion-year-old fossilized bacteria in South Africa. Then, in 1987, he and some colleagues reported that they had found the 3.465 billion-yearold microscopic fossils at a site called Warrawoona in the Western Australia outback—the ones he would display at the NASA press conference. The bacteria in the fossils were so sophisticated, Schopf says, that they indicate “life was flourishing at that time, and thus, life originated appreciably earlier than 3.5 billion years ago.”
Since then, scientists have developed other methods for detecting signs of early life on Earth. One involves measuring different isotopes, or atomic forms, of carbon; the ratio of the isotopes indicates that the carbon was once part of a living thing. In 1996, a team of researchers reported that they had found life’s signature in rocks from Greenland dating back 3.83 billion years.
The signs of life in Australia and Greenland were remarkably old, especially considering that life probably could not have persisted on Earth for the planet’s first few hundreds of millions of years. That’s because asteroids were bombarding it, boiling the oceans and likely sterilizing the planet’s surface before about 3.8 billion years ago. The fossil evidence suggested that life emerged soon after our world cooled down. As Schopf wrote in his book Cradle of Life, his 1987 discovery “tells us that early evolution proceeded very far very fast.”
A quick start to life on Earth could mean that life could also emerge quickly on other worlds—either Earth-like planets circling other stars, or perhaps even other planets or moons in our own solar system. Of these, Mars has long looked the most promising.
The surface of Mars today doesn’t seem like the sort of place hospitable to life. It is dry and cold, plunging down as far as -220 degrees Fahrenheit. Its thin atmosphere cannot block ultraviolet radiation from space, which would devastate any known living thing on the surface of the planet. But Mars, which is as old as Earth, might have been more hospitable in the past. The gullies and dry lake beds that mark the planet indicate that water once flowed there. There’s also reason to believe, astronomers say, that Mars’ early atmosphere was rich enough in heat-trapping carbon dioxide to create a greenhouse effect, warming the surface. In other words, early Mars was a lot like early Earth. If Mars had been warm and wet for millions or even billions of years, life might have had enough time to emerge. When conditions on the surface of Mars turned nasty, life may have become extinct there. But fossils may have been left behind. It’s even possible that life could have survived on Mars below the surface, judging from some microbes on Earth that thrive miles underground.
When Nasa’s Mckay presented his pictures of Martian fossils to the press that day in 1996, one of the millions of people who saw them on television was a young British environmental microbiologist named Andrew Steele. He had just earned a PhD at the University of Portsmouth, where he was studying bacterial biofilms that can absorb radioactivity from contaminated steel in nuclear facilities. An expert at microscopic images of microbes, Steele got McKay’s telephone number from directory assistance and called him. “I can get you a better picture than that,” he said, and convinced McKay to send him pieces of the meteorite. Steele’s analyses were so good that soon he was working for NASA.
Ironically, though, his work undercut NASA’s evidence: Steele discovered that Earthly bacteria had contaminated the Mars meteorite. Biofilms had formed and spread through cracks into its interior. Steele’s results didn’t disprove the Martian fossils outright—it’s possible that the meteorite contains both Martian fossils and Antarctic contaminants— but, he says, “The problem is, how do you tell the difference?” At the same time, other scientists pointed out that nonliving processes on Mars also could have created the globules and magnetite clumps that NASA scientists had held up as fossil evidence.
But McKay stands by the hypothesis that his microfossils are from Mars, saying it is “consistent as a package with a possible biological origin.” Any alternative explanation must account for all of the evidence, he says, not just one piece at a time.
The controversy has raised a profound question in the minds of many scientists: What does it take to prove the presence of life billions of years ago? in 2000, oxford paleontologistMartin Brasier borrowed the original Warrawoona fossils from the NaturalHistoryMuseum in London, and he and Steele and their colleagues have studied the chemistry and structure of the rocks. In 2002, they concluded that it was impossible to say whether the fossils were real, essentially subjecting Schopf’s work to the same skepticism that Schopf had expressed about the fossils from Mars. “The irony was not lost on me,” says Steele.