Greenland Rocks Tell New Tale of Life on Earth
|
Photo of Mark Harrison by
Photo by Allen Nutman
Photo by Allen Nutman
Photo of rocks by Scripps |
It may not have had fingers and toes, nonetheless it was life, according
to NSF-supported geochemist Mark Harrison of the University of California,
Los Angeles (UCLA). He is describing evidence of ancient life found in a
rock formation on Akilia Island in West Greenland. Harrison and colleagues
recently made a ground-breaking discovery, one that pushed the age of the
oldest known life on Earth back almost 400 million years, to some 3.86 billion
years ago.
The scientists -- from UCLA, University of California, San Diego's Scripps Institution of Oceanography and other universities -- say their research has provocative implications. "Our evidence establishes beyond reasonable doubt that life emerged on Earth at least 3.86 billion years ago," says Stephen Mojzsis, a graduate student in geochemistry at Scripps and lead author of a recent paper on the subject in the scientific journal Nature. "This is not the end of the story. We may well find that life on Earth existed even earlier." Explains Harrison, "We look in rocks like the ones from Akilia Island for a chemical suggestion of life; in this case, we found it. Although it would be wonderful to see a head and toes, while we don't have those, we have found very strong evidence for ancient life in these rocks." Scripps senior scientist Gustaf Arrhenius is the principal investigator for the project and Mojzsis' research advisor. He adds, "We don't necessarily need fingers and toes. As long as there is no other known way for nature to achieve the strong carbon isotope change found in the rock from Akilia Island, this change provides the best evidence for the existence of life." The life, according to Arrhenius and colleagues, was located in an unusual rock formation in what is now a desolate part of Greenland. The site, known to be 3.85 billion years old, is the oldest known collection of rocks on Earth. The team, which included scientists from the Australian National University and Oxford Brooks University in England, as well as Scripps and UCLA, used funding from the National Aeronautics and Space Administration to collect the samples. Back in the United States, carbon in the Akilia Island rock was analyzed with UCLA's high-resolution ion microprobe, an instrument that enables scientists to determine the exact composition of samples. Mojzsis describes the instrument, developed with support from NSF, the W. M. Keck Foundation and UCLA, as "the world's best instrument for this kind of research." The microprobe directs a beam of ions -- charged atoms -- at a sample. The sample then releases its own ions, which are in turn analyzed in an instrument called a mass spectrometer. Scientists can aim the beam of ions at specific microscope areas of a sample and analyze the results. Harrison believes the recent research was in part the result of a jump in technology. "It was a scientific problem awaiting a new-generation microprobe with a high level of resolution. No other instrument would have been sensitive enough to reveal the exact isotopic composition of carbon in the rock." "Major advances in comprehending Earth and the solar system have followed development of new analytical tools like this one," adds Dan Weill, program director in NSF's Division of Earth Sciences, which supports the ion microprobe facility. The advent of high-precision mass spectrometry, says Weill, led to new research on isotope variations. "The results of that research have forever altered our views of the origin and evolution of Earth, its moon and the solar system. Similar advances are now occurring as conventional high-precision mass spectrometry is augmented by new-generation ion microprobes capable of working on microscales." The ion microprobe facility at UCLA provides access to scientists conducting research in many earth sciences fields, "allowing NSF, the Keck Foundation and UCLA to leverage their various contributions to this facility into a whole with much more to offer than any one of these three funders could have supported alone," says Weill. To the Akilia Island rock researchers, the ion microprobe provided one of the most important pieces of evidence: a high ratio of one isotope, or type of carbon, to another. Carbon atoms come in two varieties: carbon-12, with six protons and six neutrons, and carbon-13, with six protons and seven neutrons. Since living organisms use the lighter carbon-12, rather than the heavier carbon-13, a lump of carbon that has been processed by a living organism has more carbon-12 atoms than one found in other places in nature. The researchers found that the ratio of carbon-12 to carbon-13 was three percent higher than would be expected if life were not present, says Mojzsis, "and that three percent is a very large amount." They also found carbon in a phosphate mineral called apatite, the material that makes up bones and teeth. Apatite is often formed through interactions with microorganisms, but can also be formed inorganically. The association of the carbon with the apatite is "suggestive," says Arrhenius, "but does not in itself establish the existence of life." However, when linked to the evidence of excessive carbon-12, the researchers are confident that these rocks hold the remnants of ancient life. The form of life discovered was once likely a simple microorganism, although its actual shape or nature cannot be ascertained. Over time, heat and pressure have destroyed its original physical structure, "which was likely a 'blob' even then," says Mojzsis. |
It's unknown exactly when life first appeared on Earth, but it's thought it was about 4.5 billion years ago. The previous earliest evidence for life showed that, on the basis of bacteria-like fossils, primitive plant life that looks much like modern "pond scum," existed on Earth some 3.46 billion years ago.
"The evolution of lifeless matter into primitive life forms, and their organization into the complex structure of cells, represents an enormous leap, one that happened long before the Akilia sediments came into being," says Arrhenius.
The residues of ancient life the scientists discovered existed prior to the end of the heavy bombardment of the moon by large objects, approximately 3.8 billion years ago. The implication, says Harrison, is that the often assumed simultaneous bombardment of Earth did not lead to the extinction of life, if it existed prior to that time in the Akilia Island sediments and likely elsewhere.
The new research claims that life on Earth began during the first 700 million years of the planet's existence. This places an upper limit on the time needed for the creation of life, or on the time period available for it to arrive from elsewhere.
The claims are meeting both encouragement and caution from other parts of the scientific community. William Schopf, discoverer of the earliest form of physical fossils and director of the UCLA Center for the Study of Evolution and Origin of Life, told the Los Angeles Times that the findings were "very significant."
John Hayes, a leading researcher in the chemistry of early Earth who works at Woods Hole Oceanographic Institute, told the New York Times that the Nature paper was "good science," but adds, "There are potential processes other than life that could account for the Arrhenius isotope results, and even though they are long shots, we cannot entirely rule them out."
However, Mojzsis says life is everywhere. "Life is tenacious, and it completely permeates the surface layer of the planet. We find life beneath the deepest ocean, on the highest mountain, in the driest desert, on the coldest glacier, and deep within rocks and sediments.
"Since we don't know for sure what conditions are needed for the emergence of life, it's only possible to speculate about exactly when it first began on earth, or its existence elsewhere in the universe." An important contribution to the answer to this latter question, he says, could come from exploration of the surface of Mars for traces there of extinct life.
An equally interesting question being studied here on Earth, says Arrhenius, is how life could originally arise from lifeless molecules and evolve into the forms recorded in the Akilia Island rocks. They may have been blob-like, and not have had fingers and toes, but, says the scientist, "they were pretty sophisticated nonetheless."
