| LIFE ON EARTH BEGAN AT LEAST 3.85 BILLION YEARS AGO, 400 MILLION YEARS
EARLIER THAN PREVIOUSLY THOUGHT, SCIENTISTS SAY November 6, 1996 RELEASE: 96-230 Life on Earth began at least 3.85 billion years ago, an international team of scientists reports in the cover story of the Nov. 7 issue of the journal Nature. The scientists, from UC San Diego's Scripps Institution of Oceanography, UCLA's Department of Earth and Space Sciences, the Australian National University and England's Oxford Brookes University, present evidence that pushes back the emergence of life on Earth by 400 million years.The evidence comes from a rock formation discovered on Akilia Island in southern West Greenland that is at least 3.85 billion years old. The research -- funded primarily by the National Science Foundation and NASA -- has provocative implications. "Our evidence establishes beyond reasonable doubt that life emerged on Earth at least 3.85 billion years ago, and this is not the end of the story," said Stephen J. Mojzsis, a graduate student in geochemistry at Scripps and the lead author of the article. "We may well find that life existed even earlier." "We look in rocks like this for chemical suggestions and isotopic evidence, and we found both," said T. Mark Harrison, professor of geochemistry at UCLA and director of UCLA's W.M. Keck Foundation Center for Isotope Geochemistry. "It would be wonderful to see a head and toes, and while we don't have those, we have found very strong isotopic evidence for ancient life." "But in the cases of Earth's most ancient rocks and minerals, we are actually better off relying on this type of isotopic evidence -- chemofossils -- rather than on the shape of life-like objects with which nature has often been deceiving the unwary," said Gustaf Arrhenius, professor of oceanography at UC San Diego and principal investigator for the research project. The carbon inclusions in the rock were analyzed with UCLA's high-resolution ion microprobe -- an instrument that enables scientists to learn the exact composition of samples -- which Mojzsis described as the "world's best instrument" for this research. The microprobe shoots a beam of ions -- charged atoms -- at a sample, releasing from the sample its own ions that are analyzed in a mass spectrometer. Scientists can aim the beam of ions at specific microscopic areas of a sample and analyze them. The team of scientists, Mojzsis; Arrhenius, who is his research adviser; Harrison; Kevin McKeegan, a researcher in UCLA's Department of Earth and Space Sciences; Allen Nutman, a research fellow at the Australian National University; and Clark Friend, a geologist at Oxford Brookes University, presents the following evidence for the ancient life: Most importantly, a high ratio of one form -- an isotope -- of carbon to another, which provides a "signature of life," Mojzsis said. The carbon aggregates in the rock have a ratio of about 100 to one of 12C (the most common isotope form of carbon, containing six protons and six neutrons) to 13C (a rarer isotopic form of carbon, containing six protons and seven neutrons). "The light carbon, 12C, is more than three percent more abundant than scientists would expect to find if life were not present, and three percent is, in this case, a very large amount," Arrhenius said; The inclusion of the carbon in a phosphate mineral called apatite, which is also the material of which bones and teeth are made. Apatite is often formed by microorganics, but it can also be formed inorganically. The association of the carbon with the apatite is "suggestive, and not surprising, but does not in itself establish life," Arrhenius said. The form of life discovered was probably a simple micro- organism, although its actual shape or nature cannot be ascertained, Mojzsis said, because heat and pressure over time have destroyed any original physical structure of the organisms. Harrison, who directs UCLA's ion microprobe, said of the research, "This was a scientific problem that was waiting for a new generation microprobe of this resolution. The individual samples are very small, and no other instrument would have been sensitive enough to reveal precisely the isotopic composition and location of the carbon inclusions in the rock." It is unknown when life first appeared on Earth, which is approximately 4.5 billion years old. The previous earliest evidence for life was presented by UCLA paleobiologist J. William Schopf, who showed that on the basis of bacteria-like fossils, primitive life, much like modern "pond scum," existed on Earth 3.46 billion years ago. "The evolution of lifeless matter into primitive life forms, and their organization into the complex structure of cells like those found by Schopf, represent an enormous development in the earliest history before the deposition of the Akilia sediments," Arrhenius said. The residues of ancient life that the scientists have discovered existed prior to the end of the "late heavy bombardment" of the Moon by large objects, which ended approximately 3.8 billion years ago, Harrison said. The implication, he added, is that the often assumed simultaneous bombardment of Earth did not lead to the extinction of life. This research shows that life on Earth began during the first approximately 700 million years after the formation of the planet, placing an upper limit on the time needed for the creation of life on Earth, or on the time period available for it to arrive here from elsewhere, the scientists said. "Life is tenacious, and it completely permeates the surface layer of the planet," Mojzsis said. "We find life beneath the deepest ocean, on the highest mountain, in the driest desert and the coldest glacier, and deep down in the crustal rocks and sediments. Not knowing what conditions are needed for the emergence of life, it is only possible to speculate about its existence elsewhere in the universe. An important contribution to the solution of this problem could come from exploration of the surface of Mars for traces there of extinct life." An equally interesting question, the scientists agreed, that is currently studied in laboratories on Earth is how life originally could have arisen from lifeless molecules, and evolved into the already sophisticated isotope fractioning life forms recorded in the Akilia rocks. Mars Ocean Hypothesis Hits the Shore "The ocean hypothesis is very important, because the existence of large bodies of liquid water in the Martian past would have had a tremendous impact on ancient Martian climate and implications for the search for evidence of past life on the planet," said Dr. Kenneth Edgett, a staff scientist at MSSS. Features in earlier Mars probes, in particular the startling Viking images, led a number of researchers to look for remnants of ancient coastlines and further raised the possibility that such a body of water once existed.
"So things now with respect to the oceans," said Dr. Michael Carr, of the US Geological Survey, 'they're kind of in limbo right now. I have looked at a lot of the MOC images in places where the shorelines are supposed to be and I can't find any evidence. You can see features there, but whether these are shorelines or not is kind of difficult."
Beginning in 1998, MSSS scientists Michael Malin and Kenneth Edgett set out to answer this question with higher resolution cameras—five to ten times better than Viking. Initially the team targeted about 2% of the MOC images in places that would test shorelines proposed by others in the scientific literature. With the researchers' visual identification at higher resolution, none of these features appeared to have been formed by the action of water in a coastal environment. Their analysis first appeared in Geophysical Research Letters, in a paper entitled "Oceans or Seas in the Martian Northern Lowlands: High Resolution Imaging Tests of Proposed Coastlines."
Fourteen images were analyzed of areas that had been indicated, from Viking images, to be candidates for shorelines. Whether a larger image sample or confirming data will bear out the visual interpretation of what expectations of a Martian shoreline should look like, remains a scientific conclusion ripe for debate. The Mars Global Surveyor carries onboard the Mars Orbiter Laser Altimeter instrument (MOLA), which uses infrared laser pulses to measure the surface below. "The MOC images we took in the late '90s do not show any coastal landforms in areas where previous researchers—-working with lower resolution Viking images—proposed there were shorelines." As presented in the Geophysical Research Letters paper, the analysis focused on four different areas that had been proposed as coastlines. One of these areas is northwest of the great volcanoOlympus Mons (Figure 3 ). Viking images of the linear feature separating the western margin of the Lycus Sulci from the lower, smoother Amazonis plains (upper left in Figure 4 ) led some researchers to conclude that the two surfaces were in contact along a cliff. Previously, since the proposed cliff faces toward the smooth plains, it was suggested that this feature might be the signature of a cliff that forms from erosion by waves in a body of water as they break against a coastline.
Three MOC images were acquired along this proposed shoreline, covering the areas indicated by the white boxes in Figure 4. Each image was targeted to straddle the feature, a rise that runs diagonally across the scene from near the lower left toward the upper right. The middle section of the central image, shown in Figure 5, was taken in July 1998. The Lycus Sulci uplands (lower half) here are roughly-textured while the flat Amazonis plains (upper half) appear more smooth. This image in particular shows that the contact between Amazonis and Lycus Sulci is clearly not a wave-cut cliff, and that there are no features that can be unambiguously identified as coastal landforms. "But what bothers me," said Carr, "is that throughout this latitude band where the ocean shorelines are being mapped, the surficial geology is very complicated. There's a lot going on. You get all kinds of very complicated morphology."
"Even on Earth, looking for ancient shorelines from the air or space is a challenge," said Dr. Malin. "But, despite the difficulties in identifying ancient coastlines remotely, we believe these MOC images of the proposed shorelines are of a high enough resolution that they would have shown features indicative of a coastal environment had there been an ancient ocean on Mars." Martian seas would have been influenced by only one third
the gravity of Earth's seas and would not have been subject to strong tidal forces, like
that arising from the Earth's Moon. Because of the Earth's active erosion, there are fewer
chances to compare an ancient Mars coast with a present eroding Earth coast as such a rift
would appear seen from space.
While the suggestion that Mars at one time had oceans cannot be ruled out, the foundation for the "ocean hypothesis" developed in the 1980s on the basis of suspected shorelines appears now to require a broader scan of any apparent beachfront real estate on Mars. However, it should be understood that there is significant other evidence of water on Mars in the past, both from Mars Global Surveyor and from previous missions. To search for clues to the very important question of the role of water in the evolution of Mars, the MOC continues to acquire new high resolution pictures. Related Links Original NASA/JPL/Malin Space Science Systems Press Release Additional details and more images from the Malin and Edgett paper Malin, M. C., and Edgett, K. S., 1999. Oceans or Seas in the Martian Northern Lowlands: High Resolution Imaging Tests of Proposed Coastlines, Geophys. Res. Letters, V. 26, No. 19, p. 3049-3052 Possible
Configuration of Ancient Oceans on Mars
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