Residue of Life
|Rocks on Greenland’s Akilia Island. Scientists report evidence that supports life on Earth more than 3.8 billion years ago. They analyzed cross-cutting igneous rocks formed by intrusion, such as the light rock in the center (with hammer).
Ten years ago, an international team of scientists reported evidence, in a controversial cover story in the journal Nature, that life on Earth began more than 3.8 billion years ago–400 million years earlier than previously thought. A UCLA professor who was not part of that team and two of the original authors report that the evidence is stronger than ever.
Craig E. Manning, lead author of the new study and a professor of geology and geochemistry in the UCLA Department of Earth and Space Sciences, painstakingly mapped an area on Akilia Island in West Greenland where ancient rocks were discovered that may preserve carbon-isotope evidence for life at the time of their formation. Manning and his co-authors–T. Mark Harrison, a UCLA professor of geochemistry, director of UCLA’s Institute of Geophysics and Planetary Physics, and University Professor at the Australian National University; and Stephen J. Mojzsis, assistant professor of geological sciences at the University of Colorado, Boulder–conducted new geologic and geochemical analysis on these rocks. Their findings were recently reported in the American Journal of Science. Harrison and Mojzsis were co-authors on the Nov. 7, 1996, study in Nature.
"This paper shows, with far greater confidence than we ever had before, that these rocks are older than 3.8 billion years," said Manning, who has conducted extensive research in Greenland. "We have shown that the rocks are appropriate for hosting life.
"Everything from the basic geology to the analysis in the original report (in Nature) has been challenged," said Manning, who has expertise in areas that have become central to the debate, including the chemistry of water and the interaction of water and rock. "The chemical evidence for life has been challenged, as have been the minerals to determine whether life was present, whether the rocks have the origin that was originally attributed to them, and whether the rocks were as old as originally envisioned. We didn’t go to Greenland in response to the criticism. We went to learn the age of the rocks and to make a better geologic map of the area than any that existed."
At the time of the 1996 Nature paper, there was no reliable map showing the geology of the area, Manning said. So he created one.
|A rock sample from Akilia Island off the coast of Greenland. A zircon crystal determined to be 4.4 billion years old may make it the world’s oldest known terrestrial material. Zircon is a mineral commonly used to determine the geological age of rocks. Chemical analysis of this grain suggests that the Earth was cool enough to have water, a hydrosphere and, possibly, life much earlier than previously thought.
"I wandered around that outcrop for two-and-a-half weeks–it’s not a big area–with a clipboard, maps, a compass and grid paper. We mapped it like an archeologist would map it," Manning said. "It became clear that these rocks that hosted life line up into two beautiful, coherent layers. They are not randomly distributed, as you might expect if the alternative interpretation is right. I’m very confident about that. I went to Greenland with some skepticism, but I became more and more confident as time went on that the original interpretation was right."
"It could have gone any way," Harrison said. "We could have placed the claim on much firmer footing, or we could have proved ourselves wrong. We found a much more compelling cross-cutting relationship in the rocks than we originally thought."
The new research is a comprehensive response to the critics, Harrison said.
"We’ve been holding our fire rather than fire away at each criticism in a piecemeal way," he said. "We’ve gone back to Greenland and done the study from the ground up, with much more data than existed at the time of the original paper. I’m much more confident today than I was in 1996 about the likelihood that this is evidence of early life. This is not ‘smoking gun’ evidence–we are not seeing fossils–but in every case, the model has come through with flying colors."
Manning agrees, saying he is confident the rocks contain evidence of ancient life, but "it’s not a slam dunk."
Why is there doubt? After more than 3.8 billion years, the rocks are severely damaged.
"They have been folded, distorted, heated and compressed so much that their minerals are very different from what they were originally," Harrison said.
Why Akilia Island in Greenland?
"Akilia Island was not the best place to search for evidence of early life; it’s simply the place where it turned up," Harrison said.
"There’s nothing special about Akilia Island," Manning said. "If life was there, it should have been abundant on Earth 3.83 billion years ago. The only place where that’s been tested so far, also in Greenland, has come up positive."
One of the key methods for dating the rocks is by carefully analyzing cross-cutting intrusions made by igneous rocks, Manning said, adding, "Whatever is cross-cut must be older than that which is doing the cross-cutting. We went there to find these cross-cutting relationships, which we did."
The research on the Akilia rocks is federally funded by the National Science Foundation (http://www.nsf.gov/) and the NASA Astrobiology Institute (http://nai.arc.nasa.gov/), a partnership between NASA, 12 major U.S. teams and six international consortia.
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Scientists look for evidence of life in ancient rocks like those from Akilia Island by searching for chemical suggestions and isotopic evidence. The very strong isotopic evidence for ancient life found in the 1996 study included a high ratio of one form of carbon–an isotope–to another, which provides a "signature of life," Mojzsis said.
The carbon aggregates in the rocks 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 3 percent more abundant than scientists would expect to find if life were not present, and 3 percent is very significant, Harrison said.
Carbon inclusions in the rocks were analyzed with UCLA’s high-resolution ion microprobe–an instrument that enables scientists to learn the exact composition of samples. The microprobe shoots a beam of ions, or charged atoms, at a sample, releasing from the sample its own ions, which are then analyzed in a mass spectrometer. Scientists can aim the beam of ions at specific microscopic areas of a sample and analyze them.
While critics noted there are ways to make light carbon in the absence of life, Harrison considers those possibilities to be "extremely unlikely," especially in light of another discovery of rocks in Western Greenland, not far away, of the same age, and a similar ratio of 12C to 13C.
The scientists see light carbon inclusions in a phosphate mineral called apatite, which is also the material of which bones and teeth are made.
The form of life the researchers believe they have discovered was probably a simple microorganism, 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.
|Lunar Clementine mission shows the South Pole of the Moon. The permanently shadowed region center showed earlier evidence of meteor cratering and ice never exposed to direct sunlight. Credit: NASA/DOD Clementine|
Harrison said of UCLA’s ion microprobe and the research: "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 residue of ancient life that the scientists believe they have found existed prior to the end of the "late heavy bombardment" of the Moon by large objects, a period which ended approximately 3.8 billion years ago, Harrison noted.
"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."
An unanswered question 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.
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Advances in our Understanding of Life
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