"Long before it's in the papers"
October 19, 2015


Early Earth wasn’t so hellish—and life was here, scientists say

Oct. 19, 2015
Courtesy of University of California - 
Los Angeles
and World Science staff

Scientists say they have found ev­i­dence of life on Earth at least 4.1 bil­lion years ago—300 mil­lion years ear­li­er than pre­vi­ous re­search in­di­cat­ed. That would sug­gest life be­gan shortly af­ter the plan­et formed 4.54 bil­lion years ago, and the early planet wasn’t ne­cess­arily as hellish a place as tra­dit­ional books depict.

Traditional depictions of the early Earth as an almost unimaginable hell­hole—such as the above artist’s con­ception—may be off the mark, new stu­dies suggest. (Credit: NA­SA God­dard Space Flight Center Con­cep­tual Im­age Lab)

“Twenty years ago, this would have been he­ret­i­cal; find­ing ev­i­dence of life 3.8 bil­lion years ago was shock­ing,” said Mark Har­ri­son, a co-author of the re­search, and a ge­o­chem­is­t at the Uni­vers­ity of Cal­i­for­nia Los An­ge­les.

“Life on Earth may have started al­most in­stan­ta­neously,” added Har­ri­son. “With the right in­gre­di­ents, life seems to form very quick­ly.”

The re­search, pub­lished to­day in the on­line early edi­tion of the jour­nal Pro­ceed­ings of the Na­t­ional Acad­e­my of Sci­ences, sug­gests life ex­isted be­fore a mas­sive bom­bard­ment of the in­ner so­lar sys­tem that formed the moon’s large craters 3.9 bil­lion years ago.

“If all life on Earth died dur­ing this bom­bard­ment, which some sci­en­tists have ar­gued, then life must have restarted quick­ly,” said Pat­rick Boehnke, a co-author of the re­search and a grad­u­ate stu­dent in Har­ri­son’s lab­o­r­a­to­ry.

Sci­en­tists had long be­lieved the Earth was dry and dead in that era.

“The early Earth cer­tainly was­n’t a hell­ish, dry, boil­ing plan­et; we see ab­so­lutely no ev­i­dence for that,” Har­ri­son said. “The plan­et was probably much more like it is to­day than pre­vi­ously thought.”

The re­search­ers, led by Eliz­a­beth Bell, a post­doc­tor­al schol­ar in Har­ri­son’s lab­o­r­a­to­ry, stud­ied more than 10,000 zir­cons orig­i­nally formed from mol­ten rocks, or mag­mas, from West­ern Aus­tral­ia. Zir­cons are heavy, tough mine­rals re­lat­ed to the syn­thet­ic cu­bic zir­co­ni­um used for imita­t­ion di­a­monds. They cap­ture and pre­serve their im­me­di­ate en­vi­ron­ment, mean­ing they can serve as time cap­sules.

The sci­en­tists an­a­lyzed 79 zircons with Ra­man spec­tros­co­py, a tech­nique that shows the mo­lec­u­lar and chem­i­cal struc­ture of an­cient mi­croor­gan­isms in three di­men­sions.

Bell and Boehnke, who have pi­o­neered chem­i­cal and mine­ralogical tests to de­ter­mine the con­di­tion of an­cient zir­cons, were search­ing for car­bon, the key com­po­nent for life. One of the zir­cons con­tained two bits of graph­ite—pure car­bon. That car­bon must have been there ev­er since the zir­con formed, Har­ri­son said—that is, 4.1 bil­lion years ago, based on its ra­tio of ura­ni­um to lead. 

“There is no bet­ter case of a pri­ma­ry in­clu­sion in a mine­ral ev­er doc­u­ment­ed,” Har­ri­son said. “No­body has of­fered a plau­si­ble al­ter­na­tive ex­plana­t­ion for graph­ite of non-biological or­i­gin in­to a zir­con.”

The re­search sug­gests life in the uni­verse could be abun­dant, Har­ri­son said. On Earth, sim­ple life seems to have formed quick­ly, but it likely took many mil­lions of years for it to evolve the abil­ity to pho­to­syn­the­size, or con­vert sun­light in­to en­er­gy.

The car­bon con­tained in the zir­con al­so has a char­ac­ter­is­tic sig­na­ture that in­di­cates the pres­ence of photosyn­thet­ic life, the sci­en­tists said. That sig­na­ture con­sists of a spe­cif­ic ra­tio be­tween two forms of car­bon, car­bon-12 to car­bon-13.

“We need to think dif­fer­ently about the early Earth,” Bell said.

* * *

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Geochemists say they have found evidence of life on Earth at least 4.1 billion years ago—300 million years earlier than previous research indicated. That would suggest life began shortly after the planet formed 4.54 billion years ago. “Twenty years ago, this would have been heretical; finding evidence of life 3.8 billion years ago was shocking,” said Mark Harrison, co-author of the research and a professor of geochemistry at the University of California Los Angeles. “Life on Earth may have started almost instantaneously,” added Harrison. “With the right ingredients, life seems to form very quickly.” The research, published today in the online early edition of the journal Proceedings of the National Academy of Sciences, suggests life existed before a massive bombardment of the inner solar system that formed the moon’s large craters 3.9 billion years ago. “If all life on Earth died during this bombardment, which some scientists have argued, then life must have restarted quickly,” said Patrick Boehnke, a co-author of the research and a graduate student in Harrison’s laboratory. Scientists had long believed the Earth was dry and dead in that era. “The early Earth certainly wasn’t a hellish, dry, boiling planet; we see absolutely no evidence for that,” Harrison said. “The planet was probably much more like it is today than previously thought.” The researchers, led by Elizabeth Bell, a postdoctoral scholar in Harrison’s laboratory, studied more than 10,000 zircons originally formed from molten rocks, or magmas, from Western Australia. Zircons are heavy, tough minerals related to the synthetic cubic zirconium used for imitation diamonds. They capture and preserve their immediate environment, meaning they can serve as time capsules. The scientists identified 656 zircons containing dark specks that could be revealing and closely analyzed 79 of them with Raman spectroscopy, a technique that shows the molecular and chemical structure of ancient microorganisms in three dimensions. Bell and Boehnke, who have pioneered chemical and mineralogical tests to determine the condition of ancient zircons, were searching for carbon, the key component for life. One of the zircons contained two bits of graphite—pure carbon. That carbon has been there ever since the zircon formed, 4.1 billion years ago, Harrison said, pronouncing himself “very confident” of that. The zircon was dated based on its ratio of uranium to lead, the researchers explained. “There is no better case of a primary inclusion in a mineral ever documented,” Harrison said. “Nobody has offered a plausible alternative explanation for graphite of non-biological origin into a zircon.” The research suggests life in the universe could be abundant, Harrison said. On Earth, simple life seems to have formed quickly, but it likely took many millions of years for it to evolve the ability to photosynthesize, or convert sunlight into energy. The carbon contained in the zircon also has a characteristic signature that indicates the presence of photosynthetic life, the scientists said. That signature consists of a specific ratio between two forms of carbon, carbon-12 to carbon-13. “We need to think differently about the early Earth,” Bell said.