Wednesday, September 02, 2009

Did Life Arise 3.5 Billion Years Ago?

J. William Schopf is a paleontologist at the University of California, Los Angeles (USA). He became famous in the 1990s for his studies of the Apec chert—ancient rocks in the northern part of Western Australia near the town of Marble Bar. Parts of these rocks have been reliably dated to 3.465 billion years ago.

Schopf claimed to have discovered bacteria fossils in these rocks. He published his results in a highly cited Science paper back in 1993 (Schopf, 1993). The title of the paper "Microfossils of the Early Archean Apex chert: new evidence of the antiquity of life" establishes his claim.

It's worth quoting the abstract of the paper because it shows the confidence Schopf exuded. Not only did he claim that the 3.5 billion year old Apex chert contained bacterial fossils but, even more astonishingly, he identified eleven different species and clearly stated that they resembled cyanobacteria.
ResearchBlogging.org
Eleven taxa (including eight heretofore undescribed species) of cellularly preserved filamentous microbes, among the oldest fossils known, have been discovered in a bedded chert unit of the Early Archean Apex Basalt of northwestern Western Australia. This prokaryotic assemblage establishes that trichomic cyanobacterium-like microorganisms were extant and morphologically diverse at least as early as approximately 3465 million years ago and suggests that oxygen-producing photoautotrophy may have already evolved by this early stage in biotic history.
The data were immediately challenged. There were two problems, First, many paleonotologists questioned whether the "fossils" were really fossils. They suggested that the structures could easily be inorganic in nature and not remnants of living organisms. Secondly, the presence of cyanobacteria—among the most complex bacteria—is inconsistent with molecular data. Even though the early tree of life is complicated, the available evidence indicates that cyanobacteria arose late in the evolution of bacterial taxa. It's very unlikely that the earliest forms of life could be cyanobacteria, or even photosynthetic bacteria.

The publicity associated with the presumed discovery of the earliest forms of life was too much to resist. In spite of the criticisms, the "fact" of these "fossils" made it into the textbooks within months of the discovery. The original figures have often been purged from more recent editions but the widespread claim that life originated 3.5 billion years ago persists.

Schopf defended and promoted his work in a trade book—The Cradle of Life— published in 1999. In that book he appeared to address most of his critics. He insisted that his "fossils" met all the rigorous tests of science.

The Fossils Aren't Fossils


Over the years, the challengers became more and more emboldened. In 2002 Martin Brasier published a re-analysis of Schopf's original fossils and noticed that the published images were not as complete as they could be. In the figure shown here, Brasier et al. (2002) compare Schopf's original images ("b" and "c") with a larger view of the same material.

The "fossils" look much more like inorganic inclusions that just happen to resemble strings of bacteria, according to Brasier. A debate between Martin Brasier and Bill Schopf took place in April 2002 and it was widely perceived to have resulted in victory for Brasier. The "fossils" aren't fossils.

A report in Nature presented the bottom line (Dalton, 2002).
The textbooks say that oxygen-producing microorganisms evolved some 3.5 billion years ago. But as that claim and its author come under attack, the history of life on Earth may have to be rewritten...

Supporters and critics of Schopf alike describe him as a driven and tenacious character — nicknamed 'Bull' Schopf by some — whose energy and enthusiasm has done much to raise the profile of micropalaeontology, and to draw funding into the field. "He has a driving ambition to be in the limelight, and he doesn't like to admit he's wrong," says one former colleague. But these traits have led Schopf into conflict with his collaborators on at least one previous occasion.
A similar piece in Science helps drive the point home (Kerr, 2002).
The search for fossils in rocks formed before the Cambrian explosion of life 540 million years ago "has been plagued by misinterpretation and questionable results," leading paleontologist William Schopf of the University of California, Los Angeles (UCLA), once noted. Now Schopf's own claim for the oldest known fossils--fossils that have entered textbooks as the oldest ever found--is under attack as a misinterpretation of intriguingly shaped but purely lifeless minerals.

A paper in this week's issue of Nature argues that the microscopic squiggles in a 3.5-billion-year-old Australian chert are not fossilized bacteria, as Schopf claimed in a 1993 Science paper (30 April 1993, p. 640), but the curiously formed dregs of ancient hot-spring chemistry. "There's a continuum [of putative microfossils] from the almost plausible to the completely ridiculous," says lead author Martin Brasier, a micropaleontologist at the University of Oxford, U.K. "Our explanation is that they are all abiogenic artifacts."

If true, the analysis calls into question the fossil record of life's first billion years. It would also raise doubts about the judgment of Schopf, the man chosen by NASA to set the standard for distinguishing signs of life from nonlife at the press conference unveiling martian meteorite ALH84001 (Science, 16 August 1996, p. 864). But Schopf says that such speculation is unwarranted. "I would beg to differ" with Brasier's interpretation, he says. "They're certainly good fossils."
The latest paper by Pinti et al. (2009) extends earlier observations of the Apex chert that re-interpret it as a hydrothermal vent. Temperatures reached 250° during formation of the vent and the alternation between molten and cooler forms of material was not conducive to life. Furthermore, deposits of iron oxides and clay minerals could be mistaken for microfossils .

Organic Traces of Early Life?


One of the early signatures of life is trace organic matter. In theory, it is possible to distinguish between organic molecules that form by chemical processes and organic molecule that are synthesized by living organisms. The key is the ratio of the two isotopes of carbon; 12C and 13C. The common isotope is 12C and living organisms preferentially incorporate 12C when they synthesize carbohydrates, lipids, and other molecules of life.

The result is that organic molecules made in cells have a smaller percentage of the heavy isotope, 13C. The presence of "lighter" organic molecules is evidence of life—or so the story goes.

Even this evidence of early life is being challenged. For example, a review of the evidence for life in the 3.7 billion year old rocks of western Greenland points out two potential problems (Fedo et al., 2006). First, the material has probably been misidentified—it is not what it was claimed to be. Recent evidence suggests that the rocks are igneous, not sedimentary. Secondly, the isotope ratios may not be accurate and/or they can be explained by non-biological processes. Isotope ratios are not an unambiguous indication of life.

These problems, and others, with the Akilia rocks of western Greenland have been known for many years. They were discussed in a hard-hitting Nature News and Views article by Stephen Moorbath in 2005. You may not understand the technical details (I don't) but there's no mistaking the tone when Moorbath says ...
This persuasive discovery seems an almost inevitable, yet highly problematic, consequence to the increasing scientific doubts about the original claim. We may well ask what exactly was the material originally analysed and reported? What was the apatite grain with supposed graphite inclusions that figured on the covers of learned and popular journals soon after the discovery? These questions must surely be answered and, if necessary, lessons learned for the more effective checking and duplication of spectacular scientific claims from the outset.

To my regret, the ancient Greenland rocks have not yet produced any compelling evidence for the existence of life by 3.8 billion years ago. The reader is reminded that another debate on early life is currently in progress on 3.5-billion-year-old rocks in Western Australia, where chains of cell-like structures, long identified as genuine fossils10, have recently been downgraded by some workers11 to the status of artefacts produced by entirely non-biological processes. To have a chance of success, it seems that the search for remnants of earliest life must be carried out on sedimentary rocks that are as old, unmetamorphosed, unmetasomatized and undeformed as possible. That remains easier said than done. For the time being, the many claims for life in the first 2.0–2.5 billion years of Earth's history are once again being vigorously debated: true consensus for life's existence seems to be reached only with the bacterial fossils of the 1.9-billion-year-old Gunflint Formation of Ontario12.
There's another, potentially more serious, problem with using isotope ratios as evidence of early life. Gérard et al. (2009) have recently documented the presence of modern bacteria in drillcore samples of rocks that are 2.7 billion years old. They detected trace amounts of ribosomal RNA that were sufficient to identify more that ten diverse species of bacteria living in these subsurface formations.

If modern bacteria can invade and colonize ancient rocks then it's highly likely that more ancient bacteria can also live in ancient rocks. Over the course of millions of years, these colonizers can leave traces of organic molecules. But those molecules do not show that life existed in those places at the time when the rocks were formed. In other words, just because you have "light" organic molecules in rocks that are billions of years old does not mean that the cells that created those molecules lived billions of years ago.

The conclusion of the Gérard et al. (2009) paper is worth quoting,
Our results strongly suggest that contemporary bacteria inhabit what are generally considered exceptionally well-preserved subsurface Archaean fossil stromatolites of the Hamersley Basin, Western Australia. They are possibly in very low numbers, their distribution confined to microfractures where water may circulate (perhaps only intermittently), and their metabolic activities might be extremely low. However, upon geological timescales spanning 2.7 Gy, even such low cell numbers must have contributed significantly to the pool of biogenic signatures associated to these rocks, including microfossils, biological isotopic fractionation and lipid biomarkers. Although our results do not necessarily invalidate previous analyses, they cautiously question the interpretation of ancient biomarkers or other life traces associated to old rocks, even pristine, as syngenetic biogenic remains when bulk analyses are carried out.
What does all this tell us about early life? It tells us that the evidence for life before 3 billion years ago is being challenged in the scientific literature. You can no longer assume that life existed that early in the history of Earth. It may have, but it would be irresponsible to put such a claim in the textbooks without a note of caution.

What else does this story tell us? It tells us something about how science is communicated to the general public. The claims of early life were widely reported in the media. Every new discovery of trace fossils and trace molecules was breathlessly reported in countless newspapers and magazines. Nobody hears about the follow-up studies that casts doubt on those claims. Nobody hears about the scientists who were heroes in the past but seem less-than-heroic today.

That's a shame because that's how science really works. That's why science is so much fun.


Brasier, M.D., Green, O.R., Jephcoat, A.P., Kleppe, A.K., Van Kranendonk, M.J., Lindsay, J.F., Steele, A., and Grassineau, N.V. (2002) Questioning the evidence for Earth's oldest fossils. Nature 416::76-81. [PubMed]

Dalton, R. (2002) Microfossils: Squaring up over ancient life. Nature 417:782-784. [doi:10.1038/417782a]

Fedo, C.M., Whitehouse, M.J. and Kamber, B.S. (2006) Geological constraints on detecting the earliest life on Earth: a perspective from the Early Archaean (older than 3.7 Gyr) of southwest Greenland. Phil. Trans. R. Soc. B 361:851-867. [doi: 10.1098/rstb.2006.1836]

Gérard, E., Moreira, D., Philippot, P., Van Kranendonk, M.J., and López-García, P. (2009) Modern Subsurface Bacteria in Pristine 2.7 Ga-Old Fossil Stromatolite Drillcore Samples from the Fortescue Group, Western Australia. PLoS ONE 4: e5298. [doi:10.1371/journal.pone.0005298]

Pinti, F.L., Mineau, R., and Clement, V. (2009) Hydrothermal alteration and microfossil artefacts of the 3,465-million-year-old Apex chert. Nature Geoscience 2:640-643. [doi: 10.1038/ngeo601]

Schopf, J.W. (1993) Microfossils of the Early Archean Apex chert: new evidence of the antiquity of life. Science 260:640-646. [PubMed]



Gérard, E., Moreira, D., Philippot, P., Van Kranendonk, M., & López-García, P. (2009). Modern Subsurface Bacteria in Pristine 2.7 Ga-Old Fossil Stromatolite Drillcore Samples from the Fortescue Group, Western Australia PLoS ONE, 4 (4) DOI: 10.1371/journal.pone.0005298

Pinti, D., Mineau, R., & Clement, V. (2009). Hydrothermal alteration and microfossil artefacts of the 3,465-million-year-old Apex chert Nature Geoscience, 2 (9), 640-643 DOI: 10.1038/ngeo601

18 comments :

  1. Larry,

    How long will it take for science to realize that all the "iron clad" evidence for life before 6000 years is a big Lie?

    :)

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  2. It's funny how the big numbers stick in your brain. Funny thing for me is: I'm pretty sure I'd heard rumblings of *both* these disputes, but somehow it didn't quite go home that without these two, you're back to Gunflint and 1.9 billion... If anyone hadda asked me 'How old's the first life' I might have hedged at something around or past 3 billion, mentioned there's some squabbling over the validty of the oldest... But it's dead cert I wouldn't have come out with 'Well, 1.9 billion is the oldest widely-recognized find...' Wouldn't have even occurred to me.

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  3. I heard Schopf speak back in the '90's. He is an excellent speaker and gave the impression of very broad knowledge.

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  4. Fantastic post, maybe your best ever. I had no idea there was so much controversy over those fossils. All this time I have been looking at them in class and thinking, "This is it?, this picture shows early life? Am I not seeing something, it looks like it could be anything" Once again I read Sandwalk and learn a valueble lesson about taking things at face value.

    Thanks for the great post.

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  5. "What else does this story tell us? It tells us something about how science is communicated to the general public. The claims of early life were widely reported in the media. Every new discovery of trace fossils and trace molecules was breathlessly reported in countless newspapers and magazines. Nobody hears about the follow-up studies that casts doubt on those claims. Nobody hears about the scientists who were heroes in the past but seem less-than-heroic today."

    "That's a shame because that's how science really works. That's why science is so much fun."

    It also gives the lie (yet again) to creationists who posit a scientific conspiracy to bring out only the evidence in favor of evolution. The creationist worldview surely doesn't include scientists working hard to show supposed early evidence of life is wrong.

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  6. The science books that go towards education should then state that the clearest sign of bacterial life has been found to have existed 1.9 billion years ago with many investigations that could put the date as far back as 3.5 billion years.

    Start with what is certain and then tack on what is possible or probable (within context). I think that approach opens the doors to those that may want to jump onto that particular field and attempt to tackle the problem.

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  7. Thank you.

    That's a good end line, about what it is that makes science so wonderful, attached to a story I found interesting, but one I have no way of knowing anything about.

    I'm like you... I wasn't clear on what he wrote, but your ending was magic.

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  8. Larry, have you seen this relatively recent paper also in Nature Geoscience. It supports a life at 3.5byr scenario

    Hoashi, M. et al. (2009) Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geoscience, 2, 301 - 306.

    the abstract:

    The timing of the origin of photosynthesis on the early Earth is greatly debated. It is generally agreed, on the basis of the presence of biological molecules found in shales from the Hamersley Basin, Australia, that oxygenic photosynthesis had evolved 2.7 billion years (Gyr) ago. However, whether photosynthesis occurred before this time remains controversial. Here we report primary haematite crystals and associated minerals within the marine sedimentary rocks preserved in a jasper formation of the Pilbara Craton, Australia, which we interpret as evidence for the formation of these rocks in an oxygenated water body 3.46 Gyr ago. We suggest that these haematite crystals formed at temperatures greater than 60 °C from locally discharged hydrothermal fluids rich in ferrous iron. The crystals precipitated when the fluids rapidly mixed with overlying oxygenated sea water, at depths greater than 200 m. As our findings imply the existence of noticeable quantities of molecular oxygen, we propose that organisms capable of oxygenic photosynthesis evolved more than 700 million years earlier than previously recognized, resulting in the oxygenation of at least some intermediate and deep ocean regions.

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  9. Following up on SteveF's comment, Chris Rowan at HighlyAllochthonous has a rather long post about that paper. Very interesting.

    http://scienceblogs.com/highlyallochthonous/2009/09/free_oxygen_on_the_archean_ear.php

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  10. I see Divalent beat me to part of this post, but here goes.

    It does seem that exuberance may be a bit of a problem in overstating when first life appeared. The 1.9 Ga Gunflint Formation may be the most definitively recognized fossil evidence of early life. Nonetheless, the general attitude of most geologists and paleontologists is that there is ample circumstantial evidence that the earliest life forms could very well have formed long before that, and that geochemistry and mineralogy were extremely important factors in the origin of life. Volcanoes were abundant in the Archean, providing feldspar-rich rocks which were readily weathered to form clay minerals and zeolites that provide catalytic surfaces for organic molecules which could easily have been produced by lightning from intense volcanic activity. Geologists see no need to invoke “seeding” from comets as the only mechanism for the origin of life on earth. Check out Elements: An International Magazine of Mineralogy, Geochemistry, and Petrology. Special Issue: Rocks, Minerals, and the Geochemical Origin of Life (V. 1, No. 3, June 2005) [This should be available at most university geological libraries]. Here are the articles:

    Cairns-Smith, A. G.: Sketches for a Mineral Genetic Material.

    Cody, G. D.: Geochemical Connections to Primitive Metabolism.

    Ferris, J. P.: Mineral Catalysis and Prebiotic Synthesis: Montmorillonite-Catalyzed Formation of RNA.

    Hazen, R. M.: Genesis: Rocks, Minerals, and the Geochemical Origin of Life.

    Smith, J. V.: Geochemical Influences on Life’s Origins and Evolution. Elements.

    For a geoscience perspective on oxygen in the Archean, check out what blogger Chris Rowan has to say concerning Haoshi’s paper. The mineralogical and textural evidence (especially for geologists) seems to strongly indicate that hematite (in this particular geological environment) formed as the primary sedimentary mineral and has not been altered, which is an extremely important implication for origins of life research:

    http://scienceblogs.com/highlyallochthonous/

    Hoashi, M., Bevacqua, D., Otake, T., Watanabe, Y., Hickman, A., Utsunomiya, S., & Ohmoto, H. (2009). Primary haematite formation in an oxygenated sea 3.46 billion years ago Nature Geoscience DOI: 10.1038/NGEO465

    WKM

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  11. Another recent paper worth pointing out in this debate relates directly to the Apex material and argues for a biogenic origin:

    De Gregorio, B.T. et al. (2009) Biogenic origin for Earth's oldest putative microfossils. Geology, 37, 631-634.

    Carbonaceous microbe-like features preserved within a local chert unit of the 3.5 Ga old Apex Basalt in Western Australia may represent some of the oldest evidence of life on Earth. However, the biogenicity of these putative microfossils has been called into question, primarily because the sample collection locality is a black, carbon-rich, brecciated chert dike representing an Archean submarine hydrothermal spring, suggesting a formation via an abiotic organic synthesis mechanism. Here we describe the macromolecular hydrocarbon structure, carbon bonding, functional group chemistry, and biotic element abundance of carbonaceous matter associated with these filamentous features. These characteristics are similar to those of biogenic kerogen from the ca. 1.9 Ga old Gunflint Formation. Although an abiotic origin cannot be entirely ruled out, it is unlikely that known abiotic synthesis mechanisms could recreate both the structural and compositional complexity of this ancient carbonaceous matter. Thus, we find that a biogenic origin for this material is more likely, implying that the Apex microbe-like features represent authentic biogenic organic matter.

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  12. A billion years here and a billion years there and pretty soon you are talking about real difference :-)

    Seriously though, when I was a student well before Schopf's 1993 paper, we were taught "life on Earth arose sometime 2-3 BYA". So even with Schopf, the difference between solid and totally out of your mind is only two-fold...

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  13. Does not compute:
    "oldest positive signs of life in the 1.9 Ga Gunflint"

    and

    "modern bacteria in 2.7 Ga stromatolites"

    Unless you're an abiogenic stromatolite person, your oldest stromatolite is your oldest life- that's the 3.43 Ga Warrawoona.

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  14. First, the material has probably been misidentified—it is not what it was claimed to be. Recent evidence suggests that the rocks are igneous, not sedimentary.

    RE: Fredo et al., 2006

    Maybe not. Manning et al.*, 2006, have put up a rather spirited defense of the chemical sedimentary origin and age of the supracrustal rocks used to obtain the apatite (graphite) originally evaluated as representative of an early life proxy.

    *including Steve Mojszis as second author, who's 1996 paper (Mojzsis et al., 1996) started the 3.7-3.8 bya early-life talk.

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  15. Larry writes: "The claims of early life were widely reported in the media. Every new discovery of trace fossils and trace molecules was breathlessly reported in countless newspapers and magazines. Nobody hears about the follow-up studies that casts doubt on those claims. Nobody hears about the scientists who were heroes in the past but seem less-than-heroic today."

    No so, Larry! I covered the same main points you make here (the challenges to Schopf's microfossils, and doubts over Greenland isotopic evidence - and stating the same conclusion, that the Gunflint Chert at 1.9 Ga was the only evidence *everyone* could still agree on unequivocally) in a feature in the popular science magazine New Scientist in *2003*. That magazine has a readership of 750 000 lay people around the world. So "nobody hears about the follow-up studies" really is a sweeping generalisation, worthy of a tabloid journalist!

    The media love a controversy - if the recent challenges to previously-held orthodoxy have not been widely reported, it is more likely because those publishing those papers have not widely press released their work. It's all to easy to blame "the media", but we as researchers have a responsibility to initiate communication with them if we feel we have something that the wider world needs to know.

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  16. Thanks for this insight into fossils. It really is surprising how we take in all the information and never really care whether the data has been refuted. I'm glad you brought up this problem.

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