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.

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.

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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]

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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