When did life first appear?

The Earth’s rock record begins around 3.8 billion years ago with a period that is known as the Archaean.  The Archean can be split into different eras, where the early Archaean extends from 3.8 to 3.6 billion years ago, the Paleoarchaean extends from 3.6 to 3.2 billion years ago, the Mesoarchaean extends from 3.2 to 2.8 billion years ago, and the Neoarchaean extends from 2.8 to 2.5 billion years ago.  At this point, a new period known as the Proterzoic begins and it will extend all the way until about 550 million years ago, the time of the Precambrian.

I have long assumed that life appeared on this planet approximately 3.5 billion years ago.  But lately, the evidence for such ancient life seems to be evaporating.  The crown jewel among the ancient microbial fossils has been the filamentous cyanobacteria from 3.5-billion-old Australian chert that were first described by William Schopf from UCLA.  Yet in 2002, Martin Brasier and colleagues made a strong case that those fossils are not remnants of living things, but represent the activity of ancient and exotic geochemical processes [1].  And a recent study has just confirmed these are not fossils (HT to Joe):

Twenty years ago the palaeontological community gasped as geoscientists revealed evidence for the oldest bacterial fossils on the planet. Now, a report in Nature Geoscience shows that the filament structures that were so important in the fossil descriptions are not remnants of ancient life, but instead composed of inorganic material.

This finding is an important reminder that the evidence for such ancient life is not as strong as I had assumed:

Their finding is not stirring feelings of jubilation. “After nearly 30 years of effort at pushing evidence for life to or beyond 3.5 billion years ago, we are reminded that the ancient record is more fraught with complications than we ever thought,” says geologist Stephen Mojzsis at the University of Colorado, Boulder.

Olcott Marshall suggests that this carbonaceous material may have been accidentally sampled by other research teams and played a part in them identifying the filaments as biological. Brasier disagrees, saying instead that people wanted to find life so badly that they ignored the obvious. “There is a willful blindness about these structures that sometimes has more to do with local politics than global truth,” he says.

Then there was another study that came out 3 years ago that also erased some evidence for ancient biotic processes (HT to Valery):

Whatever their origin, the biomarkers must have entered the rock after peak metamorphism 2.2 Gyr ago and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78–1.68 Gyr ago and 2.15 Gyr ago, respectively. Our results eliminate the evidence for oxygenic photosynthesis 2.7 Gyr ago and exclude previous biomarker evidence for a long delay (300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45–2.32 Gyr ago.

Perhaps Stephen Moorbath, from the Department of Earth Sciences at Oxford University, says it best:

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 Ontario. [2]

Or, from another review of the fossil evidence:

In general, the most ancient microfossils are dated as follows: (a) the indisputable remains of cyanobacteria found in the Belcher Supergroup are about 2.0 billion years old; (b) the possible microfossils of early eukaryotes found in the Negaunee Iron Formation are slightly older than 1.9 billion years; (c) the very probable remains of cyanobacteria found in the Transvaal Supergroup are 2.52–2.55 billion years old; (d) possible remains of cyanobacteria and heterotrophic bacteria are 2.69–2.76 billion years old (the Fortescue Series) or even 3.4–3.5 billion years old (the Onverwacht, Fig Tree, and Warrawoon Series). [3]

Anyone who has read my book might recognize the distinctions between the possible, the plausible, and the probable.  That is, the safe date for life’s first appearance is about 2 billion years ago, things become probable around 2.5 billion years ago, but once we get beyond that, we enter the realm of the possible.

So while I am not expert on this subject, nor can I claim to following this area of research closely, I must say that I have begun to take a more tentative stance on life’s first appearance.  Is there any solid evidence that life is really older than 2.5 billion years?  And if so, how do we know that evidence will not evaporate in the next decade or so?

1. Brasier MD, Green OR, Jephcoat AP, Kleppe AK, Van Kranendonk MJ, Lindsay JF, Steele A, Grassineau NV. 2002. Questioning the evidence for Earth’s oldest fossils. Nature 416:76-81.

2. Moorbath, S. 2005. Dating earliest life. Nature 434: 155-156.

3. Sergeev VN, Gerasimenko LM, Zavarzin GA. 2002. Proterozoic history and present state of cyanobacteria.  Microbiology 71: 623-637.

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7 responses to “When did life first appear?

  1. Valery Anisimov

    Mike, I think that on the time been one of the most professional description of this problem may be found in the Schopf’s paper “Fossil evidence of Archaean life”: http://rstb.royalsocietypublishing.org/content/361/1470/869.full.pdf
    One interesting thing which may be discovered from this work is that it looks like that nearly 3.500 millions years ago it was some “Archaean explosion” in some points similar to the later “Cambrian explosion”. Before this date any traces of life practically absent but immediately after this point we can see numerous signs of life of different nature (microfossils, stromatolites, rocks with increased light carbon level). So per example if we are thinking about panspermia option this time may be a very good candidate for the Earth “infection”.

  2. Yes, there is a very large body of evidence from rocks older than 2.5 billion years — stromatolites (from at least 50 different geologic units), many hundreds of individual microfossils, more than a thousand carbon and sulfur isotopic analyses, and biologically complex organic biomarkers — indicating that microbial life was flourishing on Earth at least as early as 3.5 billion years ago (with hints of life at 3.8 billion years).

    There is so much evidence of life earlier than 2.5 billion years, gathered around the world by nearly 100 scientists from some 20-30 countries over the past 40 years (since 1965) — and the evidence is so very similar to/essentially identical with that known from rocks 2.5 to 0.5 billion years in age — that there is not the remotest possibility that the evidence will ever “evaporate” — not “in the next decade or so,” not ever.

  3. Gts,

    Larry Moran weigs in</b

    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.

    he then says:

    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.

    And this is from a guy who you would think would want there to be evidence for cellular organisms 3 billion + yrs ago.

  4. I would think that people who favor abiogenesis would prefer a longer period of time for life to develop from non-life.

    On the other hand, I think a shorter period of time from the appearance of prokaryotes to eukaryotes would fit in better with FLE.

  5. Cynic that I am, I think people who favor abiogenesis in the sense of ‘it was not design’ will be happy either way.

    If it’s a shorter period of time, then clearly the appearance of life must be easy, non-special.

    If it’s a longer period of time, then clearly the appearance of life must be difficult, a freak accident.

  6. Joe, sometimes the structures that are looked at are “entrapped” in chert – which is a chemical precipitate with tiny interlocking grains. Other types of sediments with large pore spaces, fractures, or planes of potential microbe intrusion between detrital layers definitely have the potential to include younger organisms or at least organic products from them. But that appears not to be the case for primary cherts.

    Many cherts are within a sequence that has been well dated as being Archean and the cherts themselves are considered to be primary deposits (in other words, that they did not form after the sediments above them). And many such deposits are known from the Precambrian.

  7. Valery Anisimov

    I can notice that “hints of life at 3.8 billion years” looks more and more doubtful. See per example:
    The Akilia Controversy: field, structural and geochronological evidence questions interpretations of >3.8 Ga life in SW Greenland:
    http://jgs.lyellcollection.org/cgi/content/abstract/166/2/335
    or
    Reassessing the evidence for the earliest traces of life:
    http://www.nature.com/nature/journal/v418/n6898/full/nature00934.html

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