How do we know Gabon’s ‘multicellular’ fossils are 2.1 billion years old?

ResearchBlogging.orgA post by Chris RowanThe fossil record prior to 550 million years ago is so patchy that every discovery is going to cause some fanfare. That is certainly case with these odd looking things, which have been proclaimed in Nature as the oldest mulitcellular organisms ever found.

Gabon_multicell_fossil.jpg
A 2.1 billion year old fossil atop the bed it was preserved in. Source: Albani et al., Fig. 2

These flat, dish-like fossils are found at a number of horizons within a black shale unit of the Francevillian Group in southeast Gabon. They grew in a marine delta environment, and following a rapid burial event, sulphate-reducing bacteria got to work decomposing them. One by-product of sulphate reduction is pyrite, and as a result decomposition left a durable, mineralised impression of what was consumed.
To my admittedly inexpert eyes, these things do not immediately scream ‘multicellular!’ at me (in contrast to, say, the enigmatic Stirling fauna) – they look like some kind of bacterial mat. However, Albani et al. have used high resolution X-ray scans to reveal a complex internal structure, and what they argue is evidence of coordinated growth patterns, both of which suggest a higher degree of organisation than a bacterial colony [Update: Go and read Ed Yong's write-up for a number of semi-sceptical expert opinions].
The age of the Francevillian Group is given as 2.1 billion years (2100 million), and it is this great age that makes these fossils so potentially significant: if they are multicellular, then they are the oldest known large multicelluar organisms by a margin of about 200 million years (it’s not clear in the article, but I think that the Stirling fauna is what has been usurped). It seems not everyone is so impressed by this*, but it does push the possible origins of multicellularity back much closer to the oxygenation of the Earth’s atmosphere between 2400 and 2300 million years ago, which must have had a significant evolutionary impact on life. If this discovery pans out, a hypothesised connection between the two becomes slightly less tenuous.
Because precisely dating such old rocks is a tricky business, I was curious to know exactly how good the age constraints on the Francevillian Group were. As it turns out, they are actually pretty good. By a lucky happenstance, the unit just above the fossil-bearing layers has a zircon-bearing tuff which has been extremely precisely dated as forming 2080 million years ago. This provides a minimum age for the units below – they had to have already been there when the tuff erupted. Another minimum age comes from the Oklo uranium ore deposit, which is found at a slightly lower stratigraphic level, is pretty famous in its own right, and is known to have formed around 2050 million years ago. Because the ore body is the result of later mineral growth some time after deposition of the host rock, the host rock must be older than the ore. Diagenetic clays just below the fossil-bearing layers, which probably formed shortly after deposition, have also been less accurately dated at around 2100 million years ago.
A final chronological clue is provided by the observation that carbonate minerals within the fossil-bearing sequence have elevated levels of carbon-13 with respect to carbon-12. A similar trend (known as the Lomagundi excursion after the place in Zimbabwe it was first identified) has been found in sequences between 2220 and 2100 million years old from Africa, South America, North America, Scandanavia and Australia. Correlating to what seems to have been a global change in seawater chemistry therefore provides maximum and minimum estimates for when the fossil organisms lived and died. When combined with the other age constraints described above, particularly the age of the overlying tuff, an age towards the younger end of this interval seems to be the most reasonable interpretation, hence the 2100 million year estimate given by the authors.

Francevillian_chrono.png

So, whilst the significance of these Gabonese fossils can be debated, they are almost certainly around 2.1 billion years old.
*only people used to working in the Archean and Proterozoic can be blasé about such lengths of time. 200 million years from the present day, dinosaurs were still at the beginning of their reign, and all the continents were collected together into Pangaea, so it’s not an insignificant interval in terms of evolution or geology. Although to be fair to Daniel, he’s mainly responding to a media write-up that propagates the whole ‘nothing biologically interesting before the Cambrian – except THIS!’ trope.
Albani, A., Bengtson, S., Canfield, D., Bekker, A., Macchiarelli, R., Mazurier, A., Hammarlund, E., Boulvais, P., Dupuy, J., Fontaine, C., F?ºrsich, F., Gauthier-Lafaye, F., Janvier, P., Javaux, E., Ossa, F., Pierson-Wickmann, A., Riboulleau, A., Sardini, P., Vachard, D., Whitehouse, M., & Meunier, A. (2010). Large colonial organisms with coordinated growth in oxygenated environments 2.1‚ÄâGyr ago Nature, 466 (7302), 100-104 DOI: 10.1038/nature09166

Categories: fossils, geology, paper reviews, past worlds, Proterozoic

Comments (11)

  1. Ray Ingles says:

    While it would be exciting, I kinda almost hope these things aren’t multicellular. The whole “Great Filter” explanation for why we don’t see aliens out there implies there’s something that keeps intelligent life from spreading through the universe.
    The jump to multicellularity seemed promising, since the fact that it took almost three billion years to happen on Earth would argue that it was pretty unlikely. But if it actually happened much earlier, that’d make it more likely to have happened elsewhere, too…

  2. Daniel says:

    Hehe, yes, as you said… Its not the theory Im arguing about, its the way the news was presented in the newspaper. The large amount of actual errors is not ok for such a big newspaper.

  3. harold says:

    A rather obvious “great filter” that would keep us from interacting with alien civilizations even if there were any is the speed of light.
    We have no reason to think that anything resembling a space ship will ever be able to travel faster than the speed of light. Neither can any method of communication that we can envision for the forseeable future.
    Once eukaryotic cells exist (another potential “great filter”, perhaps), the “jump to multicellularity” is hardly very improbable.

  4. Andrew says:

    I’ve been looking at concretions for so long, brought to me by dozens of readers around the world, that I just can’t see the fancy fossil images in the paper as anything but concretions. If they have other evidence, like this bit about cell-wall sterols, then that is more persuasive than a pseudo-organic shape.

  5. Ray Ingles says:

    Harold – even if ships travel below light-speed, if the population is exponentially expanding, they should be here already. Look up the calculations behind the “Fermi Paradox”.

  6. Christopher Gwyn says:

    On another site the announcement/analysis seemed to suggest that if (if!!!!) this is an example of multi-cellular life that multi-cellular life may have gone extinct and that we, therefore, would be descended from the second evolution of multi-cellular life. How reasonable or unreasonable an interpretation of the data does that seem to be?

  7. Chris says:

    I think the fossils are highly significant and – along with the Stirling fauna, shore up confidence in eukaryotic multicellular organisms in the palaeoproterozoic. There are more pictures on the BBC and ScienceDaily websites that show the fossils in splendid detail and internal structure:
    http://news.bbc.co.uk/1/hi/science_and_environment/10471599.stm
    http://www.sciencedaily.com/releases/2010/06/100630171711.htm
    Great post, as usual Chris!

  8. Ed Yong says:

    @6 – Entirely reasonable, given that multicellularity has evolved independently many times.

  9. Adam F says:

    Faster than light travel or communication may be impossible, but the speed of light is no permanent barrier to interstellar communication. Even at our current technological state we are perfectly capable of sending and receiving radio and laser messages with nearby stars.

  10. Lab Lemming says:

    Re: 4.
    Andrew, the concretion hypothesis isn’t publishable in Nature. It is probably out there in the literature somewhere, probably in a decades-old paper, quite possibly in French. But there is no Nature Publishing Group marketing muscle behind it, so chances are it has never been blogged about.
    Personally, my hypothesis is that this is a fossil of the intelligent multicellular life form that built the Oklo reactors before blowing themselves off the planet.

  11. Chris Rowan says:

    Andrew @4: The sulphur isotopes, and the presence of pyrite, seem to me to be fairly convincing evidence that the mineralisation is due to the bacterially mediated decomposition of something biological in origin, rather than a largely inorganic precipitation as is the case with a concretion. Going beyond that is where it seems you move into more speculative realms.
    Lab Lemming @10: You’re right: finding these things in proximity to a ‘natural’ nuclear reactor is just to big of a coincidence to ignore ;-)