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Life’s Persistence through the Bombardment

None of us would have wanted to be around during the Late Heavy Bombardment, that frenetic bashing of our planet as the young Solar System worked out its debris problems between 4.1 and 3.8 billion years ago. The Hadean period was a time when enormous asteroids pummeled our world over a span lasting as long as 200 million years, an ongoing series of events one would have assumed lethal for whatever organisms may have evolved by then.

But was the Late Heavy Bombardment really the deadly rain we’ve always assumed? A new paper in Nature questions the idea, basing its results on computer modeling of the Earth’s heating during the bombardment. Oleg Abramov and Stephen J. Mojzsis (University of Colorado) argue that our planet’s surface would likely have been sterilized during this period, but microbial life below the surface or in underwater conditions would almost certainly have survived.

“Our new results point to the possibility life could have emerged about the same time that evidence for our planet’s oceans first appears,” said Mojzsis, principal investigator of the project.

If so, it’s conceivable that life has been through a continuous process of development here, rather than one marked by sudden extinctions and re-starts. The theory is given weight by our growing understanding of the role of hydrothermal events deep beneath the oceans, where early life might well have lingered. Abramov thinks the finding has implications for life elsewhere:

“Even under the most extreme conditions we imposed on our model, the bombardment could not have sterilized Earth completely. Our results are in line with the scientific consensus that hyperthermophilic, or ‘heat-loving,’ microbes could have been the earliest life forms on Earth, or survivors from an even more ancient biosphere. The results also support the potential for the persistence of microbial biospheres on other planetary bodies whose surfaces were reworked by the bombardment, including Mars.”

It takes but a glance at the Moon through a small telescope to see the extent of the bombardment on a surface that has not, because of internal activity, resurfaced itself in billions of years. And it’s worth speculating, as we look at nearby stars like Tau Ceti, that the abundance of impactors that may be present in a system like this may not preclude the continuing development of life. That’s a conclusion that’s cheering to this writer, for life once started seems determined to last.

The paper is Abramov and Mojzsis, “Microbial Habitability of the Hadean Earth during the Late Heavy Bombardment,” Nature 459 (21 May 2009), pp. 419-422 (abstract).

Comments on this entry are closed.

  • Adam May 21, 2009, 17:55

    Arthur C. CLarke and Stephen Baxter imagined intelligent beings in Earth’s early days before the LHB, in “The Light of Other Days”, who preserved a sample of what life they could (i.e. hyperthermophiles) in the face of an impending giant comet bombardment. Does make you think: what would we do if faced with a giant comet?

  • David May 21, 2009, 22:32

    Tau Ceti has a Kuiper Belt that is much more massive (> 10 times) than the one
    around our solar system, is that why you mentioned it ?

  • Administrator May 22, 2009, 8:14

    David, yes, I mentioned Tau Ceti because of the evidence for heavy bombardment within the system suggested by that dense outer belt. It’s interesting to think that heavy bombardment doesn’t necessarily rule out life.

  • Ronald May 22, 2009, 8:37

    Ref. Tau Ceti: it has, however, been suggested, that, because of it’s (very) low metallicity (estimates vary from 22 – 74 % of solar, but most and most recent ones in my knowledge tend toward the lower end of this range), TC may not ven have a true planetary system at all, but just a failed one, consisting of planetesimals and dust. This what may have been observed. Future observation will tell.

  • Athena Andreadis May 22, 2009, 10:31

    Bacteria as a group are virtually impossible to eradicate. I suspect that many planets will have bacterial life, even in hostile conditions. The evidence is strong that once the life we know arose on Earth, it was never interrupted. However, that does not negate the multiple extinction events that claimed more complex life forms. The bell tolled, all right — just not for everyone present at each tolling!

  • bigdan201 May 23, 2009, 12:12

    i agree that microbial life is relatively common in the universe. id say that the more complex a lifeform is, the less common it is.

    weve found extremophiles on earth than can live anywhere from hot geysers, to arctic ice, to radioactive nuclear waste sites. considering that, microbial life on a planet like venus wouldnt surprise me. and oceans are a very good environment for life – for one thing, it would provide natural defense from radiation or cosmic bombardment.

  • Ron S May 24, 2009, 14:38

    bigdan201: “i agree that microbial life is relatively common in the universe. id say that the more complex a lifeform is, the less common it is.”

    Not necessarily. Once you get a lifeform that is so complex that it is able to adapt itself to a wide range of environments it may become very common. For example, humans. When we successfully get off-planet we could become as common as bacteria in the universe.

    Maybe “commonality” looks a bit like an inverted bell curve when plotted against complexity (for a suitable definition of complexity).

  • ljk May 26, 2009, 9:34

    Ron S. said:

    “When we successfully get off-planet we could become as common as bacteria in the universe.”

    Which as Fermi once famously asked, if this is the case, where are all the other
    “bacteria”? Or is life on Earth the result of a visit from long ago and we just
    beginning to figure this out now?

  • amphiox May 28, 2009, 20:45

    Ron S – I understand your argument, but we humans will never be as common as bacteria simply because each one of us carries several trillion of them around with us all the time.

  • Ron S May 29, 2009, 22:20

    I didn’t equate commonality with quantity of organisms, but rather with the diversity of places they can be found.

  • ljk June 24, 2009, 15:42

    The evidence for and against astronomical impacts on climate change and mass extinctions: A review

    Authors: C.A.L. Bailer-Jones (Max Planck Institute for Astronomy, Heidelberg)

    (Submitted on 24 May 2009 (v1), last revised 23 Jun 2009 (this version, v2))

    Abstract: Numerous studies over the past 30 years have suggested there is a causal connection between the motion of the Sun through the Galaxy and terrestrial mass extinctions or climate change.

    Proposed mechanisms include comet impacts (via perturbation of the Oort cloud), cosmic rays and supernovae, the effects of which are modulated by the passage of the Sun through the Galactic midplane or spiral arms.

    Supposed periodicities in the fossil record, impact cratering dates or climate proxies over the Phanerozoic (past 545 Myr) are frequently cited as evidence in support of these hypotheses. This remains a controversial subject, with many refutations and replies having been published.

    Here I review both the mechanisms and the evidence for and against the relevance of astronomical phenomena to climate change and evolution. This necessarily includes a critical assessment of time series analysis techniques and hypothesis testing.

    Some of the studies have suffered from flaws in methodology, in particular drawing incorrect conclusions based on ruling out a null hypothesis. I conclude that there is little evidence for intrinsic periodicities in biodiversity, impact cratering or climate on timescales of tens to hundreds of Myr.

    Furthermore, Galactic midplane and spiral arm crossings seem to have little or no impact on biological or climate variation above background level. (truncated)

    Comments: 51 pages, 7 figures, 140 references. To appear in the International Journal of Astrobiology. For hyperref version with full resolution figures see this http URL

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Galaxy Astrophysics (astro-ph.GA); Populations and Evolution (q-bio.PE)

    Cite as: arXiv:0905.3919v2 [astro-ph.EP]

    Submission history

    From: Coryn Bailer-Jones [view email]

    [v1] Sun, 24 May 2009 20:21:24 GMT (404kb,D)

    [v2] Tue, 23 Jun 2009 11:54:46 GMT (405kb,D)


  • ljk August 20, 2009, 23:03

    Research Reveals Major Insight Into Evolution Of Life On Earth

    Los Angeles CA (SPX) Aug 21, 2009

    Humans might not be walking on Earth today if not for the ancient fusing of two microscopic, single-celled organisms called prokaryotes, NASA-funded research has found.

    By comparing proteins present in more than 3000 different prokaryotes – a type of single-celled organism without a nucleus – molecular biologist James A. Lake from the University of California at Los Angeles’ Center for Astrobiology showed that two major classes of relatively simple microbes fused together more than 2.5 billion years ago.

    Lake’s research reveals a new pathway for the evolution of life on Earth. These insights are published in the Aug. 20 online edition of the journal Nature.

    Full article here: