Late Heavy Bombardment Intensity and the Origin of Life

Late Heavy Bombardment Intensity and the Origin of Life

In previous editions of Today’s New Reason to Believe my colleagues Jeff Zweerink and Fuz Rana reported on a new computer simulation that indicated the Late Heavy Bombardment (LHB) may not have been as damaging as scientists once thought.1 The simulation demonstrated that rather than Earth’s entire surface transforming into a lava ocean, there may have existed refuges of liquid water where temperatures fell a little below 230° F (110° C). Such sanctuaries would’ve permitted the survival of hyperthermophiles (bacterial species capable of tolerating temperatures a little above water’s boiling point).

The LHB rendered the entire planet surface sterile of all possible life and subterranean layers sterile of all mesophiles (organisms that thrive at moderate temperatures, typically between 50° and 105° F or 10° and 40° C). Nonetheless, as the new simulation shows, it may not have been a total sterilization event. This possibility has raised hopes within the origin-of-life research camp that the maximum time window for the beginning of life could be moved from between the end of LHB and the earliest chemical signatures for life (just a few million years) to perhaps as much as several hundred million years. This increased time span allows for a much greater chance for naturalistic evolutionary processes to bring about an origin of life.

Like any new scientific claim that challenges previous understanding, the LHB simulation needs to be tested. Simulations are theoretical. For one to have any scientific credibility it must be affirmed by observations. The team performing this particular simulation cited terrestrial zircons with various dates (between 4.38 and 3.85 billion years old) as indications of the presence of liquid water conditions–at least at the individual times and geographical locations in which the zircons were found.2 (They also cited the craters on the Moon, Mercury, and Mars as evidence for the LHB. These solar system bodies still bear the marks of bombardment while Earth’s aggressive geological activity has erased any sign of the event.)

These few zircons, plus high-temperature-resistant rocks3 found near Great Slave Lake in Canada and dated no earlier than 4.03 billion years ago (bya), are the only known terrestrial survivors from the era dating previous to 4.0 bya. But this argues against the hypothesis that several hundred million years were available for the origin of life. If indeed liquid water continually existed on and in the crust of Earth from 4.4 to 3.83 bya (when the first undisputed isotope signature of life appeared),4 then scientists should possess more than just the few high-temperature-resistant zircons and rocks.

As Fuz and I described in our book, Origins of Life, the existence of liquid water conditions within a few limited refuges at intermittent times throughout 4.38 to 3.85 bya provides a superior explanation for the zircon and rock remains.5 This scenario leaves open the possibility that God intervened every time, or nearly every time, liquid water was present on Earth to create life. When that life was destroyed by a bombardment event, God simply waited for the liquid water to reappear to create life again. (This is why we used the word “origins”–as opposed to “origin”–in our book title.) In More Than a Theory, I suggest that God might have chosen this repeated origins-of-life strategy as a tool to jumpstart the chemical transformation of Earth’s atmosphere.6

The repeated origins-of-life scenario even more seriously challenges naturalistic explanations for where life came from. Instead of needing an explanation for just one rapid emergence of life from nonlife, naturalists would now need to explain multiple rapid emergences of life.

All naturalistic models for the emergence of life possess four essential requirements:

1. a large, concentrated prebiotic soup;
2. prebiotic sugars that are 100 percent right-handed;
3. prebiotic amino acids that are 100 percent left-handed;
4. adequate time for the sugars and nucleotides to naturally assemble into the necessary RNA and DNA molecules and for the amino acids to naturally assemble into needed proteins. 

Carbon, nitrogen, and sulfur isotope ratios fail to provide any undisputed evidence that Earth ever possessed even a diluted prebiotic soup. This lack of evidence is due to the oxygen-ultraviolet paradox. Even a tiny amount of oxygen in Earth’s environment will halt prebiotic chemistry. However, without a large amount of oxygen in Earth’s environment, intense ultraviolet radiation will stream in from the Sun, and will, likewise, stop any prebiotic chemistry.

For 40 years geologists and astronomers have searched for a possible terrestrial or astronomical mechanism that will generate homochirality, the production of sugars or amino acids that display only a right- or left-handed configuration. No such sources have been found and not even the theoretical possibility for their discovery exists.7 The bottom line: the new LHB simulation does nothing to relieve the problem that all four requirements for an evolutionary explanation for life’s origin remain unmet.

Yet another problem throws a wet blanket over the raised hopes within the naturalistic camp. The simulation team presumed that life can originate under hyperthermal conditions and that hyperthermophilic life can easily evolve into mesophilic life. But in 2004, a different team demonstrated that extremophiles are irrelevant to a naturalistic origin of life.8 While life, if appropriately designed, can survive under extreme physical and chemical conditions, it cannot originate under those conditions.

High temperatures are especially catastrophic for evolutionary models. The higher the temperature climbs, the shorter the half-life for all the crucial building block molecules. For example, at an ideal neutral pH, neither acidic nor alkaline, a sample of ribose (a five-carbon sugar and crucial component in every nucleotide in RNA) will break down, losing half of its molecules in 44 years at 32° F (0° C). At 212° F (100° C–the boiling point of water) ribose’s half-life drops to just 73 minutes.9 Such a drastically low half-life guarantees that the concentration of ribose will remain too low to sustain any hope of a naturalistic pathway for RNA assembly. (The same could be said even for the longer half-life of 44 years).

Additionally, nowhere on early Earth was the ideal temperature range for hyperthermophilic existence sustained for a long period of time. As the simulation research team noted in their paper, a 2,000 kilometer (1,250 mile)-wide impact crater (and the surrounding crustal material) would take ten million years to cool while a 20 kilometer (12.5 mile)-wide crater would take one hundred thousand years to cool.10 In each case the time window during which hyperthermophile-friendly temperatures are maintained remained far too brief for even the most optimistic naturalistic origin-of-life models.

In one context, the more tepid, longer-lasting LHB proposed by the researchers makes things worse for scientists adhering to a naturalistic explanation for life’s origin. A more spread out and varied bombardment means that everywhere on Earth the temperature would be changing too erratically for either the emergence or survivability of life. Whereas, an intense, concentrated bombardment results in a lava state followed by general cooling and stabilized temperatures suitable for liquid water.

Hyperthermophiles pay a price for their capacity to live at temperatures above 212° F (100° C). They must devote a large portion of their biochemical machinery to repairing the chemical damage generated by the high temperatures. The severity of these limitations increases for hyperthermophiles living deep underground. One or two miles below the surface the availability of nutrients declines dramatically, slowing down metabolic and reproductive rates, while exposure to radiation from radioactive decay rises. All these constraints impair both the rate and the degree to which these species can undergo evolutionary change. The hypothesis that such deep underground hyperthermophiles evolved into mesophiles is not credible.

The research team that produced the new simulation acknowledges that the assumptions built into the simulation need to be tested and refined. They presumed, for example, that the LHB colliders came from the Main Asteroid Belt, asteroids dwelling between the orbits of Jupiter and Mars. However, a different bombardment configuration would’ve resulted if the LHB arose as a result of Jupiter and Saturn achieving a 1:2 resonance in their orbits as they interacted with the more distant and much larger Kuiper Belt. The researchers also presumed that the presently visible craters on the surfaces of Mars, Mercury, the Moon, and Venus provide an adequate basis for determining the damage Earth received from the LHB. But it is still not well understood how much of the damage evidence on these celestial bodies has been erased. Also, the timing and the duration of the LHB is not yet well established.

There are reasons to be hopeful, though, that the LHB’s precise nature will be pinned down soon. Better modeling of the dynamics of the early history of the solar system should establish the actual cause of the LHB. Research on extrasolar comet-asteroid belts may provide additional help in reaching this goal. Studies of debris clouds (a.k.a. asteroid and comet belts) around solar-type stars that manifest the age of the LHB-era Sun indicate that gas-giant-planet orbital realignment can produce massive disruptions in the debris clouds.11 Researching LHB craters on the Moon could further help confirm the LHB’s cause. Once its cause is unambiguously determined, scientists can begin to improve their measurements of its timing, duration, and intensity.

Missions to the Moon can help accomplish another goal. As I described in our magazine New Reasons to Believe, fossils of Earth’s first life can be recovered from the Moon. Chemical signatures confirm that life was abundant on our planet as far back as 3.83 billion years ago, but no fossil or fossil fragment older than 3.47 billion years remains on Earth.12 The planet’s own plate tectonics and erosion forces have destroyed beyond recognition the remains of first life. However, between 4.4 and 3.5 bya bombardment of Earth transferred millions of tons of crustal material to the Moon. Since much of this material arrived on the Moon at low-velocity oblique angle trajectories, and because the Moon manifests very little geological activity, abundant fossils of Earth’s first life should be present in pristine form in the upper layers of the lunar regolith.

Future manned or unmanned voyages to the Moon can recover, identify, and analyze these fossils, which will settle the origin-of-life issues raised by the simulation research team. These ancient remains will also definitively establish whether atheists’ origin-of-life predictions or RTB’s are correct. We eagerly await this test. Meanwhile, nothing in the new LHB simulation causes us to make any major change in our model for the origin of life.

  1. Oleg Abramov and Stephen J. Mojzsis, “Microbial Habitability of the Hadean Earth during the Late Heavy Bombardment,” Nature 459 (May 21, 2009): 419-22; Lynn J. Rothschild, “Life Battered but Unbowed,” Nature 459 (May 21, 2009): 335-36.

  2. Simon A. Wilde et al., “Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago,” Nature 409 (2001): 175-78; Stephen J. Mojzsis, Mark Harrison, and Robert T. Pidgeon, “Oxygen-Isotope Evidence from Ancient Zircons for Liquid Water at the Earth’s Surface 4,300 Myr Ago,” Nature 409 (2001): 178-81.

  3. Samuel A. Bowring and Ian S. Williams, “Priscoan (4.00-4.03 Ga) Orthogneisses from Northwestern Canada,” Contributions to Mineralogy and Petrology 134 (January, 1999): 3-16.

  4. Kevin D. McKeegan, Anatoliy B. Kudryavtsev, and J. William Schopf, “Raman and Ion Microscopic Imagery of Graphic Inclusions in Apatite from Older Than 3,830 Ma Akilia Supracrustal Rocks, West Greenland,” Geology 35 (July, 2007): 591-94.

  5. Fazale Rana and Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off (Colorado Springs: NavPress, 2004), 85-92.

  6. Hugh Ross, More Than a Theory: Revealing a Testable Model for Creation (Grand Rapids: Baker, 2009), 145-46.

  7. Rana and Ross, 123-33.

  8. H. James Cleaves II and John H. Chalmers, “Extremophiles May Be Irrelevant to the Origin of Life,” Astrobiology 4 (March 2004): 1-9.

  9. Rosa Larralde et al., “Rates of Decomposition of Ribose and Other Sugars: Implications for Chemical Evolution,” Proceedings of the National Academy of Sciences, USA 92 (1995): 8158-60.

  10. Oleg Abramov and Stephen J. Mojzsis: 420.

  11. A. Gáspár et al., “The Low Level of Debris Disk Activity at the Time of the Late Heavy Bombardment: A SPITZER Study of Praesepe,” Astrophysical Journal 697 (June 1, 2009): 1578-96.

  12. J. William Schopf, “The Oldest Known Records of Life: Early Archean Stromatolites, Microfossils, and Organic Matter,” in Early Life on Earth, Nobel Symposium No. 84, ed. Stefan Bengston (New York: Columbia University Press, 1994): 191-206; J. William Schopf, “Microfossils of the Early Archean Apex Chert: New Evidence of the Antiquity of Life,” Science 260 (1993): 640-46; Yuichiro Veno et al., “Early Archea (ca. 3.5 Ga) Microfossils and 13C-Depleted Carbonaceous Matter in the North Pole Area, Western Australia Field Occurrence and Geochemistry,” in Geochemistry and the Origin of Life, ed. S. Nakashima et al., (Tokyo: Universal Academy Press, 2001): 203-36.