The Case for a Creator, Chapter 7
Gonzalez's next assertion strikes me as highly dubious. He claims that, if the Earth were larger than it is, the higher surface gravity would tend to smooth out mountains and ocean basins, producing a perfectly spherical planet with little surface relief. (He provides no numbers on how much bigger the planet could be before this happens.) This would result in a "water world" whose surface was evenly covered by a shallow ocean, and "a water world is a dead world" [p.181] because there would be no continental weathering to wash mineral nutrients into the oceans. "In a water world, many of the life-essential minerals would sink to the bottom. That's the basic problem." [p.181]
Yet again, evolution is smarter than the creationists. Vital minerals sinking to the ocean bottom would suit hydrothermal vent communities just fine - the ocean-bottom ecosystems that use minerals spewing from volcanic fissures in the ocean floor and subsist on chemical energy rather than sunlight. Some biologists even believe that these "black smokers" are where life on Earth began.
"Besides, the salt concentration in a water world would be prohibitively high. Life can only tolerate a certain level of saltiness." [p.181]
Gonzalez's argument here is that in continental environments like marshes, ocean water can evaporate and leave salt deposits behind, preventing the seas from becoming too salty, but this couldn't happen in a water world.
This reminds me of those young-earth creationist lists that offer various "proofs" which directly oppose each other. One of my favorites was the list that claimed, on the one hand, that the Earth must be young because if it was old, erosion would have worn away all the mountains, and on the other hand, the Earth must be young because if it was old, volcanism would have created mountains much higher and larger than we see today.
Gonzalez, allegedly a scientific authority, has made the same elementary blunder here. Salt comes from continents! If we lived on a water world, with no continents to erode and wash material into the ocean, there would be no source of salt. And, yet again, Gonzalez overlooks the fact that some species of Earthlife already can cope with the very salty environments that he claims would make life impossible. They are called halophiles. The green alga Dunaliella salina, for example, can live in water with a 30% salt content (ocean water is about 3% salt).
Next on the list is plate tectonics, which Gonzalez claims is a necessary ingredient for life. The book correctly describes the cause: the natural heat of radioactivity, which keeps the earth's interior hot and causes the continental plates to drift on an underlying sea of semi-molten rock. The churning of the Earth's iron core also creates a dynamo effect that's responsible for the planet's magnetic field.
"The magnetic field is crucial to life on Earth... If we didn't have a magnetic shield, there would be more dangerous radiation reaching the atmosphere." [p.183]
But every few hundred thousand years, the Earth's magnetic field reverses - in other words, the magnetic north and south poles exchange places. (We know this from the geological record: iron crystals in lava align with the geomagnetic field like tiny compass needles, then are frozen in place when the lava cools and hardens.) A full magnetic reversal takes several thousand years from start to finish, and while it's happening, our planet has a greatly weakened magnetic field. Life has obviously survived these events, and no mass extinctions are known to be correlated with pole reversals.
Gonzalez also says that plate tectonics plays a vital role in the carbon cycle, subducting carbonate minerals into the mantle and then reemitting them from volcanoes as carbon dioxide. This does play a role in creating the environment for life on Earth, but again, there's no reason to believe it's an absolute necessity. Carbon dioxide levels have been much higher in past geological periods, and this too did not lead to the extinction of all life.
The Galactic Habitable Zone
Gonzalez's final assertion has to do with the Earth's location in the galaxy. He says that Earth is located in a "safe area" [p.169] of the Milky Way, far from the central supermassive black hole and from active star-forming regions, both of which would have dangerously high levels of radiation. On the other hand, the galaxy's outer regions and globular clusters are composed mostly of older, cooler stars and lack the heavy elements that are cooked up by supernovae and that are needed to form planets and life:
"...[Y]ou can have a whole globular cluster with hundreds of thousands of stars, and yet there won't be a single Earth." [p.170]
Clearly, Gonzalez has a strong point here. Globular clusters lack heavy elements, are too gravitationally unstable, and in all other ways are completely unsuitable for planets. Therefore, we can't possibly have discovered PSR B1620-26b, an extrasolar planet orbiting a dual-star system in the globular cluster Messier 4.
Now, I'll grant that Gonzalez is not entirely wrong: the low metallicity of globular clusters does make them a poor environment for planetary formation. (Another factor may be the stronger ultraviolet radiation in globular clusters that would dissipate protoplanetary disks of gas and dust.) But the existence of PSR B1620-26b shows that it is not impossible. In fact, not only does this planet exist, it's thought to be extremely old - over 12 billion years, over twice the age of the Earth - and presumably formed in an era of the universe when heavy elements are sparser than they are now. This strongly implies that something is wrong with Gonzalez's confident assertions about the probability of planet formation, and very probably indicates that the process is not as difficult or as unlikely as he implies.
As far as extremes of radiation - as with extremes of temperature and salinity, which were discussed in the previous part - once again the creationists have underestimated life's adaptability. There are already Earth species that can survive levels of radiation well in excess of what humans can tolerate. The reigning champion is Deinococcus radiodurans (see also), a bacterium which can survive a dose of 15,000 grays (10 grays is lethal to a human). D. radiodurans has been found living in the cooling water of nuclear reactors. The microscopic animals called tardigrades are nearly as resilient, able to withstand not just high doses of radiation but also high pressure, the vacuum of space, and temperatures well above boiling or just above absolute zero.
Of course, these are microscopic creatures, not large, complex life. But how do we know that the adaptations they possess couldn't have been transferred to creatures more like us? The Earth's environment has never been so extreme as to provide an evolutionary pressure in that direction, but there's no obvious reason to believe it's impossible. (Withstanding high doses of radiation is mainly a matter of DNA repair mechanisms.)
The creationists who parade these horribles before us want us to believe that life is fragile, just barely clinging to existence, and even a small perturbation to the environment would spell our doom. (One wonders why they're not more fervent about opposing global warming, if that's so.) But the evolutionary record reveals precisely the opposite: life is a tenacious phenomenon, able to survive in a wide variety of environments from the frozen arctic to the boiling-hot vents on the ocean floor, from arid deserts to salt-saturated ponds, and even in the vacuum of space. It's worth wondering whether, if creationists like Strobel were willing to acknowledge the true breadth and depth of life's history, they might be willing to give it more credit for being able to flourish in unlikely places.
Other posts in this series: