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Prior Fitness for Life: The Problem of Boron

Photo credit: NPS, Patrick Taylor.

Boron is not an element often considered in lists of essential ingredients for life. It is not incorporated into any animal enzymes, fats, proteins, or nucleic acids, and few of us look at boron content in our food labels. Arizona State University’s list does not even include boron making up any minute fraction of body weight. And yet, surprisingly, there would be no life without boron: no plants, no bones, and no brains. Why do we need it? And where does it come from?

The roles of elements Michael Denton describes in his “Privileged Species” series of books and videos, particularly in The Miracle of the Cell (2020) and The Miracle of Man (2022), are truly fascinating: especially the metalloenzymes like iron, copper, and magnesium, to say nothing of the exacting requirements for the common elements like oxygen, nitrogen, and (of course) carbon. It was never his intent, though, to discuss every element in the periodic table. His work can prompt others to help augment the case for the “prior fitness” of the universe for complex life. Recently we discussed one other element he passed over — phosphorus — that adds to the argument. Boron has a similar story to tell.

Nothing Boring About Boron

Boron, element 5 in the periodic table, is called a “metalloid” because, standing between metals and non-metals, it shares some properties with both. Containing 5 protons and 3 to 9 neutrons (with boron-10 and boron-11 being the most common isotopes in nature), boron has three valence electrons in its outer shell that can give rise to many compounds. It is never found in its elemental form naturally, but over 100 boron-containing compounds with hydrogen, oxygen, carbon, nitrogen, sodium, chlorine, and even aluminum are known. Many boron compounds, like boric acid, are water soluble. In its amorphous elemental form, boron is a brown powder that burns green in flame (see a demonstration video below). In fact, the green color in many fireworks displays comes from boron. Boron compounds have long been used to ignite rocket fuels from the days of Apollo up through today’s SpaceX missions.

Before getting into its roles in biology, you might be interested to know that boron compounds have many uses in everyday life: in hand soap (Boraxo), roofing tile, charcoal, glass, ceramics, nuclear shielding, makeup, semiconductors, magnets, and much more, as the U.S. Borax Company likes to boast. Many have heard about the historic 20-Mule Teams that hauled borax in Death Valley and delivered it to Mojave, 165 miles away, a ten-day ordeal for intrepid miners and their mules in the 1880s. Farther southwest, there’s a small town named Boron in the Mojave Desert that is the site of the world’s largest borax mine. It supplies half the world’s borates and boric acid. The other half is supplied by Turkey, where even larger deposits may exist untapped.

A Surprisingly Rare Atom

For such a simple atom, boron is surprisingly rare in the universe. Atomic physicists believe it is produced in small amounts by spallation reactions with cosmic rays or in supernova explosions, but not by stellar nucleosynthesis. This raises questions about how Earth got its supply — a subject we will return to shortly. Natural boron minerals called borates can be found throughout Earth’s crust, on the ocean floor, and in volcanic deposits. If it’s depleted in soil, leaves turn yellow, but too much is toxic to plants. Farmers know that supplemental boron in fertilizer can increase crop yields to a point. In general, biology does not appear starved for boron.

So why is boron not incorporated into biomolecules? It stands right next to carbon in the periodic table but is extremely different in its actions. Like bromine, boron takes part in the synthesis of important compounds without residing in them. An essential trace element, boron acts as a regulator and facilitator of important biochemical pathways; for instance, it can extend the half-life of vitamin D and thereby increase its bioavailability. It plays essential roles in hormone production. Plants depend on boron for construction of their cell walls, and animals depend on it during bone formation. U.S. Borax explains its many roles in plant life:

Boron is an essential micronutrient, integral to a plant’s life cycle. Required only in small amounts, boron is necessary in plants to control flowering, pollen production, germination, and seed and fruit development.Boron also ensures the healthy transport of water, nutrients, and organic compounds to growing portions of the plant….

As plants draw borates from the soil, the boron is distributed throughout the stems, leaves, roots, and other structures. When people eat plant-derived foods — such as fruits, vegetables, nuts, and legumes — they routinely absorb small amounts of boron. [Emphasis added.]

Most people get sufficient boron from plant sources like apples, coffee, legumes, and potatoes. We only need about 1.2 to 3 mg of boron per day, but there is “nothing boring about boron,” wrote Lara Pizzorno in the Journal of Integrative Medicine. Consider her astonishing list of benefits we get from the tiny amounts of this element that we ingest:

Boron has been proven to be an important trace mineral because it (1) is essential for the growth and maintenance of bone; (2) greatly improves wound healing; (3) beneficially impacts the body’s use of estrogen, testosterone, and vitamin D; (4) boosts magnesium absorption; (5) reduces levels of inflammatory biomarkers, such as hs-CRP and TNF-α; (6) raises levels of antioxidant enzymes, such as SOD, catalase, and glutathione peroxidase; (7) protects against pesticide-induced oxidative stress and heavy-metal toxicity; (8) improves brain electrical activity, cognitive performance, and short-term memory in elders; (9) influences the formation and activity of key biomolecules, such as SAM-e and NAD+; (10) has demonstrated preventive and therapeutic effects in a number of cancers, such as prostate, cervical, and lung cancers and multiple and non-Hodgkin’s lymphoma; and (11) may help ameliorate the adverse effects of traditional chemotherapeutic agents. 

Geological Availability of Boron

Now that we are convinced of boron’s benefits, some may wish to monitor their boron intake or even ask their doctors about supplementation if they are at risk. But where did Earth’s boron come from? As stated earlier, it is relatively rare in nature, and so there should not have been large amounts in the solar nebula from which the rocky planets are believed to have accreted. This has led some to speculate that boron was delivered to earth in a “late veneer” of chondrites. That seems odd, though, because one could ask where those objects got it if not from the solar nebula. However the Earth got its boron, it’s here now. One might assume that plate tectonics would recycle it, as happens in other elemental cycles (e.g., carbon, nitrogen).

Even taking Earth’s current boron budget as a given, though, another issue was raised in a recent paper by Liang Yuan and Gerd Steinle-Neumann in Geophysical Research Letters. According to their models and computations, most boron should have sunk to the Earth’s core, because at high temperatures and pressures, it clings to iron.

Plate tectonics promotes the transport of surface rocks into the mantle, producing much of its chemical heterogeneity. Boron, a quintessential crustal element, is often used as a proxy for crustal contributions when found in mantle rocks and is, therefore, one of the central tools in geochemistry to trace recycling/mixing in the mantle. Using quantum mechanical calculations, we find that the chemical behavior of boron changes from lithophile (rock-loving) to siderophile (iron-loving) under pressure–temperature conditions relevant to core formation. Thus, much boron may have been transported to the core, and the core may be Earth’s largest boron reservoir, rather than the crust.

In other words, molten iron as it sank to the core should have carried most of this scarce element with it. Indeed, the two researchers believe that half of Earth’s boron budget is stored in the core now. How can it get up into the crust where plants and animals depend on it?

Denton’s “Prior Fitness”

This opens a question that might interest design advocates looking for more evidence of Denton’s “prior fitness” argument. Does the circumference and mass of our planet determine the availability of boron? Was there an issue of timing that prevented a runaway depletion of boron to the core? As the authors state, “As metallic iron is present predominantly in the core and likely at percent level throughout the mantle, its impact on Earth’s boron budget merits consideration.” Equal consideration must be given to requirements for any habitable planet capable of supporting complex life. I have not seen boron availability discussed by Denton or in The Privileged Planet by Gonzalez and Richards (2004).

The authors mention that certain diamonds (Type IIb) contain excess boron. Other geochemists have taken that to be a proxy for tectonic recycling, but these authors challenge that interpretation. “Rather than boron in Type IIb diamonds representing crustal recycling, its predicted siderophile nature suggests the fingerprint of a metallic reservoir.” The reservoir cannot be in the core, though:

The hypothesis of a core contribution to the boron signature of Type IIb is highly conjectural as it requires more than 2,000 km of vertical migration of dense core components with minimal dilution of boron signatures.

They suggest that molten iron moved into the mantle, carrying boron with it, and that the diamonds were erupted from there. (Diamonds can arrive suddenly at the surface from the mantle in rapid volcanic explosions called kimberlite eruptions.) Although they suggest some isotopic evidence for mantle reservoirs, their solution seems highly conjectural as well. Unless evidence for a self-sustaining boron cycle can be established, one might have to surmise that complex life appeared on Earth at a special time due to the limiting factor of boron availability.

The fact remains that Earth’s surface now seems to have plentiful boron for living organisms, even if industrial demand requires mining as much as can be found in isolated locations like California deserts and certain provinces in Turkey. In that regard, the boron budget resembles the phosphorus budget as support for Denton’s “prior fitness” argument.