The vital role of vitamin D in evolution

 The vital role of Vitamin D in Evolution

The Earth came into being 4,500 mya (million years ago). It is assumed to have been initially hot and fiery, but after 500my (million years) the surface temperature had dropped to between 0 and 100 degrees Celsius and therefore water was present in liquid form. This enabled life to begin. There are suggestions that life originated elsewhere in the universe, or in the Solar System, and arrived on Earth via comets. This begs the question as to where and how life began. Water is fundamental to life as we know it and so it is reasonable to assume that life began in the oceans.

In the beginning

The origin of life is obviously uncertain and speculative, but organic life must have had inorganic origins, from rock and water. To quote Professor Brian Cox, “The origin of life is a transition from geochemistry to biochemistry”.  

Life requires energy, electro-chemical energy. Nick Lane from Imperial College, London, presents the plausible hypothesis that the electrochemical energy was provided by an electrical gradient in deep ocean alkaline vents in which the alkaline fluid is separated from the acidic ocean by thin rock in a sponge-link structure. “Lost City” in the mid-Atlantic, discovered in 2000, is the most well-known example.

"Lost City"

The vital part of life is enzymes, which enable metabolic processes. Many enzymes have metal at the core, suggesting catalysts whose origin was in rock. Examples are iron, copper, magnesium, cobalt, manganese, zinc, and molybdenum, present at the core of organic proteins including enzymes that are called metaloproteinases.

2,500 mya saw the appearance of prokaryotes, micro-organisms containing some DNA but without a defined nucleus. The two branches are achaea and bacteria.

Cyanobacteria contained pigments that absorb sunlight energy and convert it into chemical energy, combining water and carbon dioxide and releasing free oxygen into the atmosphere. The main pigment was green chlorophyll, with an atom of magnesium at its core. It has a structure with a feature in common with haemoglobin, which has an atom of iron, and cyanocobalamin (vitamin B12), which has an atom of cobalt.

The next step was the evolution of eukaryotes, single cells with a defined nucleus. At one stage, perhaps only once, a eukaryote engulfed a cyanobacterium thereby creating an internal chloroplast, which would multiply inside the cell. This process is called endosymbiosis, and cell division would lead to more of these cells, the start of plant life. It would create a great evolutionary advantage by providing the cells with the ability to generate large amounts of photosynthetic energy in the form of glucose. It would also release more oxygen into the atmosphere with subsequent evolutionary benefits.

A second endosymbiotic event was the engulfment of a eukaryotic cell by another, multiplying and becoming internal mitochondria. Mitochondria are the energy-producing organelles of the cells of the animal kingdom, and so once again the new cell had a great evolutionary advantage in respect of the internal energy released from engulfed food. 

There became a energy metabolic cycle of the chloroplasts within plant cells capturing sunlight energy and storing it in the form of glucose, with the release of oxygen, and then animal cells feeding on glucose, with mitochondria using oxygen to release the stored sunlight energy for metabolic use.

All our food comes indirectly from sunlight energy.

Plants capture energy from the Sun with water and carbon dioxide to become the glucose molecule, which is then stored as a polymer. One polymer is starch, which can be used by the plant as an energy source for future growth. Starch also becomes a source of food for many vegetarian animals including ourselves. Starch is broken down into glucose and then re-polymerised  into glycogen for storage in the animal body, much of it in the liver. Carniverous animals obtain glucose and other nutrients by eating vegetarian animals. The process of Kreb's Cycle in the mitochondria of animal cells extracts the energy from glucose and returns carbon dioxide and water to the atmosphere. 

Plants can form glucose obtained from photosynthesis into an alternative polymer, a linear form called cellulose. This becomes the structure of the plants, and it is interesting that wood is made from glucose. When wood is burned the stored sunlight energy is released.

The appearance of plankton

Evolution moved on from single-celled to multicellular organisms, and the next vital step was the appearance of a wide range of aquatic organisms called plankton. Some of these, phytoplankton, have the ability for photosynthesis, whereas others, zooplankton, feed on algae and the remains of dead plankton.

Plankton are present in vast numbers in the oceans and seas, and also in fresh water, and they have a wide range of shapes. Many are fluorescent and this can be obvious at night on the ocean and sometimes at the sea-shore.

When plankton first evolved they had no predators and this remained so for 1,000my. Their numbers must have been much greater than today as it was not until the Cambrian explosion of more advanced sea creatures 550mya that predators appeared. 

Plankton

There is the theoretical supposition that when they appeared plankton would be in danger of attack by pre-existing bacteria, but this appears not to have happened. Plankton have a physical defence in the form of an outer shell, but they seem to live in a symbiotic relationship with bacteria. Plankton produce carbon-based nutrients upon which bacteria can feed.

The history of the Earth has seen frequent restructuring, with tectonic plate movement, mountain building, and great amounts of sedimentation in the seas and oceans. Between the layers of ocean sediment that produce strata when elevated above the sea level, there are thin layers of a carbon-rich material which are derived from dead plankton. It is this layer that allows the strata to slide upon each other and allow the mountain building with which we are familiar.

Strata in the of sea cliff, Hartland, North Devon

Defence against solar UV

Although there was an absence of predators, plankton are surface dwelling and so they are in danger from damaging UV radiation from the Sun. Defence mechanisms must be necessary.

The first is “Diurnal Vertical Migration”. In order to avoid UV damage by day, plankton sink below the surface. By night they will return to the surface, hence the nocturnal fluorescence displays. This movement would appear to have been triggered by variation of sunlight, but it is now encoded in DNA and in the body clock. When taken out of the sea and transported to a laboratory without sunlight variation, diurnal vertical migration continues.

The second defence of plankton against damaging UV radiation is the synthesis of a sunscreen. This is the four-ringed steroid molecule 7-dehydrocholesterol (7-DHC), which is derived from squalene, a long-chain fatty acid also known as shark oil. Evolution created a long cascade of biochemical reactions (each step with its own enzyme) to finally produce the important squalene and 7-DHC.

7-dehydrocholesterol

7-DHC absorbs UV energy (UVB wavelength between 280 and 315 nanometres), and the energy splits a specific inter-atomic bond. This results in the molecule changing shape and becoming what we know as cholecalciferol, also as vitamin D3.

Cholecalciferol, vitamin D

D3 is the form of vitamin D derived from plankton and animal sources. D2 is ergocalciferol, obtained from fungi. I will simply refer to vitamin D.

Plankton produced vitamin D 1,500mya, but it was simply a waste product as they had no use for it. It was not until the Cambrian explosion of new complex aquatic life-forms that vitamin D was to find its role in evolution.

The importance of defensive immunity

Life-forms more complex and advanced than plankton might have appeared, but challenge by bacterial infection would have led to their immediate extinction. 

1898 CE saw the publication of HG Wells’ novel “War of the Worlds”. In this fictional story there is an invasion from Mars centred on London. The invaders in their large tripod machines caused devastation, but then the devastation and the machines came to a sudden halt. The invaders had died, and the cause of death was found to be the “Bacillus”. The previous two decades had seen the works of Robert Koch and Louis Pasteur, which achieved general acceptance as the “Germ Theory”. HG Wells was quick to explain the potential of bacteria to cause death to aliens.

What HG Wells unwittingly described was that the indigenous populations would have immunity to local bacteria, but without such immunity (herd immunity) aliens would succumb to disease. There were similar occurrences when European invaders arrived in Central America, leading to widespread death from smallpox among the indigenous people.

Snowball Earth

Snowball Earth occurred between 720 and 630mya. At that time there was no life on land, neither plant nor animal, but some aquatic life  continued in the oceans, and this included plankton. It was no coincidence that the Cambrian explosion of complex life-forms appeared after the end of Snowball Earth, as the climate would have allowed the opening of the oceans to the skies and the appearance of liquid water on surface of the land. The climate was what was required for the evolution of life to continue.

Complex life-forms might have appeared during the course of 1,000my since the evolution of plankton, but they would have been without appropriate immune defence mechanisms. They would have died rapidly from bacterial infection, as did the invaders from Mars. Remember that when we die we rapidly decompose under the influence of the trillions of bacteria and other micro-organisms that live on and within us. It is our life-force, our metabolism that maintains our defence, most importantly defensive immunity.

Defensive immunity

It is as well at this stage to think about defensive immunity. There are two forms, innate immunity and adaptive immunity. Innate immunity is a non-specific reaction, creating inflammation (redness, heat, swelling, pain) which is driven mainly by the cytokine TNFα (Tumour Necrosis Factor Alpha). 

Adaptive immunity is delayed by about 48 hours but it rapidly becomes specific to the invading micro-organism. It has a memory providing a more rapid response should the same microbial antigen be encountered in the future. Adaptive immunity involves several cell types including T-cells, the activity of which is controlled by what we now call the VDR gene. 

The VDR gene provides the instructions for (encodes) the production of the VDR protein within cells, and this controls the activity of many genes including those that are necessary for the immune response. VDR is the key gene and molecule, and the number of VDR molecules can be increased many-fold to meet the defensive demand. The key defensive cytokine released from the cells becomes TGFβ which will take over from TNFα leading to tissue recovery.

VDR, the incomplete molecule

The strange thing about VDR is that it is a complex molecule that is synthesised within the cells from its genetic template in a form that is incomplete and it is therefore inactive. VDR is produced without a vital building block which the cells are unable to synthesise. The missing component is what we know as cholecalciferol, vitamin D, and we have seen that is derived from 7-DHC by the action of UV wavelength 280 – 315 nanometres, an energy that it appears cannot be met by metabolic activity of living cells.

But vast amounts of cholecalciferol / vitamin D are available from plankton, the basis of the aquatic food chain. Following ingestion cholecalciferol / vitamin D is taken to the liver where it is hydroxylated to 25(OH)D, the form in which it circulates in the blood as a reservoir, and it is also known as calcifediol. It is taken up as required by the cells of immunity and immediately converted by second hydroxylation into the active form 1,25(OH)D. 

This is the missing component required to complete the VDR molecule: VDR stands for Vitamin D Receptor. When it is complete VDR can form a dimer with RXR (Retinoid X Receptor) and then bind to and activate specific DNA sequences that will escalate defensive immunity and control other genetic processes.

The Cambrian explosion, the evolution of immunity 

During the time between the appearance of plankton 1,500mya and the Cambrian explosion of new life-forms 550mya, evolution was progressing and immunity was developing. Before VDR evolved and the orchestra of immune processes that it controlled, new aquatic animal forms would have appeared but would have succumbed to bacterial damage. But the time was reached that immunity had matured. VDR had evolved and the missing component was readily available thanks to plankton. What was then required was the appropriate physical climate for life to prosper. That time was the end of Snowball Earth.

The aquatic life-forms that characterised the Cambrian explosion must have had defensive immunity to have survived and thrived. Most of the new animal life-forms we know only from fossil records and they did not survive. We know of them because of rich fossil records. Rather than an absence of immunity, the reason why so many did not survive was the rapid evolution of carniverous predators. Sharks and other cartilaginous fish evolved about 450mya, and so were obviously related to other new species living at that time. They have defensive immunity.

Cambrian life-forms

Fossils from the Cambrian explosion of new life-forms indicated segmented bodies, but initially invertebrates without a spine. Vertebrates would have a cranium and a spinal cord protected by a vertebral column. The first were jawless fish, agnatha, hagfish, which still survive as scavengers in the oceans. Hagfish have the VDR gene, with the related lamprey having the polymorphism lVDR. Cholecalciferol / vitamin D attaches to both. Clearly the immune system had developed by the time of the Cambrian explosion and the availability of vitamin D enabled this to be active. 

Evolution of bone

Fossils dated to 420mya have shown fish with a bone skull, but this will have been calcification of soft tissue, membranous bone. Evolution had led to the appearance of sharks 450mya, with cranium, spinal cord, vertebral column and other bones that were composed of cartilage. The later evolution of bony fish in  was of immense importance as ultimately a bone skeleton would be essential for the evolution of land-based vertebrates about 370mya. 

Bone can be formed from foetal cartilage, especially long bones,  or from soft tissues, membranous bones that form the skull. In both, the critical bone-forming cell is the osteoblast.  Osteoblasts are derived mainly from mesenchycmal stem cells in the bone marrow, but also from other tissue sources. All osteoblasts contain VDR and as with immune cells, they require cholecalcferol / vitamin D for VDR to be complete and to activate the appropriate genes. Bone is always in a state of reconstruction, requiring a balance of osteoblasts that create bone and osteoclasts that reabsorb (“destroy”) bone. Osteoclasts also contain VDR and require vitamin D for activation. Osteoclasts are derived from monocytes, the fundamental cells of immunity.

Evolution of immunity and bone had in common VDR genes and intracellular VDR, and therefore a need for vitamin D so that the VDR protein can become active. In the oceans increasingly complex aquatic life had a large and continuous supply of dietary vitamin D in the form of plankton. When the development of bone allowed vertebrates to move from the sea on to the land, there was a problem. VDR in the cells of immunity and bone required vitamin D to complete the VDR molecule, but plankton were not immediately available on land. A diet containing fish that would have consumed plankton would have been adequate for semi-aquatic animals, but another evolutionary development was necessary for species that were exclusively terrestrial.

Vitamin D production in terrestrial animals

The metabolism of plankton included the important synthetic chain leading to farnesyl-pyrophosphate (F-PP). The next step leads to squalene (and then to ubiquinones, heme-A, sterol, dolichols, prenylated proteins). We have seen that squalene folds to become 7-DHC (7-dehydrocholesterol), and that solar UV acts on this to produce colecalciferol / vitamin D. There is also an enzyme (7-DHC reductase) conversion of 7-DHC to cholesterol and then its derivatives. This metabolic pathway had evolved in plankton and it persisted in subsequnt animal life. It is essential in humankind, and it is interesting to note that statin drugs partially block the pathway at F-PP.

This pathway is present in all animal species with the oil 7-DHC being produced internally and also in the skin. As a result, and by what we can regard as a success of evolution, terrestrial animals have been able to produce vitamin D in or on the skin, allowing the completion of the vital VDR molecule without a diet of sea-food. Evolution could then continue with essential defensive immunity and a skeleton composedof bone. 

Problems for humankind

There were problems with the cutaneous production of vitamin D that came to be appreciated by humankind in the 20th century. Serious atmospheric pollution during the early industrial era blocked UV penetration to ground level, seriously diminishing vitamin D production. This resulted in rickets (impairment of bone formation) and tuberculosis (due to impairment of immunity). This was compounded by indoor work and indoor life in general, and also extensive clothing, sometimes cultural. It was recognised that in the elderly the synthesis of 7-DHC in the skin declines progressively, leading to dry skin and an inevitable reduction of vitamin D production. 

Evolution brought about people living in Africa having a black melanin-rich skin, the melanin being a natural sun-screen absorbing the damaging high intensity UV from the overhead Sun, but allowing adequate vitamin D production in the skin. People living in the tropical zones in Asia were also selected to have a melanin-rich dark skin, with protection against UV. 

People living in Europe, especially the north-west parts, closer to the North Pole than to the Equator, became selected to have a white skin, but the white skin had the potential to darken on exposure to the solar UV during the summer months. With UV energy at sea level being so much less at these latitudes, the necessity for protection against UV was much less than for people living in the tropics. However there was a greater necessity for the production of vitamin D in the skin, favoured by the absence of genetic melanin.

Population changes in the late 20th century brought about movement of people of Black African and South Asian ethnicity to north west Europe and north America. The evolutionary advantage of a melanin-rich skin was lost when they moved to a zone with much reduced UV energy at ground level, and it became a positive disadvantage as the dark skin was much less efficient in producing vitamin D from 7-DHC.

The people of Black African and South Asian ethnicity living in the UK in particular have developed several health disadvantages compared to the indigenous ethnic white population. The high prevalence of vitamin D deficiency is well-established, and the health disadvantages are mainly result of impaired immunity.

Conclusion

The Cambrian explosion of advanced life-forms 550mya followed the end of Snowball Earth, allowing a clement climate that included liquid water. Pre-existing plankton would have provided the basis of the food-chain for the new aquatic life.The appearance of photosynthesising green plants on the newly exposed land would have increased the oxygen content of the atmosphere and provided food for terrestrial animals.  

The survival of complex aquatic life-forms identified in the Cambrian explosion depended on defensive immunity against pre-existing bacteria. The major player in immunity was the intracellular molecule VDR and its encoding gene. The VDR molecule was synthesised within cells incomplete, lacking the building block molecule cholecalciferol / vitamin D, which could not be synthesised by animal life-forms as it required the input of high energy from solar UV. 

Plankton had evolved 1,000my earlier, and as part of their defence against solar UV they synthesised the oil 7-DHC that absorbed UV energy, converting 7-DHC into the waste product cholecalciferol / vitamin D. This had no function until evolution had brought about defensive immunity with the incomplete VDR.

VDR became the controlling molecule in the evolution of immunity, and 100my later in the control of bone production. In both these processes vitamin D was essential for the completion and activation of the VDR molecule. 

Aquatic animals were able to obtain vitamin D from plankton, the basis of the aquatic food chain. In addition to eating fish, land animals became able to obtain vitamin D by the action of solar UV on 7-DHC synthesised in the skin.

The evolution of VDR has been critical, but also vitamin D to complete and thereby activate VDR. Without vitamin D, evolution beyond plankton in the form that we know it would not have taken place.

It is important that we understand how human behaviour can disrupt evolutionary effects leading to serious vitamin D deficiency with impaired immunity and bone production (rickets and osteomalacia). Evolution provided humankind with large brains, so that in 1920 vitamin D was identified and its importance appreciated. VDR was identified in 1969, but its importance in the escalation of defensive immunity remains under-appreciated by most medical scientists, so that millions of people with vitamin D deficiency stilll exist, with suboptimal immunity and at risk of serious health disadvantages.