Pattern of Elements Changes During Evolution

Radiation of metazoans took place quite a period of time (about 1.55 to 1.25 bio.

years ago according to molecular dating [Feng et al. 1997]) after both eukaryotic organisms evolved and cell colonies became arranged in a more stable way starting to recognize particular cells.⁹This radiation was during a period of time during which no substantial changes in atmospheric or geochemistry could be detected so far. Radiation among multicellular organisms and diversification of patterns of

9As for “primitive” animals having just three or four different kinds of cells, sponges can be dispersed down to the cellular level. Of course, one can then mix suspensions of cells of different species of sponges. When such a mixture is left alone for a few days, the unlike cells will unmix again to reconstruct little sponges each of which consists of cells taken from just one of these species. Obviously the cells already bear an immunological signature which here causes them to separate (and gather with their closest relatives, reconstituting the individual organs and species) rather than to agglutinate. Both trace metal (e.g. accumulation of Ti [outside the SiO₂needles fortifying the entire structure]) and organic biochemistry (formation of terpenoid isocyanides, -isothiocyanates, -formamides etc.) of marine sponges are noteworthy and peculiar. Probably isocyanides R-NC (R¼ terpene residue) do not just produce a disagreeable smell but also peculiar heavy metal ion enrichments in marine sponges.

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essential elements probably were not brought about by some thorough change of environmental conditions to which organisms would respond in different manners, with their metabolic modes become increasingly unlike to each other.

The argument that patterns of essential elements may be influenced by environ-mental conditions as well as adaptive radiation can be traced back further from phylogeny to ontogeny: ontogeny in biology uses to reproduce earlier stages of evolution: very young human embryos first strongly resemble coelenterates (the blastocyst), then fish larvae, to become typical monkey embryos, or rather, fetuses only much later (even in advanced fetal stages, fetuses of man and chimpanzee are still very similar). In the following the terms ontogeny and phylogeny (the bioge-netic law) will be more detailed described.

Ontogeny—literally „the way to become (old Greek genein) a being”—refers to the development from a fertilized or otherwise activated (in case of parthenogen-esis) egg cell into an adult, fully differentiated multicellular organism; commonly the term is used with (early development phases of) animals, humans rather than that of either plants or fungi while it does apply there also to some extent. In late nineteenth century, eggs of viviparous animals had been identified beyond doubt (in 1875) and certain mammals (monotremes, likePlatypus) became known to lay down small (<2 cm long) eggs very similar to those of reptiles (1884). Then, Ernst Haeckel (1834–1919) noted that such larval stages (blastocysts, embryos, fetuses) resemble “older” stages of evolution: this was called “biogenetic law”. F.e., the internal organization of the blastocyst is most similar to that of a coelenterate while young larvae of primates, other higher vertebrates (including birds) still cannot be distinguished by shape¹⁰from those of either bony or cartilagenous fishes if size is similar. Even much later, even after sexes became obvious in the fetus, human fetuses might be taken to be as such of chimps or bonobos (pygmy chimps) or vice versa, including an almost complete cover by body hair [lanugo]. Intermediate stages (late embryos, young fetuses) were allowing for a broad range of misinter-pretation among more remote taxa also. For example, take those coelacanth- or tadpole-like “paddles” of a human embryo of about seven weeks after conception which still have to undergo shaping by apoptosis which carves out either fingers, toes or the typical structures of bird- or bat wings. Here, apoptosis is incomplete leaving behind a skin extending among the entire finger bones whereas in ptero-saurs and certain birds parts of hand, arm skeletons were “set free” in much the same way as in land-dwelling vertebrates.

The “biogenetic law” does hold for interior organs also: e.g., the human heart starts as a simple S-curved artery with flip-back valves which closely resembles that simple hearts of fishes, tunicates and lampreys to reshape into a sophisticated device divided into four chambers only later. Often the foramen ovale, the hole between both main parts of the heart which is typical of amphibians and reptiles

10The entire argument relies on morphology and histology exclusively: DNA, of course, is unique from the very moment of conception while kind and series of gene activation will differ (or not) in different animal larvae producing the above results.

keeps open even after birth in humans, deteriorating cardiovascular performance of the individual (it uses to be closed in mammals, birds, and crocodiles). In fetuses, lungs start as simple hollow bags (like those of frogs or air-breathing fishes) only later to form alveoles, etc.

Thus, observation of ontogeny, taking weeks to two years (elephants, whales) in total (from egg fertilization to delivery) at most, provides a kind of “fast-forward movie” version of (higher animal) evolution. While there were arguments that

“ontogeny does create phylogeny rather than recapitulate it” (Garstang 1922), the biogenetic law is quite useful for our purposes:

• The processes to shape organs, by differentiated cell growth as well as apoptosis, are chemical processes from the beginnings to the very end. They are controlled by a DNA some 70 % of which is identical, and hence produce identical results up to a certain point of ontogeny among all animals;

• among animals, certain enzymes, including those required only during stages of ontogeny, like metamorphosis in amphibians, use to operate on identical metal ions (if any are involved);

• many protein sequences are well-conserved in genetic history (which includes binding preferences towards certain metal ions for enabling function in metalloproteins) while

• in arthropods, larval stages may even differ in metal kind of oxygen transport protein (either hemocyanin [Cu] or hemoglobin [Fe]) from adults, especially if the latter live in quite another environment, like dragonflies do; the “switch” can be either way round.

Given these facts, the biogenetic law suggests we are in a position to do something in an somewhat indirect manner we cannot do by studying fossils anyhow: reconstruct which functions metals and some non-metals like Se, I, had in animals which are long extinct. For non-animals, application of the biogenetic law is less reliable even for histology, and thus let alone chemistry. As the present patterns of essentiality were constructed probably step-by-step during evolution, we would anticipate the same step-by-step patterns in selective transfer of elements from egg (-liquid or -yolk), environment or maternal womb into an embryo or fetus.

The problem now obviously is about data, or clever ways to obtain or infer these data.

In marsupial mammals, very unripe, premature animals are born meaning they can grow to several hundred times the weight at birth while feeding on milk alone.

This can succeed only if milk (in its typical composition) provides everything in terms of all amino acids, sugars, lipids and trace metals the offspring is made of just before or when finally leaving the poach. Hence, e.g. the iron content of great kangaroo milk is some 100 times higher than in both human and ewe’s milk (Griffiths 1983). Assuming monotremes or marsupial mammals are forms which predated evolution of a fully functional placenta which can more effectively supply a larger fetus, the chemical requirements met by recent marsupial milk are likely to be similar to those conditions in early (Triassic to Jurassic) mammals or mammal progenitors. F.e., the high demand in iron might suggest the partial pressure of O₂ 3.1 Essentiality of Elements for Living Organisms, Taxonomy and the Environment 117

was lower in Mesozoic atmosphere except for the very last stage of Cretaceous (Maastricht). It supported active flight in pterosaurs much bigger than anything living which managed to fly on muscle power before or afterwards (Pteranodon sternbergi, Fig. 3.5a, 7.5 m wing span, estimated weight 15–26 kg) in Jurassic times; flightworthy Pliocene megabirdArgentavis magnificens (Fig.3.5b, 6.2–7 m wing span, weight 60–80 kg), man-powered ornithopters of similar weight (some 65 kg “pilot + engine” plus 40–50 kg of device). Other chemical features of kangaroo or platypus milk might reveal yet more information on Mesozoic condi-tions. The ability of recent tadpoles, newt- or salamander larvae to keep on growing while breathing through gills, becoming fertile and reproduce quite as if they had undergone “normal” metamorphosis does strongly suggest iodine which is a part of thyroxine (Fig. 3.6) and possibly also Cu were not available all the time when tetrapod vertebrates evolved. Into which environmental conditions might this translate? Analysis of organisms and their essentiality, metalloprotein features during larval stages will (likely) tell us!

Human embryos share the typical properties of almost all animals in biochem-ical terms from the very beginning. They depend on dioxygen and requiring a multitude of hormones including sources of some essential elements from the very first cell divisions; yet it is not at all certain that this applies in all conditions vertebrates, let alone, other animals or members of yet different kingdoms live and grow in: e.g. amphibians need thyroxine (and thus iodine) not during their entire life-cycles but only to accomplish metamorphosis. If the thyroid gland is resected in tadpoles they will not die (as mammals would do before long) but simply stay larvae, but these larvae then are capable to reproduce (neoteny) while growing

Fig. 3.5 (a) Painting of Pteranodon (pterosaurs) by Heinrich Harder (1858–1935); (b) Argentavis magnificens is one of the largest flying birds ever known. Courtesy images of Wikipedia

Fig. 3.6 Structure of thyroxine

much beyond the typical size at which they would else undergo metamorphosis.

Keeping in mind there are many species of amphibians which undergo metamor-phosis only if their hatching pools start falling dry but otherwise retain the neotenic mode of living including reproduction, one is posed to ask in which sense of the word iodine is essential for amphibians at all. Additionally, the metalloproteins required to introduce iodine into tyrosine amino acid to eventually make thyroxine—

depending on Cu—are not needed if metamorphosis is avoided for one reason or another. Hence in frogs or toads or newts (like Mexican axolotl) I is not really an essential element but just serves to adapt them to air-breathing semi-terrestrial life (iodine is essential thus for toad populations capable to inhabit desert regions which see rain-caused flooding only once in rather large periods of time). Requirements for Cu will have a peak only during metamorphosis while otherwise amphibians would happily go with traces of it at most.

But even this is not the entire story: What happens if organisms can still cope with quite different chemical conditions (which is more than euryoicy: man or rat can live in quite different climates, but both are obligate aerobes)? What is different with “really euryoicic” organisms in their responding to “really big” chemical challenges, such as switching from aerobic to anaerobic conditions? How do they change their metabolisms in terms of expression of metalloproteins which are needed as enzymes when using O₂ (or doing nitrate-respiration) only? This response, individual rather than evolutionary adaptation, obviously will have an impact on their gross chemical compositions even though probably not a single hitherto (i.e., in aerobic conditions) essential element will completely vanish from function or even from analytical detectability.

Examples of living creatures which can adapt to grossly different ambient oxidation potentials and can make alternative use of different terminal electron acceptors, include common baker’s yeast (Saccharomyces cerevisiae) and even small (
0.4–1 mm total length) animals of phylum Loricifera (the most recently discovered animal phylum of all). This is not a single exotic species but these fully anoxic loriciferans, living in an environment enriched by H₂S etc, are assigned to belong to novel species which represent three different genera, namelySpiniloricus (the one shown in Fig. 3.7), Rugiloricus (somewhat smaller) and Pliciloricus.

Besides, exuvia of copepods and some nematodes other relatives of which are known to endure both prolonged anaerobic and hypersaline conditions, were found in the about 40 m thick brine layer and below in sediment. It is difficult to estimate whether either corresponds to organisms actually living and even reproducing in these deep-ocean brine “pools”. Anyway, loriciferans are known to be fairly abundant in the Atlantic Ocean but had not been observed in the Mediterranean, either elsewhere or in whatever biochemical conditions.

During adaptation to anaerobic conditions their demands for metals included in some oxidoreductases will change accordingly. Effects are anticipated to be rather pronounced since respective oxidoreductases are more or less-substrate-specific.

Loricifera in the Mediterranean live in an environment of 410 mM Mg²⁺(almost eight times the average marine level) and some 5 M NaCl (nine times marine) obtained from leaching of underlying evaporates (left behind from Pliocene-times almost complete evaporation of Mediterranean Sea). As expected, their trace 3.1 Essentiality of Elements for Living Organisms, Taxonomy and the Environment 119

element will respond to this challenge also (though they cannot be directly com-pared to populations living nearby having access to O2and, moreover, represent novel species). The anaerobic Loricifera are distinguished by hydrogenosomes (Mentel and Martin 2010) which very much resemble those of strictly anaerobic unicellular organisms rather than having mitochondria in their cells which trans-lates to more Ni and less Mn therein. In these Loricifera changes between “normal”, oxygenic populations dwelling in Atlantic Ocean sand interspace fauna and those in sulfate- and H₂S-rich anoxic brines at the bottom of the Mediterranean (some 3,300 m deep) near Crete are considerable, to say the least. The data given in Table3.1are relative contents of Na, Mg, Si, P, S, Ca, Fe, Cu, Zn and Br in the body parts of Loriciferans from the Atalante basin compared to the Atlantic Ocean (the analyzed elements given here add up to 100 %, regardless of the almost sure presence of yet other elements and probable differences in total content).

Regrettably, neither Mn nor Ni were measured in this experiment. Atlantic Ocean-derived, that is, aerobic, Loriciferans totally lack Mg and Fe while the Cu content is larger except for the abdomen. The role of Br in anoxic populations is enigmatic, whereas higher (any) Fe levels in parts of body of the latter (Mediter-ranean, anoxic) would be involved in sulfate reduction. The latter process also requires P for activation while total S levels are similar among either population.

Given this it is likely the lower relative P levels in anaerobic Loricifera indicate lower levels of both metabolism and reproduction rates than with dioxygen-breathing cousins, as is common in ecological stoichiometry [P levels refer to reproduction rates (Sterner and Elser 2002)] and would be expected from poorer Fig. 3.7 Anaerobic

Loriciferans which (can) spend their entire generational cycle without oxygen in a sulfate-brine medium (Danovaro et al. 2010). These are the only animals (and hence metazoans) so far known to be capable to do so while there are lots of facultatively anaerobic monocellular organisms (Mentel and Martin 2010)

Table3.1ElementalcompositionofLoriciferansfromtheL’AtlantebasinandoxygenatedNorth-EastAtlanticdeep-seasediments RegionBodypartNa%Mg%Si%P%S%Ca%Fe%Cu%Zn%Br% AtalantebasinAbdomen0.00.022.39.630.64.50.022.510.60.0 Posterior0.06.531.39.919.72.65.25.21.817.7 Whole4.66.521.19.136.42.05.22.61.011.5 AtlanticOceanAbdomen15.90.020.48.9229.516.30.09.220.00.0 Posterior0.00.04.217.129.433.10.07.29.00.0 Whole4.940.010.712.331.028.40.05.96.80.0 Reportedaretherelativecontents(giveninpercentage)ofNa,Mg,Si,P,S,Ca,Fe,Cu,ZnandBrintheabdomen,posteriorloricaandwholebodyof loriciferans(Danovaroetal.2010).Thevaluesgivenarenotabsoluteconcentrationsofthe10elementsbutatupto100%,hencearerelativeshares.Relative amountofzerohowever,i.e.,Mg,Feimpliesanabsoluteconcentrationofzero,too

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metabolic performance (energy [¼ATP] yield) of sulfate vs. O2reductions. (Much) lower Ca levels likewise suggest decreased cell budding rates in anaerobic populations, too, in accord with P levels. Cu rather than Fe would be used for O₂ transport in these animals, while in other animals rather modified haems (based on Fe porphyrines) are used to control and transport. Insofar occurrence of Fe in parts of anaerobic Loriciferans is quite telling.