Anatoly. M. Belyaev, Paul V. Yukhalin
Paleovirusology group, Sidose LLC, St. Petersburg, Russia
Email: email@example.com; firstname.lastname@example.org,
Despite the diversity of viruses and the role they play in the biosphere, their fossilized remains have not yet been found in ancient rocks. This is due to the fact that, in essence, viruses (like all skeletal unicellular organisms) are tiny droplets of protoplasm in a protein shell, which after death experience postmortal transformations – lysis (dissolution) or collapse, and when buried and heated decompose into water, carbon dioxide, nitrogen oxide and phosphate ion. In addition, most paleontologists adhere to the dogma (not substantiated by factual material) that ancient viruses, as well as modern ones, were extremely small, and therefore they cannot be detected in rocks, or distinguished from mineral formations. It is very likely that during micropaleontological studies, the remains of ancient viruses were still found, but were not identified as representatives of the living world, or attributed to the group of acritarchs – unicellular organisms of unknown origin.
Akritarchs are microscopic organisms in the form of spherical, elliptical or disc-shaped capsules. Hundreds of species of fossilized microscopic remains (microfossils) have been described in micropaleontology akritarchs. They are characterized by only two parameters – the presence of an organic shell and a central cavity, ranging in size from 8-500 µm to 1 mm. Some varieties of acritarchs have numerous and more diverse spikes, which are supposed to have served as protection from large predators!? Acritarchs are presumably cysts of fossil dinoflagellates, or unicellular algae. Because of the size, acritarchs have never been compared to viruses. At the same time, some microfossils found in sedimentary rocks of the Precambrian and Early Paleozoic, and attributed to acritarchs, are morphologically similar to modern influenza RNA viruses, rotaviruses and coronaviruses (Fig. 1.1-1.3).
Acritarchs of Peteinosphaeridium armatum from the Ordovician deposits of China with an age of more than 400 million years (Fig. 1.4. Tongiorgi et al., 1995), and acritarchs of Shuiyousphaeridium macroreticulatum with an age of 1700 – 1400 million years (Fig. 1.5; Agić, 2016), have RNA-like outer spherical shells and numerous villi. At their ends, protein molecules may have been located for attaching ancient viruses to cell shells and penetrating viral RNA or DNA into them, as in modern viruses.
Of course, one cannot expect complete morphological similarity between modern RNA viruses and ancient virus-like structures that existed hundreds of millions of years ago, and moreover modified during fossilization. They have only some close morphological features – spherical shape, and villus, which presumably had similar functions to viruses. However, microfossils of virus-like structures with dimensions of 60-90 micrometers across (1 millimeter — 1000 micrometers) are 400-1000 times larger than RNA viruses in linear dimensions. At first glance, with such a difference in size, there can be no question of their relationship or common nature. But the paleontological record shows that size is not the main factor in determining the relationship of organisms. So, the extinct giant dinosaurs were, at least, distant relatives of modern birds and reptiles. And even before the appearance of dinosaurs, giant insects reigned on land and in the sea – sea scorpions and land centipedes-arthropleuros, reaching two and a half meters in length. Therefore, it is possible that the ancient ancestors of viruses could have been full–fledged cells – facultative parasites, which in the course of evolution decreased in size and lost the ability to reproduce independently, turning into obligate parasites – modern viruses.
In the twentieth century, viruses with sizes from 20 to 300 nanometers were known to science. However, in the twenty-first century, giant viral structures were unexpectedly discovered, surpassing traditional viruses in linear dimensions by hundreds of times or more (Arslan, 2011; Jônatas Abrahão et al., 2018; Lviv et al. 2018). In 2013, two viruses were discovered at once: Pandoravirus salinus and Pandoravirus dulcis. They have an oval shape, with thick walls and a hole at the end (Fig. 2.1). Because of their size (1×0.6 µm), initially these viruses were mistaken for bacteria. They have 1.9 and 2.5 million base pairs, respectively, whereas most viruses contain from three to several hundred genes. At the same time, more than 93% of the genome of these giants has no analogues among living organisms. The discoverers of Pandora viruses suggest that they originated from full-fledged cells. (Abergel, 2015).
Microfossils very similar to pandoraviruses were found in Karelia in Archean tufogenic sedimentary rocks with an age of more than 2600 million years (Astafieva, 2006). Structures with “flattened-oval shapes” are only six times larger in size than pandoraviruses (Figure 2.2).
In recent decades, several species of giant viruses of the Mimiviridae family have been found parasitizing amoebas. (Arslan, 2011; Jônatas Abrahão et al., 2018; Lviv et al., 2018). These viruses are hundreds of times larger than traditional viruses in linear dimensions and have icosahedron-shaped capsids. In cross sections, they are observed as hexagonal or pentahedral zonal structures (Fig. 2.3-2.5.).The capsids of giant viruses are covered with numerous protein strands (Fig 2.3; 2.4.), and the Tupanvirus still has a long “tail” (Fig 2.5.) (Jônatas Abrahão et al., 2018). The genomes of megaviruses include both DNA and RNA and from one to one and a half million base pairs.
Structures similar in morphology to modern giant viruses, representatives of the Mimiviridae family, were described as acritarchs from Early Paleozoic deposits of Eastern Siberia (15 µm across) and a microstructure from the Orhei meteorite (Fig. 2.6.-2.7.; Rudavskaya, 1972).
In principle, the ancestors of some modern giant viruses could have a cellular structure and the ability to reproduce independently. For example, the discoverers of giant tupanviruses and pandoraviruses admit that they originated from more complex unicellular organisms that simplified and lost some genes during the transition to a parasitic lifestyle (Abergel, 2015; Jônatas Abrahão et al., 2018).
However, to determine the viral nature of microfossils, external morphological similarity with some modern viruses is not enough. Even giant viruses have long been mistaken for bacteria. And for acritarchs, similar to viruses, there is not enough information about the details of their internal structure and, most importantly, their relationship with other microorganisms.
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