Surprisingly, recent studies in protist cell biology have shown that cyclic differentiation by asymmetric cell division are ancient eukaryotic traits 67 and stem & progenitor lineages (SPCLs) originating from the common eukaryotic ancestor, are widely distributed in the eukaryotic world. Lower eukaryotes such as primitive intestinal pathogenic amoebae - best adapted to the human host organism - are capable of polyploidisation-depolyploidisation cycles providing totipotency and stemness to the disseminated microcell progeny 8. As a result of cyclic processes, cell populations of pathogenic amoebae consist of stem and progenitor cells as well as somatic-vegetative cells changing their genotype when converting to a pathogenic state which causes liver abscesses 9101112.

In 2007 Erenpreisa and Craig 13 introduced the term cancer life cycle as an evolutionary conserved cycle of life, analogous to the life cycles of certain unicellular organisms, supporting the older statements of Sundaram et al. 14 and Rajaraman et al 15 from 2004/2006 that rightly consider stemness as a cyclic property afforded by depolyploidisation of cysts-like structures, later named aCLSs 161718 or PGCCs 1920212223.

As recently shown similarities between the protist life cycle and the development of cancer cell populations and the cancer life cycle are striking 161718. Both cancer cells and protists are capable of forming reproductive cysts or cyst like structures (aCLS, PGCCs) by polyploidisation or hyper-polyploidization (n ≥8) and the cyst progeny forms a primary stem cell line capable of differentiating into reproductive and somatic/vegetative cells and clones and sublines. Both in cancer and protists, cysts and aCLSs are formed sequentially by committed daughter cells of the reproductive sublines or occasionally by stressed vegetative cells of the somatic clones. If cancer has really an atavistic origin and performs an ancestral reproductive life cycle, it is conceivable that TP53 mutants are in fact primitive TP53 variants (a/TP53) 2425. Switching to primitive a/TP53 variants such as Ehp53 of amoebae 26 and Dp53 of Drosopihla 27 could explain why "mutated" TP53 of cancer repairs in a Dp53 manner only DNA damages of the reproductive cells but not the genotoxic DNA damage occurring in irradiated somatic cells.

Histopathologists still differentiate between well-differentiated low-grade tumors - early stages of cancer whose cells look almost normal - and poorly or undifferentiated tumors (late cancer stages) whose “immature” cells look very different. Previous cancer researchers have adopted the idea of reversed differentiation (progressive dedifferentiation) to explain cancer cell phenotypes and thought that tumor cells regress to intermediary stages of non- malignant cell development 282930.

However, appropriate research supporting this assumption is missing. While non-malignant cells during differentiation express a given set of genes in a coordinated and repeated template, only a subset of the same gene set is expressed in tumors 31 and this subset can differ from one patient tumor to another as well as between different cells of the same tumor. Differentiation patterns of malignant cells are disorganized. Cancer cell differentiation is aberrant and characteristic for tumors 31. Today it is questionable to assume cancer cells recapitulate in reverse the same developmental stages as non-malignant cells.

Are mutations merely consequences or causes of acquired cancer? Mutations are alterations of DNA sequences of genes occurring by mispairings and changes in one DNA base pair. They result either in the substitution of amino acids in the protein made by the gene or in a shortened protein that may function improperly or not at all. Other mutations occur by insertion, deletion or duplication of a piece of DNA. Genetic variations can have large effects in cancer initiation. On the other hand, certain mutations in the cancer repressor genes BRCA1 or BRCA2 greatly increase the familial risk for either breast or ovarian cancer 50.

Errors are a natural part of replication but usually repair enzymes recognize structural imperfections removing and correcting them. Some replication errors make it past these mechanisms becoming permanent mutations. Spontaneous mutations occur in the absence of stress and environmental damages such as radiation and chemicals. Moreover, when genes for DNA repair enzymes become mutated, mistakes accumulate at a much higher rate 51. After the next cell division incorrectly paired nucleotides become permanent mutations and served as templates for further replication events. Not all mutations are bad and it is not a direct correlation between mutations and cancer. Some mutations lead to genetic variation and evolution. However, many researchers sustain the idea that somatic mutations accumulated during proliferation and cell division of somatic cells may result in cancer 5152 but a growing number of researchers believe that mutations are effects and not causes of cancer. For example, many mutations detected in pancreatic cancer are present in the pancreas of older persons who never develop pancreatic cancer in their lifetime 53.

Cancer researchers have found that tumors are mosaics of mutant cells containing both genetic and epigenetic changes that distinguish them from normal cells. Mutations and epimutations are heritable: daughter cells inherit by cell division genetic and epigenetic abnormalities of the mother cell and may acquire itself new genetic and epigenetic abnormalities. Epimutations occur via DNA methylation, histone modification and RNA interference 54.However, many of the genetic and epigenetic abnormalities correspond to increased proliferation rates of mutant populations 555657. In the atavistic opinion however, the proliferation rate depends rather on oxygen contents: primitive cell types such as intestinal amoebae are capable to perform oxygenic cell cycles in less of 5-6 hrs 6712.

System instability is considered to be the major contributing factor of genetic heterogeneity in cancer. In most solid cancers (breast cancers, melanoma, and lung cancer) genome instability comes from the large frequency of mutation in the whole genome DNA sequence 585960 and from multiple cycles of clonal and non-clonal expansions. The best-understood alterations in tumor cells are the silencing or down regulation of gene expression by changes in the methylation of nucleotides. Methylation changes are thought to occur more frequently that DNA mutations; they are responsible for many changes during tumorigenesis and neoplastic progression. In cancer, loss of expression of genes occurs about 10 times more frequently by transcription silencing than by mutations 61. Transcriptional silencing may be of more importance than mutation in leading to progession of cancer. In colorectal cancer there are 600-800 genes transcriptionally silenced 6162. Another path to transcriptional repression occurs by altered expression of microRNAs 63.

There is a wide spread belief that normal human cells become cancer cells largely because mutations in their genes. On the popular internet websites of many National Cancer Societies 6465 mean mutations transform normal genes to become “cancer-causing genes” and refer to the genes having mutations linked to cancer as “cancer genes”. It is thought that (i) many mutations are needed before a cell becomes a cancer cell and (ii) cancer-causing mutations would accumulate over the course of a life time. On the other side, hereditary cancers are rare and it is believed that people inheriting mutated genes from the parents get the same type of cancer faster as a person getting sporadic cancer by acquiring a multitude of mutations during its own life period. As a consequence, hereditary cancers are more aggressive than the sporadic occurring cancers and do not respond to the typical treatments for sporadic cancer forms.

As reported by Bailey et al. 66 and Li et al. 67 researchers found more than 290 genes for which molecular lesions (mutations) are known. The “cancer genes” represented at the time more than 1% of the human genome 5. Many researchers considered that mutated genes are causativein tumorigenesis 68. Since the first report their number has increased to about 1000. Some mutations prevent a protein from being made, others may change the functionality of proteins and others up-regulate genes to overexpress a protein. Most genome-guided cancer treatments work today by blocking cancer-driving proteins. 31 targeted therapies approved by the FDA (USA) work in a manner similar to Gleevec, a terceptin related drug 69. Other drugs, such as Larotrectinib, target mutant cancer genes such TRK fusion genes 70.

Mutations affect both tumor suppressor genes and DNA repair genes that control normal cell growth and cell division, and prevent errors in DNA, but tumor suppressor genes may exhibit more frequently altered expression 68. Despite genetic and phenotypic heterogeneity observed in cancer, most tumors share certain characteristics leading to the idea of common genetic pathways that are deregulated in all cancer cells. More often than not, so called tumor-initiating mutations may predict what types of mutations occur later in the progression of certain tumors. Such genetic pathways were intially discovered in colon cancers. Each of the histological changes occurring in adenomas evolving to carcinomas is accompanied by the mutation of specific genes 71. There are subsets of mutations that correlate with specific types of cancer, and subsets of genes that correlate with the degree of malignancy 7273.

Recently more and more studies distinguish between (i) common driver mutations - which contribute to cancer initiation and development - and (ii) passenger mutations with accumulate in cells but do not contribute to carcinogenesis 66747576777879. Presumed common driver mutations - fundamental to the disease process - were identified in over 20 significantly mutated genes. Commonly mutated genes at substantially high mutated frequency are TP53, KRAS, SMAD4 and CDKN2A 5380. The dominant common mutations subject cell-cycle regulation genes (TP53, CDKN2A), DNA damage repair genes (BRCA1, BRCA2, PALB2), DNA mismatch and other intracellular processes. Many other genes are mutated at substantially lower frequency. Usually, the median number of mutated genes is about 60 per cancer 53. Persons with mutations in DNA repair genes are likely to acquire additional mutations.

Recently researcher have turned their attention to a group of genes (gene sets) derived from known pathways of protein-protein interactions which may be frequently perturbed within tumor cells and lead to acquisition of carcinogenic properties.

In the past there were not suitable methods available to detect individual driver genes carrying recurrent mutations 81. That's why researchers at the time considered driver genes as high coveragegenes of high mutual exclusivity, although they cover a high number of cancers and tend towards mutual exclusivity. A single mutation - for example the mutation of TP53 - would be usually enough to disturb one pathway. Thus, it was believed that TP53 coverage is exclusive and the p53 pathway does not occur simultaneously with other driver mutations. However, recently researchers discovered a certain cooperativity 74828384 existing between distinct mutated driver pathways (MDPs) 57858687.This coopertivity among different pathways likely occurs simultaneously in a large cohort of patients 88. It is a co-occurrence of common MDPs and individual-specific MDPs 7478. Hoadley et al. 89 found similarity among driver gene sets across distinct cancer types. According to Zhang and Zhang 78 there are eight common driver gene sets of BRCA, indicating the complexity of BRCA carcinogenesis.

Recently Marticorena et al. 77 describe universal patterns of selection; which look like driver mutations enabling cancer cells to evade normal constrains on cell proliferation to invade tissues and other organs. The number of mutations driving cancer varies considerably across different cancer types. Some of them occur in genes that are not yet identified as cancer driver genes. They are many more genes remaining to be discovered 90. The authors propose, there is a relatively small constant number of mutated genes (N= 2-11) necessary to convert a single normal cell into a cancer cell 77.

In the last 4-5 years scientists have found that the number of human genes that code for proteins is not as numerous as previously thought and put this number at less than 19000 (1-2% of the whole genome). 98% of the human DNA does not code for proteins. Scientists consider, hidden switches of regulatory elements dial gene expression up and down 4. There are hundreds of thousands of functional regions (non-coding genome portions) whose task is to control gene expression. Their number is overwhelming: there are about 3 Mio regulatory DNA regions thought to contain some 15 Mio binding sites for regulatory proteins (transcription factors) that control gene expression. On the other side, the “dark matter genome” is thought to contain numerous nonfunctional leftovers from evolutionary history 1.

Cancer biologists believe, certain regulatory elements lie hidden among the non-coding DNA driving normal gene expression, controlling the molecular mechanisms of cancer. Results suggest that disrupting a gene regulatory element has a more drastic impact on cell function than disrupting the gene itself 4. In the case of a transcription factor there are thousand genomic sites that affect p53 suppression function 96. Participants to the ENCODE project consider that 10-20% of the non-coding genome has a function that if disrupted, may be significantly perturb the cell 4. In the light of these findings Polak et al. 97 consider cancer as a “regulatory and epigenetic program that is superimposed on a cell and the result is the development of genetic and genomic instability”

One of the most exciting opinions - alternative to DNA mutation theory - comes 2017 from Australia, regarding the evolutionary origin of genes. The Trigos research group 2 mapped 17,318 human genes to a phylogenetic tree consisting of 16 evolutionary human gene phylostrata representing the major evolutionary innovations and made a transcriptional analysis regarding the age of these genes. Human genes assigned to phylostrata 1-3 date back to unicellular ancestors (UC genes), whereas genes assigned to later phylostrata emerged in multicellular ancestors (MC genes). To investigate how the expression of genes in tumors is related to evolutionary origins, researchers calculated the transcriptome age index (TAI) using RNAseq gene expression from seven tumor types and found that all tumors has consistently lower TAIs than their normal counterparts. They uncovered a close association between evolutionary gene age and expression level in RNA sequencing data. Genes conserved in unicellular organisms (UGs) were strongly up-regulated, whereas genes of metazoan origin (MGs) were inactivated. The coordinated expression of strongly interacting UGs and MGs - as occurred in primitive animals – was lost in tumors. According to the authors, 12 highly connected genes controlling UG/MG cooperation are the most important drivers of tumorigenesis.

In the last decade, cancer has been suggested to result from an atavistic process whereby the activation of primitive highly conserved programs lead to molecular phenotypes and population dynamics similar to unicellular organisms 569899100. Similarly it was suggested that the expression of highly conserved genes is a feature of drug resistance in tumor cells 101. Trigos et al. 2 show patterns of co-expression between highly inter-connected cellular processes that are disrupted in tumors. The findings suggest that deeper understanding of the differences in the expression and regulation of ancient UC genes and more recently evolved MC genes will be crucial for uncovering the molecular basis of cancer initiation and providing of new therapeutic strategies.

Briefly described, the life cycle of protists begins in the small intestine with cysts hatching and microcell dissemination 6. The eight totipotent microcells progress to the colon giving rise to a primary cell line of undifferentiated stem cells. Amoebic stem cells start in the colon two antagonistic sublines, depending of the intestinal oxygen gradient (0.1 to 6.0% O2 content) or the oxygen content of the resident niche 67102. Stem cells reaching more oxygenated capillary zones convert to reproductive ACD+ clones (progenitor sublines) that forms ACD cysts by asymmetric cell division and cyclic differentiation; in more hypoxic zones stem cells convert to somatic- vegetative clones, that also divide by asymmetric division but do not generate cysts (ACD- subline). In cultures of changing oxygen levels hypoxic proliferating stem cells convert to ACD+ and ACD- sublines, depending on the O2 content. Increased hypoxia converts the hypoxic stem cell line to the ACD- subline, while oxygenation favors stem cell conversion to oxygenic ACD+ sublines, fast cell cycling and cyclic differentiation. By increasing hypoxia the process of differentiation to cysts is delayed and the oxygenic ACD+ subline converts finally into a ACD- subline. Environmental oxygen content is the pivotal driver of stem cell conversion, proliferation and differentiation.

Daughter cells produced in cultures by asymmetric cell division are non-identical. In the case of the reproductive ACD+ subline,  one of two daughter cells is the self-renewing cell and the second is the committed precursor cell that exits cell cycle by the G1/G0 checkpoint; it enters polyploidisation and differentiation (cyst formation).  In contrast, the vegetative subline ACD- does not commit the second daughter cell to differentiation; the non-committed daughter cell arrests in a pre-differentiated state of G0/G1. These pre-differentiated MAS cells produced by the somatic ACD- subline are the counterpart of committed precursor cells produced by the ACD+ subline. MAS cells may reenter the mitotic cell cycle or differentiate to cysts under conditions of stress and nutrient depletion. Switching from one cell state into the other cell state occurs by epigenetic reprogramming, not by mutations. In the course of the disease (amoebiasis), early somatic clones change to more invasive and virulent genotypes capable of invading the liver and other organs 161718.

We believe that the decision of whether a cell commits for differentiation or not, or becomes a self-renewing pre-differentiated cell (such as MAS cells) depends on a molecular switches. We assume that a differentiation switch (DS) decides whether the second cell produced by asymmetric division becomes a MAS cell or commits for final cyst differentiation. The molecular DS switch regulates the cell fate to mitotic proliferation (DS/OFF) or differentiation (DS/ON). Differentiation commitment means DS/ON, proliferation DS/OFF. In the process of cyclic differentiation the regulatory switch is always open (DS/ON) while in the case of induced differentiation of MAS cells it must change from DS/OFF to DS/ON. Somatic MAS cells express their hidden differentiation potential by opening the regulatory DS switch.

Maybe that Eh/P53 (the p53 variant of amoebae) 26 has a pivotal role in commitment and differentiation of amoebic cells that derive from the early G1 or G1/G0. Stressed MAS cells, in a state of G2/M, finish first mitotic cell cycle and form committed daughter cells (two G1 cells) that differentiate post-mitoticly to cysts.

In summary, amoebic cells show: (1) all cells have differentiation potential: ACD+ cells express the differentiation potential autonomously while ACD- cells express the hidden differentiation potential only by stress; (2) differentiation always occurs from a state of G1; (3) under conditions of stress a mitotic blocked cell (such as the MAS cell) escapes cell death bypassing to an amitotic reproductive solution; it forms multiple microcell progeny by polyploidisation and depolyploidisation; (4) most of the individual cell states may turn into each other; (5) switching from one amoebic cell state to the other, including the transition from the stressed MAS cells to the reproductive process of polyploidisation and depolyploidisation, is not mutational but epigenetically accomplished.

The life cycle of cancer is quite similar. In recent years more researchers have tried to clarify the relationship between hyperpolyploidy (n>16) and cancer 1920212223 however, the key role of PGCCs (aCLSs) in starting cancer’s reproductive life cycle was little understood. We suppose that initiation of cancer starts from a genomic dysregulated cell blocked in G1 or G0/G1 that is in a certain sense analogous with the MAS cell of amoebae. This dysregulated human cell is capable of reactivating the atavistic gene module initiating the atavistic life cycle of cancer.

In the recent years cancer researchers have experimentally induced polyploid giant cancer cells (PGCCs, aCLSs) by irradiation and chemotherapeutics 1920212223. This is not de novo initiation of cancer from normal human cells but induced PGCC differentiation from cancer stem cells resistant to irradiation or chemotherapeutics. Stressed induced siCLS, and genotoxic induced giCLS start either from the primary stem cell pool or from reproductive aCLS+ clones (~ 1-2% of the treated cell population) and not from the somatic aCLS- subline (~ 98% of cells). We suppose that some of the self-renewing progenitor cells or committed precursor cells (Figure 1) may repair DNA damage caused by irradiation, likely with the help of an atavistic a/p53 variant. a/p53 variants such as Ehp53 or Dp53 2627 cannot repair DNA damage of the somatic aCLS- subline. Reproductive cells and stem cells after DNA repair form giCLS that give rise to new resistant stem cells capable of metastasis 103.

Proliferating somatic cancer cells are “facultative” stem cells. Unfavorable growth conditions and environmental stress reprogram some of them to secondary stem cells (cell plasticity). There is a lively exchange of information between somatic and reproductive sublines, with or without aCLS formation (Figure 1). On the other hand somatic cells change genotype forming numerous aberrant phenotypes. Reprogramming events are cell-state changes not caused by mutations but many mutations occur as effects of the aberrant phenotypes. We suppose that most of the aberrant phenotypes occurring in tumorigenesis are consequences of the great genetic control conflict that occurs between up regulated UG genes and down regulated MG genes.