The present review addresses studies based on behavioral similarities between cancer cells in a metazoan organism and unicellular organisms in an ecosystem and between malignant tumors and some primordial colonial forms of life, from which metazoans evolved. Unicellular organisms and their colonies can exist only by compensating for their losses by the proliferation of remaining cells. Therefore, proliferation or readiness to proliferate is a priority for cells. Metazoans evolution was associated with increasing the stringency of the control of this priority. In this perspective, the common set of cancer hallmarks, which emerges in the courses of progression of different cancers, which feature different initial combinations of mutations in oncogenes and antioncogenes and exist under different initial conditions possible in various organs, results not from the convergent evolution but rather from the recapitulation of the evolutionary primordial mode of cell life. The malignant transformation is a manifestation of the disinhibition of a gestalt of traits required for the survival of cell populationin an ecosystem, which is what a tumor host eventually turns to for the tumor. Such ecological attitudes to malignant growth suggest analogies betweenthe therapeutic resistance of cancer with the resistance of pests to pesticides or of bacteria to antibiotics and between metastases and invasive species. An important means used by unicellular organisms to survive in noxious conditions is increasing the rate of mutation to produce different variants, among which some may happen to fit the current situation. This is what occurs upon cancer therapy, which increases the genetic diversity of cancer cells and acts as a factor of the positive selection of cells for resistance to the very same therapy. Because the resulting cell clones compete for body resources, the elimination of the cells that respond to therapy is favorable for the cells that are resistant to it. Therefore, it is unwise to use chemotherapy for tumor elimination. It is more prudent to aim at achieving a balance of cancer cell populations, which makes it possible to turn cancer into a chronic rather than fatal disease. These arguments justify the current efforts to develop regiments of the so-called adaptive therapy for cancer.