sperm fertilizing egg//© nobeastsofierce, Adobe Stock
From ScienceDaily:
One of the greatest enigmas of evolutionary biology is that while sex is the dominant mode of reproduction among multicellular organisms, asexual reproduction appears much more efficient and less costly. However, in a study publishing on June 6 in the open-access journal PLOS Biology, researchers suggest that sexual reproduction is favored by selection because, unlike asexual reproduction, it not only provides important evolutionary advantages in constantly changing environments, but also prevents the invasion of transmissible cancer, or “cheater” cells. …
Not only did first multicellular organisms have to deal with their own cheater cells, they also had to evolve adaptations to prevent them being colonized by foreign malignant cells (i.e. infectious ones). Because asexual reproduction leads to identical (“clonal”) organisms, this mode of reproduction is risky due to the possibility of being invaded by clonal infectious cell lineages (i.e. transmissible cancers). Conversely, sexual reproduction decreases the compatibility of contagious cancer cells with their hosts, limiting individual infection risk, as well as the risks of transmission between parent and offspring. Sexual reproduction also generates genetic variation that facilitates the detection of foreign cells, the first and critical step of immune protection.
Although relatively rare, transmissible cancers do exist (e.g. Tasmanian devils, dogs, bivalves), and increasing evidence suggests that most, if not all, malignant cells are potentially transmissible provided a suitable transmission route is offered. Given the ubiquity of cancer in multicellular organisms, in combination with the plethora of potential transmission routes, sexual reproduction may have been favoured as a less risky, more profitable option to produce viable offspring despite its associated costs. Paper. (open access) – Frédéric Thomas, Thomas Madsen, Mathieu Giraudeau, Dorothée Misse, Rodrigo Hamede, Orsolya Vincze, François Renaud, Benjamin Roche, Beata Ujvari. Transmissible cancer and the evolution of sex. PLOS Biology, 2019; 17 (6): e3000275 DOI: 10.1371/journal.pbio.3000275 More.
It’s an interesting theory but the obvious problem is that transmissible cancers are, as the authors admit, rare. They may always have been rare, relative, say, to predation or extinction—whether sex was part of a life form’s organization or not. And sex is a big development, with many consequences, including a heightened risk of extinction.
Cancer cells, on this view, are seen as a form of parasite:
… As early cells banded together to form single, eukaryotic organisms, these organisms would have needed to guard against member cells that refused to subordinate themselves to the whole—“internal cheater cells,” or cancer cells. Early multicellular organisms would also have needed to develop defenses against invading malignant cells from other organisms, or transmissible cancers…
Such early immune systems would have had an easier time differentiating between healthy cells and malignancies, the study argues, if sexual reproduction created offspring that were genetically distinct from surrounding organisms. Targeting cancerous cells could have created an evolutionary pressure to embrace sex, similar to the pressure from parasites and other pathogens.
Jon Kelvey, “The Evolution of Sex Could Have Provided a Defense Against Cancer Cells” at Smithsonian Magazine
Ah yes, that “evolutionary pressure” again. It says nothing about how exactly the machinery develops. The question isn’t what would benefit an organism but for what paths did resources actually exist?
When we get down to that, we are talking about science. The rest is all speculation.
See also: Can sex explain evolution?
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