Sex Evolved Because It Protects Against Rare Contagious Cancers, Scientists Think

Sex is a risky business. And yet, for some reason, so many organisms—including humans—do it. Now, a team of scientists argue it could be partly because the process protects against certain types of cancer.

Asexual reproduction doesn't require a partner, so it's much faster than the sexual type. It also avoids the potential costs associated with finding a mate in the first place. But sexual reproduction, meanwhile, is useful for a number of reasons: It involves two parents and so prevents harmful gene mutations from being passed on, and it helps with resistance to pathogens and parasites.

In an essay published in the journal PLOS Biology, scientists presented a theory which they say could answer a 50-year question of why sexual reproduction is preferred by 99 percent of eukaryotes: organisms whose nucleus is wrapped inside a membrane, from humans to mushrooms.

The scientists argue it could stop what are known as cheater cells, or transmissible cancer cells. Rare transmissible cancers exist in some animals including dogs, Tasmanian devils, and water-dwelling molluscs called bivalves. Cheater cells put the rest of a population of cells at risk by exploiting the community for selfish gain.

Organisms have evolved to prevent cancers—where cells multiply uncontrollably—with the help of processes such as those run by the immune system.

But in order to survive and reproduce, the first multi-cellular organisms had to fight internal cheater cells as well as infectious malignant cells from the outside. And as asexual reproduction makes identical organisms, this raises their risk of being invaded by transmissible cancers, the authors think.

Sexual reproduction, meanwhile, lowers the chance of an organism not only catching an infection, but passing it on to their children. Transmissible cancers are also less likely to be compatible with their host's cells if they are made sexually. This could help the immune system pick up and kill nasty invaders.

Frédéric Thomas, study co-author of the IRD in France, told Newsweek: "The high prevalence of sexual reproduction in the natural world indirectly suggests that the selective forces behind the evolution of sex must be strong and pervasive."

Until recently, explained Thomas, ecologists have ignored the roles that parasites and the microbiota (the bugs like bacteria, fungi and viruses which live on plants and animals) might play on the biology and evolution of their hosts.

Leaps forward in technology have enabled scientists to explore this link, and have made it impossible to ignore the role they might play, argued Thomas.

As processes which cause tumors in multicellular organisms are ubiquitous and the potential routes of transmission for invaders is diverse, evolution seems to favor sexual reproduction as way to make viable offspring with fewer roadblocks to their survival, "despite its associated costs," said Thomas.

"To our knowledge, this selective scenario for the initial evolution of sex across the tree of life is novel," he argued.

"More generally we hope that our perspective will pave the way for exploring the underestimated role of oncogenic processes [when a tumor grows] in shaping the biology, ecology and evolution of their multicellular hosts."

Thomas said the research could one day help to find natural defenses against cancer.

"For instance, if our theory is correct, ancient asexual lineages that have persisted for millions of generations, such as oribatid mites, darwinulid ostracods and bdelloid rotifers, probably possess mechanisms conferring a high resistance to oncogenic processes."

These could one day be applied to cancer treatments, he said.