“At its heart, sex is a process of genetic mixing: it creates unique sets of genes and trait combinations different from either of the two parents.”
“Evolutionary theorists agree that cloning, in many ways, is a more efficient mode of reproduction, which, in a world governed by the rules of natural selection, should readily outcompete sex.”
“Unlike bacteria, very few eukaryotic species revert to strict asexuality, and those that do seem to be relatively short-lived on the evolutionary timescale. Sex is costly, but it also appears to be essential for the long-term survival of complex life.”
“The last common ancestor of all eukaryotes and the first truly complex cell was already sexual – which is why the ultimate solution to the evolutionary enigma of sex should be sought within the origin of eukaryotes themselves.”
“The early evolution of complex life is still a long way from being fully understood, but we do have a basic idea of how it started. Around 2 billion years ago in a world ruled by microbes, a bacterial species formed a close symbiotic partnership with another simple cell – an archaeon. The interaction was so tight that bacterial symbionts eventually colonised the insides of archaea and were gradually transformed into mitochondria – the organelles of our cells specialising in energy production. The chimeric cell grew and expanded, using the genetic material of both partners and the newly available mitochondrial energy source to forge a cell of unparalleled complexity, inventing countless eukaryotic features along the way – including sex.”
“But Nick Lane, the author of Power, Sex, Suicide (2005) and The Vital Question (2015), thinks that sex was far more than just one of many pre-eukaryotic inventions – it was the key to survival of the emerging complex cell. The transition to eukaryotes was beset with challenges – not only because a bacterium had to survive within a foreign cell, but also because of the massive genomic revolution that was taking place: pieces of bacterial DNA were routinely migrating into the host genome, while its circular chromosomes were broken up into linear pieces, each containing only a part of the cell’s vital genetic repertoire.”
“Fusions facilitated gene mixing through recurrent recombination, stabilised the genome, and promoted its further expansion.”
“Quite unexpectedly, the evolutionary puzzle of sex comes down to the origin of cell fusion, not just recombination, in the early stages of the transition to complex life. Why would pre-eukaryotic cells start fusing with each other? There are several possibilities. The evolutionary biologist Neil Blackstone of Northern Illinois University developed a hypothesis in which host cell fusions are induced by bacterial symbionts, unhappy with the internal environment of their current host. My own theoretical analysis suggests that fusion evolved to promote mixing among the internalised symbiont populations, covering up the detrimental effects of faulty or selfish symbionts. Critically, both hypotheses predict that the ability to fuse was a direct consequence of the bacterial invasion into the archaeal host, putting the origin of the first sexual trait well before the development of the eukaryotic cell was complete.”
“Regardless of what the initial benefit of sexual cell fusion was, the repercussions were immense. Sex rescued the emerging eukaryotic cell when it was most vulnerable; without sex, the evolutionary transition would have plainly failed.”
“While sex appears to be critical for the long-term survival of contemporary eukaryotes, it was not itself an invention of the eukaryotic cell. It is far more likely that the eukaryotic cell evolved only because sex – through cell fusion and recombination – was invented in ancestral chimeric lineages leading to the last common ancestor of all complex life.”