A brief history of sex – with no pictures

Printed from: https://newbostonpost.com/2016/02/19/a-brief-history-of-sex-with-no-pictures/

There’s a classic Gary Larson cartoon showing two amoebas sitting at home in their living room chairs, with one, holding a beer, telling the other, “Hey, I got news for you, sweetheart! … I am the lowest form of life on earth!”

Part of the humor of this is that these single-celled critters are known to reproduce asexually. They come from a family of single-celled organisms that were around on this planet long before sex. And indeed, from what we know of the history of life, the invention of sex was comparatively late in the game and occurred for multiple reasons (none of them very romantic).

This month, as the NewBostonPost discusses sex and gender on campus, it seems more than appropriate to give Darwin a nod and recall that had it not been for the invention of sex, life on campus would not have anything like the diversity (and confusion) we see. The world would likely still be just a globe teeming with single-celled microbes.

According to Nick Lane, professor of biochemistry at University College London, and author most recently of “The Vital Question: Energy, Evolution and the Complex Origins of Life,” how sex originated was considered the “queen” of biological problems in the 20th century.

With the advent of molecular genetics, we now have a good appreciation of why it started. The short answer: sex is better than cloning (though not by much).

“Sex breaks up rigid combinations of genes,” Lane writes, “allowing natural selection to ‘see’ individual genes, to parse all our qualities one by one. That helps in fending off debilitating parasites, as well as adapting to changing environments, and maintaining necessary variation in a population.”

And here Lane employs a delightful metaphor: “Just as medieval stonemasons once carved the back of sculptures that are hidden in the recesses of cathedrals, because they were still visible to God, so sex allows the all-seeing eye of natural selection to inspect her works, gene by gene.”

So, sex offers “fluid” chromosomes, he sums up: “ever-changing combinations of genes (technically alleles), which allows natural selection to discriminate between organisms with unprecedented finesse.”

“Without recombination,” he adds, “selection on certain genes interferes with selection on others. By generating chromosomes with different combinations of alleles – ‘fluid chromosomes’ – sex allows selection to act on all genes individually. Selection, like God, can now see all our vices and virtues, gene by gene. That’s the great advantage of sex. ”

How did it all come about? The broad picture is that it all started with what looks suspiciously like an act of gluttony (you knew sex would never be far from the subject of food). One bacteria ingested a smaller organism, likely something like a prototype of mitochondria (the mini energy centers of our cells), and instead of exploding, the two began to work together, the one inside the other. Now described as endosymbiosis, the feat became the launch platform for an entirely new kind of cell: eukaryotes, with nucleus protecting its DNA chromosomes and the unique ability to reproduce by swapping completely reshuffled sets of chromosomes with other cells.

Again, I’m overly simplifying the picture, but from this point, life was able to build an amazing level of complexity – animals and plants – it had never been able to achieve with single-celled asexually reproducing organisms.

But this raises an interesting question: why are there only two sexes? Could there have been more than two? Could we have turned out like some alien culture in a science fiction novel where there were males, females, and some other variants that are neither?

The answer, it turns out, comes back to mitochondria, the power generators that operate inside all of our cells. According to Lane, one of the deepest distinctions between the two sexes relates to the inheritance of mitochondria – one sex (the female) passes on its mitochondria, while the other (the hapless male) sex does not.

In our case, each of us inherits the mitochondria of our mothers – and only our mothers. Dads suffer the indignity of never passing on theirs. Why? Again, the short answer, Lane points out, is that genes in the mitochondria and the nucleus need to cooperate with each other, and mutations in either genome can undermine physical fitness. If we inherited the mitochondria of both parents, we’d run the greater risk of inheriting bad mutations.

“I proposed that uniparental inheritance, in which only one sex passes on the mitochondria, might improve the coadaptation of the two genomes,” he wrote.

As he studied the question further, it turns out that the gradual variation in the genes of mitochondria inherited from only one parent can explain the evolution of multicellular organisms that have three key attributes: anisogamy (sperm and eggs), single parental inheritance of mitochondria, and a germline, in which female eggs are sequestered early in development. Taken together these attributes form the basis for all sexual differences between males and females. “In other words,” Lane writes, “the inheritance of mitochondria can account for most of the real physical differences between the two sexes.”

What’s the upshot of all of this? Sex has never not been complicated – even at the cellular level.

Whether being mindful of this is going to make it any easier the next time you offer to buy that cute female or male a drink at the bar … that’s another question.

John Farrell is the author of “The Day Without Yesterday: Lemaitre, Einstein and the Birth of Modern Cosmology” from Basic Books. He writes about science, technology and media for Forbes.