In 2002, evolutionary biologist Jenny Graves shared a controversial calculation. Two years later, she wrote in a commentary that the human Y chromosome “has run out of time.”
Over the past 300 million years, the chromosomes that determine male sex have lost 97% of their ancestral genes. Graves calculated that if this rate continued, in a few million years it might disappear.
The Y chromosome’s destined fate quickly swept the media and, in many cases, didn’t have the nuance Graves had hoped for.
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Her evolutionary thinking should not predict the “end of maleness,” or the end of the human species. They were “back-of-the-envelope” calculations in an academic paper, but produced a “hysterical response.”
“I’m surprised that there are people who worry that humans will be extinct in five to six million years,” Graves told ScienceAlert. “After all, we humans are only 100,000 years old, so I guess we’ll be lucky to survive the next century!”
Typical human inheritance. (ttsz/Getty Images)
But if Graves’s calculations are correct, what does it mean for the Y chromosome—and what does it mean for the future of men?
The good news is that similar chromosomes in other mammals, fish and amphibians have lost their sex-determining role in genetic recombination, and species continue to tell the story.
In some rodents, for example, the Y chromosome is completely and silently replaced. For example, three species of Y-less moles, Elobis Tarpis, Down’s syndrome Erobesand Elobiusnow there is only the X chromosome. The sex-determining genes on the Y chromosome are moved elsewhere.
Agouti (West German bug), at the same time, their Y chromosome is replaced by a new version, which now acts as the sex determiner.
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“If a new variant comes along…that works better than our poor old Y, it could become dominant very quickly,” Graves predicted. “Maybe it’s already in some people somewhere – how would we know?”
After all, sex-determining variants are not routinely screened in genomic studies, and if the Y chromosome’s effects were transferred to another chromosome in the population, there would be no noticeable difference. There will still be males, and they will still be able to reproduce.
The fate of the Y chromosome has captured the world’s attention for years, but beneath the surface of the sensational headlines, many people were unaware that a powerful scientific debate was brewing, bringing two incompatible views of evolution into direct conflict.
Graves subscribes to a school of thought that holds that sex chromosomes are a crumbling relic from a bygone era, destined to disappear and ready to be replaced. Another faction positions the Y chromosome as a tenacious survivor, ultimately safe and stable.
Jenn Hughes, an evolutionary biologist at MIT’s Whitehead Institute, agrees with the latter explanation. For more than a decade, Hughes and Graves disagreed over how to interpret the same evidence and were involved in a public academic debate.
In 2012, Hughes and her colleagues found that little of the core Y gene had been lost in the human lineage over the past roughly 25 million years.
Recent evidence strengthens this argument, showing that the core Y genes in primates are deeply conserved—compared with fish and amphibians, whose Y chromosomes gradually degenerate—which some scientists like Hughes interpret as a result of the long-term evolutionary stability of the primate Y chromosome.
“Our work comparing Y gene content across many mammals shows that gene loss starts out quickly but quickly levels off and gene loss essentially stops,” Hughes told ScienceAlert.
“The genes that remain on the Y play vital roles throughout the body, so the selective pressure to maintain these genes is too great for them to be lost.”
Graves disagrees with these explanations. She believes that just because a gene is highly conserved doesn’t mean it can’t be replaced.
In addition, she said, most of the extra genes discovered in the human Y sequence in recent years are duplicate copies, some of which may be inactive.
In the past, Graves has called the Y chromosome a “DNA dump.” Graves explains that creating large numbers of gene copies can increase the survival rate of at least one gene, but it can also accidentally create evolutionary “dumbs.”
It’s a bit like a game of telephone. The more information is shared, the more likely it is to persist, but also the more likely it is to be distorted.
So why does the Y chromosome behave like this?
Evolution is to blame.
“In the ancestor of placental mammals, the X and Y chromosomes were identical and had about 800 genes,” Hughes told ScienceAlert.
“Once Y became specialized for male sex determination (about 200 million years ago), X and Y stopped recombining in males and Y started losing genes. At the same time, X could still recombine in XX females, so it essentially stayed the same.”
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Today, only 3% of the genes once shared with the X remain on the human Y chromosome. But these genes are not lost at a constant rate. Hughes believes this is the biggest misunderstanding.
Graves agrees.
Her estimated extinction date of around 6 million years is based on direct, stable degeneration of the Y chromosome, but she says this is extremely unlikely, meaning there is a large margin of error in this estimate.
“Now and forever,” Graves told Science Alert. “I’m surprised it’s taken so seriously!”
While at certain moments the Y chromosome may appear to be in a stable state, Graves believes these snapshots won’t last, even though they appear to have lasted 25 million years.
“I don’t see any reason to think that Y degradation has or could have ceased in primates or any other group of mammals,” Graves said. “Progress has been slow and fitful, for reasons that are clear to us.”
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In 2011, Hughes and Graves held a public debate on whether the Y chromosome was stable or doomed, with the audience at the 18th International Chromosomes Conference voting 50/50. They are divided down the middle into correct hypotheses.
Hopefully it won’t take 6 million years to decide the winner.
