image

China science

Are men going to become extinct? Some experts give males 5 million years, but one Chinese-led team says there is still hope

Fragility of male sex chromosome has caused it to shed more than 900 genes over the course of evolution, but one recently discovered protective mechanism may yet save the day

PUBLISHED : Thursday, 04 February, 2016, 8:30am
UPDATED : Monday, 12 June, 2017, 12:53pm

The male sex chromosome, which is notoriously fragile, has been shedding genes over the course of human evolution, leading some scientists to fret that male babies will no longer be born in 5 million years’ time.

While this may sound far-fetched, the animal kingdom throws up some scary precedents. Whiptail lizards are a case in point: they have already evolved into a self-sustaining species composed entirely of females.

Moreover, women live longer than men (73.5 years vs. 68.5 years on average).

Others believe that, before such a perfectly matriarchal society could ever come to fruition on a global scale, the earth would be destroyed by nuclear Armageddon, a renegade asteroid, worldwide pestilence - or the machines will take over.

But even if the world does die, mankind takes to the skies in search of a new planetary home, and robots remain pets and personal assistants, women are unlikely to ever see men completely phased out, according to a reassuring new study by Chinese and other scientists.

The study, which was published recently in the peer-reviewed journal PLOS ONE, pours cold water on what many may consider an outlandish “doomsday” prediction concerning the end of “man” kind.

It did this by analysing the X and Y chromosomes of 72 human donors from China and Africa, which led to the discovery that male sex genes have been protected for the last 50,000 years or so by a process known as DNA methylation, an epigenetic mechanism that cells use to keep a lid on gene expression, or change.

The international research team, which was led by Professor Sun Yingli at the Beijing Institute of Genomics, claim male genes have benefited from such protection since the time the first modern humans migrated out of Africa.

Epigenetic means “above genes”, and an epigenetic mechanism is one that does not change the sequence or internal structure of genes. DNA methylation falls under this category as it adds a methyl group to genetic molecules (DNA) in a way that modifies the genes’ functions, not their sequence or structure.

As is taught widely in school, the sex of a human baby is determined by two chromosomes: X and Y. These are also known as gonosomes. The same hold true for most other mammals, as well as certain plants.

If both chromosomes are the same (XX), the baby is a girl. If they are distinct (XY), it’s a boy. As such, the Y chromosome sustains the male lineage as it passes from father to son.

Both chromosomes have been found in animals dating back 300 million years. But back then, both X and Y had over 1,000 genes apiece.

Yet slowly, over the millennia, the number of genes associated with the Y chromosome dwindled - until only about 50 remain today. In contrast, the X chromosome still has about 1,000 genes.

However, deeper scientific studies have revealed that the rate of loss slowed dramatically about 6 million years ago, when man started to evolve from chimpanzees.

At this point, the spigot through which the genes were being leaked seems to have been - if not turned off - then at least radically tightened.

The epigenetic study conducted by Sun’s team showed how DNA methylation has maintained a level of stability in the patterning of the Y chromosome.

“This indicates that the Y chromosome is not as fragile as was previously suggested,” Sun’s team wrote in the paper.

“The function and phenotypes of human males have been protected by a stable DNA methylation pattern for tens of thousands of years,” they added.

Epigenetic studies have created a lot of buzz in recent years as an epigenetic mechanism allows an individual to acquire a new trait after they are born without undergoing any genetic mutation.

But epigenetic traits were not believed to be inheritable, because most of the acquired traits would be “erased” by the reshuffling of the genes inside the embryo.

Other scientists responded to Sun’s paper by saying it creates more questions than answers.

“If the DNA methylation of the human Y chromosome has remained unchanged for tens of thousands of years, it would be a curse rather than a blessing,” said Prof. Liao Kai, a researcher with the Shanghai Institutes for Biological Sciences under the Chinese Academy of Sciences.

“If that were the case, it would cause men to lose their ability to adapt to an ever-changing environment,” he said.

This is based on the logic that epigenetic mechanisms play a crucial role in human evolution because they can respond to environmental changes much faster than the process of genetic mutation.

They do this by allowing an individual to develop new physical traits that help them adapt - but without changing their overall genetic blueprint.

“Natural selection favours species that can adapt rather than resist change,” Liao said.