'The End Of The Age Of Man?'

Christie Lynn, at the wonderful "Observations of a Nerd" blog, writes about how the Male of some species may be on the way out. Read an excerpt below. Fascinating stuff. It's up as a semi-finalist in the 3 Quarks poll of the best Science blog posts of 2009.

"No matter what our cultural influences might try to tell us, what makes a man a man isn't rippling abs, bulging biceps, or a rugged personality.

The only thing required to be a man, as far as people are concerned, is a Y chromosome.

Human beings, barring genetic anomalies, have 23 pairs of chromosomes in their genomes. One half of each pair comes from each parent. Twenty-two of these pairs of chromosomes are the same in all people. But the last set is different - they alone determine sex. You can have two possible combinations: XX and XY, and the Y is what makes a man male. It alone carries the genes for testes and sperm that separate the boys from the girls.

But the Y chromosome is behaving unlike any other chromosome in our bodies - it's shrinking. The X chromosome, Y's feminine partner, contains almost 1500 genes. It's believed that the Y chromosome, too, once contained around that many. But now, the lonely Y only contains 86 genes. It's lost almost 95% of its genetic material.

Why is the Y shrinking? The answer lies in how our cells replicate. When a cell divides, it has to make entire copies of its DNA to put into the new cell. But our cells aren't perfect - whenever they copy genes, they make a few mistakes. Some of these mistakes are caught by special enzymes which "check" the DNA, but not all of them. Those that make it past the initial checkpoints are usually fixed later by borrowing information from the partner chromosome. So if you have a flaw in a gene on your dad's copy of chromosome 18, the body replaces that gene with the corresponding one from your mom's copy of chromosome 18. But the Y chromosome has no exact partner - ever.

Over time, these mistakes accumulate, turning large portions of the DNA into junk, which is later removed - thus the chromosome slowly shrinks.

What will happen if the Y chromosome disappears? No one's entirely sure. In truth, no one's sure that it can be lost entirely. Some believe that the important "male" genes will attach to the X chromosome or another chromosome, which has happened in some species of mole vole. These animals have either two X chromosome paired together or only one, unpaired X and still have two distinct sexes. In humans, this can happen, though rarely. It's known as XX male syndrome, where somehow a few of the Y's genes have attached to an X, rendering the person still "male" despite a lack of a Y chromosome. However, in this case, the X chromosomes tend to feminize the person as well, leading to small testes, sterility and effeminate characteristics. Others believe that another chromosome will attach to the Y, giving it more genes and allowing it to continue onward. Still others believe that it will never fully disappear, and will remain indefinitely as a husk of a chromosome containing only the one or two genes absolutely necessary for manhood.

Why is it so important to have men around? After all, plenty of species breed asexually or without sex distinctions. There are even species of lizard which are entirely female. Instead of mating with men and exchanging genetic material, they hump each other and simply clone themselves. The process of reproducing without fertilization in vertebrates is called parthenogenesis. It has been documented in some species of shark, too. But never, not once, has it been found in humans. Other than a rumor of some woman named Mary about 2,000 years ago, no one has ever had a virgin birth.

Even if human beings were to be suddenly able to undergo parthenogenesis, we would lose a vital component of our reproductive process. Without that variety, we'll be far more susceptible to the onslaught of disease and parasites which evolve far more rapidly than we do.

So, if it's not bad enough that the Ys are shrinking, the men, in general, are losing their manliness due to environmental effects.

Most scientists believe that a true loss of the Y, and thus men, would spell the end of our species

There are several, vital genes, they argue, that have to come from the male. And clearly, our reproductive process is currently dependent upon men. Even if we did evolve a way around the lack of males, the loss of genetic diversity is likely to be devastating. We'll be at the mercy of fast-evolving parasites and viruses. But then again, who really knows? Even if the Y disappears, perhaps the next few million years will not spell the end of the age of humans, but solely the end of the age of man - with the age of woman just beginning.

 

Read the full post here. And vote for it here