1. Man's end?
In the age of genetic engineering, do we need two sexes any more?
The male Y chromosome has been steadily losing genes for millions of years. My male forefathers were once the proud carriers of around 1,400 genes, but today us men only have about 27 to our name. Compare this to the female X chromosome which still carries around 1,000 or more genes.
So does the size of a man's Y really matter, and what is the endpoint of this evolutionary descent: a world without men?
2. INTERACTIVE: World without men
Click or tap on the hand icon then drag the slider to see what a future G20 summit could look like without male leaders.
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President of the United States, Barack Obama, with China's President Xi Jinping. World leaders gather for the family photo in Hangzhou, China, for the 11th G20 Leaders Summit on 4 September 2016. Getty Images
3. The shrinking Y
Chromosomes are bundles of DNA stored in our cells. Your DNA is your genetic heritage or blueprint. Females have two X chromosomes (XX), males have an X and a Y chromosome (XY). A male child can only inherit a Y chromosome from his father and the X will come from his mother. A female child inherits one X chromosome from each of her parents. Despite retaining only 3% of its original genes, the male Y chromosome remains strong and resilient and plays a vital role for male specific function, instructing the human foetus to develop male sex organs, instead of defaulting to female. .
4. Virgin births
Virgin birth, known as parthenogenesis, is nature's asexual form of a world without men. Females of some species of lizards, snakes, insects and fish have spontaneously produced young, several while in captivity. In some species they don't survive very long, but others are more successful. Scroll through the gallery to discover species that don’t need males to reproduce.
5. In defence of men
Are men needed? The decline of the Y-chromosome happened millions of years ago and has slowed almost to a halt. With its shrinking days long over, the loss of the male sex seems to have been avoided, at least in most species. But why?
Y men bear gifts
It turns out that sexual competition for mates acts to weed out unfit individuals thereby ensuring the health of the gene-pool.
So-called asexual species, such as some lizards can suffer from this genetic deterioration. So sexually reproducing species generally outcompete the asexuals over long evolutionary time periods.
Sexual selection avoids extinction
Sexual selection is the process where males compete with other males to be chosen for reproduction by females.
Modern science is on the verge of being able to take a single cell from an individual – man or woman – and turn it into what is known as a stem cell that can develop into a normal foetus if implanted into a womb. But reproducing this way without the cleansing force of mate choice, will, in the long term, cause the gene pool to decay.
Y men still exist
Despite these breathtaking advances in science, it could be that human psychology and economics will also favour having two sexes. The majority of people still identify as heterosexual, and raising families on one’s own can be exhausting and unaffordable in the modern world.
Children might also prefer and benefit from having two domestic parents, although this remains a controversial topic.