Sexual Nature - A Brief Natural History Of Sex



Sex is one of Nature's great mysteries. It is dangerous, complicated and extremely inefficient compared to the way microbes reproduce. Microscopic bugs just split in two and even some female lizards have decided that sex with males is not worth the bother. These abstemious lizards just clone themselves.

But so many organisms - from humans to seaweeds - do reproduce sexually. There must be excellent reasons why some mystery bug on Earth two billion years ago invented sex and why millions of kinds of animals, plants and fungi persist with it, and with great success.

Adam Rutherford's quest for answers takes him from the banks of London's River Lea in search of alien asexual water snails to a private room at the Brussels Natural History Museum to view a couple of extinct turtles who were killed in the act and fossilised in a dying embrace.

Along with probing theories about the purpose of sex, Adam charts the milestones in the evolutionary history of sexual reproduction. Philip Larkin was wrong when he said sexual intercourse was invented in 1963. Primitive armoured fish were doing at least 380 million years ago. This is the age of the earliest known fossil male appendages, unearthed at a remote spot called Go Go in Australia.

Sex goes back along way and in many respects it is perplexing. But if sex hadn't evolved, life on Earth would have turned out extremely dull. The planet would be populated by nothing more microscopic microbes and covered in bacterial slime - not the magnificent riot of weird and wonderful biological diversity we see and are members of.

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Via gender-bending fish, the shrivelled Y chromosome and stroking James Bond's komodo dragons, Adam Rutherford explores the multifarious ways Nature uses to make females and males.

The biological Ying and Yang of the two sexes is a fundamental condition for sexual reproduction, and we have to thank sex for the evolution of the extraordinary variety of complex life on Earth (see episode 1 of this series). Without sex, the Earth would merely be a world of bacteria, oozing in microbial slime.

Given that female and male was the essential dichotomy from the origins of sex 1.5 billion years ago, you might be forgiven for thinking that Nature produces males and females in a standardised and uncomplicated way in all creatures.

Far from it - as Adam Rutherford discovers when he enters the lair of the komodo dragons at London Zoo. A few years ago, the female dragon surprised everyone there with a virgin birth of a clutch of sons. She hadn't mated with a male and her unfertilised eggs developed into young males by a process called parthenogenesis (a kind of cloning). A wild female Komodo dragon may have this asexual reproductive trick up her sleeves if she colonises a new Pacific island. Without sex, she lays some males and can then found a new population by having sex with them. This is possible because lizards have a quite different set of sex-determining chromosomes from humans and (almost) all other mammals.

Despite their fearsome reputation as the largest and most deadly of lizards, Adam strokes the zoo's male Raja and is let into the secrets of normal dragon reproduction. Raja incidentally had a cameo role in the last James Bond film, 'Skyfall'.

Keeping with the cinematic theme, Adam also investigates what makes a boy or a girl in the tropical undersea world of 'Nemo'. 'It's NOT like the movie', warns Prof Bob Warner in California, who goes on to describe and explain a sexually fluid scene of opportunistic gender transformation. Nemo to Nema, Wilma to Willy the Wrasse, and frog-faced gobies who can flip between George and Georgina - whenever it will maximise their chance of producing offspring. Testes become ovaries or vice versa in a few days.

Adam also consults weird sex chromosome supremo Professor Jenny Graves in Melbourne Australia. She's the world expert on sex determination in the duck-billed platypus - her country's aquatic mammal which lays eggs. Although the platypus is a mammal, it's more reptilian under the fur. It has 11 sex chromosomes compared to the two X and Y of other mammals and the two W and Z sex chromosomes of birds and most reptiles. When platypuses make egg and sperm cells, all the sex chromosomes perform a bizarre choreography to ensure any new embryos are viable. Jenny Graves explains how this utterly weird sex determination system illuminates the origins of our species' own sex determination which is controlled by the X and Y chromosomes.

Jenny and Adam also discuss the degeneration of the male Y chromosome. Many researchers claim it's been shrinking during the millions of years of mammal evolution. At some point, it will shrivel to nothing. Jenny gives it 5 million years at most - but it could disappear much, much sooner. What will this mean for men and the continuation of the human species?

Producer: Andrew Luck-Baker.