Sexual reproduction refers to the production of offspring by a male and a female, both parents providing half of their genes for the descendant.

Life on Earth began in the bottom of the oceans around 3.8 billion years ago, and for long, it consisted only of simple single-celled organisms – bacteria and archaea. The two first life forms lived side-by-side deep in the water, reproducing all alone by growing large enough before dividing in two.

Once, for one reason or another, an archaea swallowed a bacteria. Perhaps to the surprise of both, the bacteria did not die. Instead, it was permanently annexed inside its aggressor, and the two began to live in a mutually beneficial relationship (symbiosis). A more sophisticated cell came into being – a eukaryote.

Eukaryotic cells turned out to be remarkable. Not only could they group together to form animals, plants, and fungi in the future, but they also invented sex and sexual reproduction more than a billion years ago (the first evidence), if not much earlier, and laid the groundwork for ever more diverse life on Earth – when both parents contribute half of their genes (randomly by natural selection) to their offspring, the newborn can better adapt or mutate to changes in environments, diseases, and other threats.¹

Since eukaryotic cells began to team up into more complex life forms less than 635 million years ago, organisms have spread from one place to another by swimming, crawling, walking, or flying. Or they have been physically pulled apart when rivers have formed, mountain ranges have thrust upwards, and continents have torn into pieces. Separated populations could no longer interbreed and began to evolve in different directions, leading to myriad different species of animals, plants, and fungi, often quite unlike their common ancestors.

¹ Perhaps the most fundamental purpose for having sex is to produce offspring that can cope in the surrounding world, but many animals do it just for pleasure. They like to touch genitals in many ways, using different body parts, too. Some enjoy having sex with the opposite gender (heterosexuals), others with the same (homosexuals), if not both (bisexuals). Many do it with the living, few with the dead (necrophilia), perhaps alone (masturbation) as well. We (Sapiens) belong to the most imaginative ones, but bonobos, another great ape, are even more active. They engage in sexual acts multiple times a day but ultimately rarely have babies. Moreover, there are myriad different ways to make offspring in nature, a penis in a vagina being just one. Many animals do not engage in penetrative sex at all, not to mention plants and fungi, and some can reproduce all alone (parthenogenesis or asexual reproduction).

Bibliography

Panciroli, E. 2022. The Earth a Biography of Life The Story of Life On Our Planet Through 47 Incredible Organisms. London, United Kingdom: An Hachette UK company. 255 p. ISBN 9781529413984. Pages 21, 24-26, 28, 146.

Rutherford, A. 2014. Creation The Origin of Life. London, United Kingdom: Penguin Books. 272 p. ISBN 9781780229072. Page 47.

Brusatte, S. 2022. The Rise and Reign of the Mammals. London, United Kingdom: Picador. 500 p. ISBN 9781529034233. Page 10.

Frankopan, P. 2023. The Earth Transformed an Untold History. London, United Kingdom: Bloomsbury Publishing. 695 p. ISBN 9781526622563. Pages 27-28.

Rutherford, A. 2020. The Book of Humans a Brief History of Culture, Sex, and the Evolution of Us. London, United Kingdom: Weidenfeld & Nicolson. 240 p. ISBN 9781615195909. Pages 78-119.

Harari, Y. N. 2015. Sapiens a Brief History of Humankind. London, United Kingdom: Vintage. 512 p. ISBN 9780099590088. Page 166.

Next Story

635 MYA
Snowball Earth

Ice on our planet is more or less uncommon, but once in a while, temperatures on Earth fall low enough, and kilometres of thick glaciers begin to form on its poles. Sometimes, they spread even further and cover more and more of the surface. These periods are called ice ages. The first ice age appeared on Earth more than two billion years ago, but the most severe one occurred hundreds of millions of years later, an episode threatening the birth of all complex life forms.

Previous Story

3.5 BYA
Photosynthesis and the formation of oxygen

The atmosphere of early Earth was filled with nitrogen, greenhouse gases, and hazy vapours, and the temperature on the surface was blistering – hostile for any living being. Yet, simple microbial life was evolving in the oceans, and some of them began using the energy of sunlight to make their food from carbon dioxide and water. As a waste product, oxygen came into being. The colourless and odourless gas began to fill the oceans and the air, eventually enabling life to grow into animals, plants, and fungi.